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Publications in peer reviewed journals

84 Publications found
  • Demystifying mercury geochemistry in contaminated soil–groundwater systems with complementary mercury stable isotope, concentration, and speciation analyses

    David S. McLagan, Lorenz Schwab, Jan G. Wiederhold, Lu Chen, Jan Pietrucha, Stephan M. Kraemer, Harald Biester
    2022 - Environmental Science: Processes & Impacts, 9: 1406-1429


    Interpretation of mercury (Hg) geochemistry in environmental systems remains a challenge. This is largely associated with the inability to identify specific Hg transformation processes and species using established analytical methods in Hg geochemistry (total Hg and Hg speciation). In this study, we demonstrate the improved Hg geochemical interpretation, particularly related to process tracing, that can be achieved when Hg stable isotope analyses are complemented by a suite of more established methods and applied to both solid- (soil) and liquid-phases (groundwater) across two Hg2+-chloride (HgCl2) contaminated sites with distinct geological and physicochemical properties. This novel approach allowed us to identify processes such as Hg2+ (i.e., HgCl2) sorption to the solid-phase, Hg2+ speciation changes associated with changes in groundwater level and redox conditions (particularly in the upper aquifer and capillary fringe), Hg2+ reduction to Hg0, and dark abiotic redox equilibration between Hg0 and Hg(II). Hg stable isotope analyses play a critical role in our ability to distinguish, or trace, these in situ processes. While we caution against the non-critical use of Hg isotope data for source tracing in environmental systems, due to potentially variable source signatures and overprinting by transformation processes, our study demonstrates the benefits of combining multiple analytical approaches, including Hg isotope ratios as a process tracer, to obtain an improved picture of the enigmatic geochemical behavior and fate of Hg at contaminated legacy sites.

  • Internal tree cycling and atmospheric archiving of mercury: examination with concentration and stable isotope analyses

    David S. McLagan, Harald Biester, Tomas Navrátil, Stephan M. Kraemer, Lorenz Schwab
    2022 - Biogeosciences, 19: 4415-4429


    Trees predominantly take up mercury (Hg) from the atmosphere via stomatal assimilation of gaseous elemental Hg (GEM). Hg is oxidised in leaves/needles and transported to other tree anatomy including bole wood, where it can be stored long-term. Using Hg associated with growth rings facilitates archiving of historical GEM concentrations. Nonetheless, there are significant knowledge gaps on the cycling of Hg within trees. We investigate Hg archived in tree rings, internal tree Hg cycling, and differences in Hg uptake mechanisms in Norway spruce and European larch sampled within 1 km of a HgCl2-contaminated site using total Hg (THg) and Hg stable isotope analyses. Tree ring samples are indicative of significant increases in THg concentrations (up to 521 µg kg−1) from the background period (BGP; facility closed; 1992–present) to secondary industrial period (2ndIP; no HgCl2 wood treatment; 1962–1992) to primary industrial period (1stIP; active HgCl2 wood treatment; ≈ 1900–1962). Mass-dependent fractionation (MDF) Hg stable isotope data are shifted negative during industrial periods (δ202Hg of 1stIP: −4.32 ± 0.15 ‰, 2ndIP: −4.04 ± 0.32 ‰, BGP: −2.83 ± 0.74 ‰; 1 SD). Even accounting for a ≈ −2.6 ‰ MDF shift associated with stomatal uptake, these data are indicative of emissions derived from industrial activity being enriched in lighter isotopes associated with HgCl2 reduction and Hg0 volatilisation. Similar MDF (δ202Hg: −3.90 ± 0.30 ‰; 1 SD) in bark Hg (137 ± 105 µg kg−1) suggests that stomatal assimilation and downward transport is also the dominant uptake mechanism for bark Hg (reflective of negative stomatal-uptake MDF shift) rather than deposition to bark. THg was enriched in sapwood of all sampled trees across both tree species. This may indicate long-term storage of a fraction of Hg in sapwood or xylem solution. We also observed a small range of odd-isotope mass-independent fractionation (MIF). Differences in Δ199Hg between periods of different industrial activities were significant (Δ199Hg of 1stIP: 0.00 ± 0.03 ‰, 2ndIP: −0.06 ± 0.04 ‰, BGP: −0.13 ± 0.03 ‰; 1 SD), and we suggest MIF signatures are conserved during stomatal assimilation (reflect source MIF signatures). These data advance our understanding of the physiological processing of Hg within trees and provide critical direction to future research into the use of trees as archives for historical atmospheric Hg.

  • Large extent of mercury stable isotope fractionation in contaminated stream sediments induced by changes of mercury binding forms

    Lorenz Schwab, Florian M. Rothe, David S. McLagan, Alexandra Alten, Stephan M. Kraemer, Harald Biester, and Jan G. Wiederhold
    2022 - Frontiers in Environmental Chemistry, in press


    Mercury (Hg) release from contaminated legacy sites is a large contributor to riverine ecosystems and can represent a significant local and regional environmental issue even long after the initial site contamination. Understanding processes of in-stream species transformation is therefore important to assess the fate and bioavailability of the released Hg. In this study, we investigated in-stream Hg transformation processes with analyses of Hg binding forms and Hg stable isotopes. Stream sediments were collected downstream of a former kyanization facility (Black Forest, SW Germany), where highly soluble Hg(II)-chloride (HgCl2) was used as an anti-fouling agent to treat timber. Exfiltration of partly anoxic, contaminated groundwater with Hg concentrations of up to 700 μg L−1 into the adjacent Gutach stream is the main source of Hg to sediments. Total Hg concentrations in the stream bottom sediments (<2 mm) ranged from background values of 6.3 µg kg−1 upstream of the contaminated site to 77 mg kg−1 near the location of exfiltration of contaminated groundwater. A five-step sequential extraction procedure and Hg pyrolytic thermal desorption (PTD) analyses indicated changes in Hg binding forms in the sediments along the flow path towards a higher proportion of organically bound Hg. A large shift towards negative δ202Hg values was observed downstream of the contaminated site (change of ≈2‰) along with a minor offset in mass-independent fractionation. Binary mixing models based on Hg isotope ratios using one industrial and different natural background endmembers were tested to estimate their respective contribution of Hg to the sediments but failed to produce plausible allocations. Based on the observed changes in isotopic composition, total Hg concentrations and Hg binding forms, we propose that the large extent of fractionation observed in downstream sediments is the result of a combination of kinetic isotope effects during sorption, redistribution of Hg within the sediment and the preferential transport of Hg associated with the sediment fine fraction. These results highlight the importance of transformation processes when assessing the sources and fate of Hg in environmental systems and show limitations of using simple mixing models based on Hg stable isotopes.

  • Ligand-Induced U Mobilization from Chemogenic Uraninite and Biogenic Noncrystalline U(IV) under Anoxic Conditions

    Kyle J. Chardi, Anshuman Satpathy, Walter D. C. Schenkeveld*, Naresh Kumar, Vincent Noël, Stephan M. Kraemer, and Daniel E. Giammar
    2022 - Environmental Science and Technology, 56: 6369–6379


    Microbial reduction of soluble hexavalent uranium (U(VI)) to sparingly soluble tetravalent uranium (U(IV)) has been explored as an in situ strategy to immobilize U. Organic ligands might pose a potential hindrance to the success of such remediation efforts. In the current study, a set of structurally diverse organic ligands were shown to enhance the dissolution of crystalline uraninite (UO2) for a wide range of ligand concentrations under anoxic conditions at pH 7.0. Comparisons were made to ligand-induced U mobilization from noncrystalline U(IV). For both U phases, aqueous U concentrations remained low in the absence of organic ligands (<25 nM for UO2; 300 nM for noncrystalline U(IV)). The tested organic ligands (2,6-pyridinedicarboxylic acid (DPA), desferrioxamine B (DFOB), N,N′-di(2-hydroxybenzyl)ethylene-diamine-N,N′-diacetic acid (HBED), and citrate) enhanced U mobilization to varying extents. Over 45 days, the ligands mobilized only up to 0.3% of the 370 μM UO2, while a much larger extent of the 300 μM of biomass-bound noncrystalline U(IV) was mobilized (up to 57%) within only 2 days (>500 times more U mobilization). This work shows the potential of numerous organic ligands present in the environment to mobilize both recalcitrant and labile U forms under anoxic conditions to hazardous levels and, in doing so, undermine the stability of immobilized U(IV) sources.

  • Soil-pH and cement influence the weathering kinetics of chrysotile asbestos in soils and its hydroxyl radical yield

    Martin Walter, Gerald Geroldinger, Lars Gille, Stephan M. Kraemer, Walter D.C. Schenkeveld
    2022 - Journal of Hazardous Materials, 431: 128068


    Asbestos fibers are carcinogenic minerals that have abundantly been used in different industrial settings and in consumer applications. Much of the historically used asbestos has ended up in terrestrial environments where the fibers pose a health risk to residents.

    Stephan Kraemer from EDGE together with the former PhD student Martin Walter and university assistant Walter Schenkeveld, and colleagues from the University of Veterinary Medicine Vienna, investigated the weathering kinetics and hydroxyl radical yield of toxic chrysotile asbestos fibers in suspensions of (cement-amended) soils with different pH and organic carbon properties.

    Soil solution pH proved to be the major determinant of asbestos weathering in soil suspensions (with fiber weathering and soil solution pH being inversely related). Addition of cement to soils inhibited asbestos weathering because of its alkalinity. The hydroxyl radical yield of asbestos in soil suspensions decreased by maximally ≈75%, and fully decreased to background levels in soils to which cement has been added (due to precipitation of Fenton-inactive minerals on fibers). In low pH podzol suspensions, also increases in the fibers’ hydroxyl radical yield were observed, presumably because of an association of soil solution Fe with the asbestos fiber surface. 

  • The potential contribution of hexavalent chromium to the carcinogenicity of chrysotile asbestos

    Martin Walter, Walter D.C. Schenkeveld*, Maura Tomatis, Karin Schelch, Barbara Peter-Vörösmarty, Gerald Geroldinger, Lars Gille, Maria C. Bruzzoniti, Francesco Turci, Stephan M. Kraemer, and Michael Grusch
    2022 - Chemical Research in Toxicology, in press


    Asbestos fibers are carcinogenic minerals that induce non-malignant and malignant diseases after inhalation.

    In this publication, Stephan Kraemer from EDGE together with the former PhD student Martin Walter and university assistant Walter Schenkeveld investigated whether Cr in chrysotile asbestos leaches from the fiber surface in its genotoxic hexavalent redox state upon oxidation by H2O2 (as found in inflamed asbestos-burdened tissues) at the physiological lung pH 7.4. These investigations were contemplated by collaborations with colleagues of the Medical University and the University of Veterinary Medicine in Vienna, and the University of Turin. Apart from Cr leaching from chrysotile, we also investigated the potential of cells from typical asbestos-burdened tissues and cancers to take up Cr(VI). Finally, we investigated the potential contribution of Cr on chrysotile surfaces to the fiber-mediated generation of highly toxic hydroxyl radicals out of H2O2.

    Chromium readily dissolved from chrysotile fibers in its genotoxic and carcinogenic hexavalent redox state upon oxidation by H2O2. All investigated cells of typically asbestos-burdened tissues and cancers readily took up Cr(VI). However, chromium associated with chrysotile did not contribute to fiber-mediated hydroxyl radical generation.

  • Catalytic effects of photogenerated Fe(II) on the ligand-controlled dissolution of Iron(hydr)oxides by EDTA and DFOB

    Jagannath Biswakarma, KyounglimKang, Walter D.C.Schenkeveld, Stephan M. Kraemer, Janet G.Hering. Stephan J.Hug
    2021 - Chemosphere, 263: 128188


    Low bioavailability of iron due to poor solubility of iron(hydr)oxides limits the growth of microorganisms and plants in soils and aquatic environments. Previous studies described accelerated dissolution of iron(hydr)oxides under continuous illumination, but did not distinguish between photoreductive dissolution and non-reductive processes in which photogenerated Fe(II) catalyzes ligand-controlled dissolution. Here we show that short illuminations (5–15 min) accelerate the dissolution of iron(hydr)oxides by ligands during subsequent dark periods under anoxic conditions. Suspensions of lepidocrocite (Lp) and goethite (Gt) (1.13 mM) with 50 μM EDTA or DFOB were illuminated with UV-A light of comparable intensity to sunlight (pH 7.0, bicarbonate-CO2 buffered solutions). During illumination, the rate of Fe(II) production was highest with Gt-EDTA; followed by Lp-EDTA > Lp-DFOB > Lp > Gt-DFOB > Gt. Under anoxic conditions, photochemically produced Fe(II) increased dissolution rates during subsequent dark periods by factors of 10–40 and dissolved Fe(III) reached 50 μM with DFOB and EDTA. Under oxic conditions, dissolution rates increased by factors of 3–5 only during illumination. With DFOB dissolved Fe(III) reached 35 μM after 10 h of illumination, while with EDTA it peaked at 15 μM and then decreased to below 2 μM. The observations are explained and discussed based on a kinetic model. The results suggest that in anoxic bottom water of ponds and lakes, or in microenvironments of algal blooms, short illuminations can dramatically increase the bioavailability of iron by Fe(II)-catalyzed ligand-controlled dissolution. In oxic environments, photostable ligands such as DFOB can maintain Fe(III) in solution during extended illumination.

  • Genome wide transcriptomic analysis of the soil ammonia oxidizing archaeon Nitrososphaera viennensis upon exposure to copper limitation

    Carolina Reyes, Logan H. Hodgskiss, Melina Kerou, Thomas Pribasnig, Sophie S. Abby, Barbara Bayer, Stephan M. Kraemer, Christa Schleper
    2020 - The ISME journal, 14: 2659-2674


    Ammonia-oxidizing archaea (AOA) are widespread in nature and are involved in nitrification, an essential process in the global nitrogen cycle. The enzymes for ammonia oxidation and electron transport rely heavily on copper (Cu), which can be limited in nature. In this study the model soil archaeon Nitrososphaera viennensis was investigated via transcriptomic analysis to gain insight regarding possible Cu uptake mechanisms and compensation strategies when Cu becomes limiting. Upon Cu limitation, N. viennensis exhibited impaired nitrite production and thus growth, which was paralleled by downregulation of ammonia oxidation, electron transport, carbon fixation, nucleotide, and lipid biosynthesis pathway genes. Under Cu-limitation, 1547 out of 3180 detected genes were differentially expressed, with 784 genes upregulated and 763 downregulated. The most highly upregulated genes encoded proteins with a possible role in Cu binding and uptake, such as the Cu chelator and transporter CopC/D, disulfide bond oxidoreductase D (dsbD), and multicopper oxidases. While this response differs from the marine strain Nitrosopumilus maritimus, conserved sequence motifs in some of the Cu-responsive genes suggest conserved transcriptional regulation in terrestrial AOA. This study provides possible gene regulation and energy conservation mechanisms linked to Cu bioavailability and presents the first model for Cu uptake by a soil AOA.

  • Importance of oxidation products in coumarin-mediated Fe(hydr)oxide mineral dissolution

    Matthias Baune, Kyounglim Kang.Walter D. C, Schenkeveld, Stephan M. Kraemer, Heiko Hayen, Gu ̈nther Weber
    2020 - Springer, 33: 305–321
  • Investigation of Siderophore-Promoted and Reductive Dissolution of Dust in Marine Microenvironments Such as Trichodesmium Colonies

    Nivi Kessler, Stephan M. Kraemer, Yeala Shaked, Walter D. C. Schenkeveld
    2020 - Front. Mar. Sci, 7: 2296-7745


    Desert dust is a major source of iron (Fe) to phytoplankton in many Fe-poor ocean regions. However, phytoplankton often struggle to obtain dust-bound Fe (dust-Fe) due to its low solubility and short residence time in the euphotic zone. Trichodesmium, a globally important nitrogen-fixing, cyanobacterium, is uniquely adapted for utilizing dust as a source of Fe. Trichodesmium colonies can actively collect and concentrate dust particles within the colony core and enhance dust-Fe dissolution rates via two bio-dissolution mechanisms: reduction and complexation by strong Fe-chelators termed siderophores. Here, mimicking bio-dissolution in Trichodesmium colonies, we studied the kinetics of desert dust dissolution by a siderophore and a reductant in seawater. By concurrent measurements of dissolved Fe, silica (Si), and aluminum (Al) we recognized two major mineral pools that released Fe into seawater over an 8-day period: Fe(hydr)oxides and aluminosilicates. In the presence of the siderophore desferrioxamine-B, we observed two stages of dissolution: a short stage of fast Fe dissolution followed by a lasting stage of slow Fe dissolution that was highly correlated to Al and Si dissolution. In the presence of the reductant, ascorbate, Fe dissolution was not correlated to Al and Si dissolution and was relatively slow. Based on these observations and on dust mineralogy, we constructed a conceptual model for dust-Fe dissolution by a siderophore and a reductant from two major mineral pools: reductive and siderophore-promoted dissolution of Fe(hydr)oxides and slow continuous dissolution of Fe-bearing clays in the presence of a siderophore. Our findings highlight the importance of clays as an Fe source to Trichodesmium and possibly to other marine phytoplankton and can be further used to assess the contribution of dust to the Fe requirements of natural Trichodesmium colonies. Combining our measured bio-dissolution rates with dust concentrations retained within colonies from the Gulf of Aqaba, we calculated the supply of dissolved Fe from dust to single Trichodesmium colonies. Applying published Fe-quotas and growth rates we calculated the Fe requirements of the colonies under Fe-limited and Fe-replete conditions. The calculated dissolved Fe supply from dust retained within colonies can fulfill the Fe requirements of slow growing Fe-limited colonies, but cannot support fast growth and/or higher cellular Fe quotas. We conclude that despite these bio-dissolution mechanisms, dust-Fe availability to Trichodesmium is low and propose that it employs additional mechanisms to actively mine Fe from dust.

  • Linking Isotope Exchange with Fe(II)-Catalyzed Dissolution ofIron(hydr)oxides in the Presence of the Bacterial SiderophoreDesferrioxamine‑B

    Jagannath Biswakarma, Kyounglim Kang, Walter D. C. Schenkeveld, Stephan M. Kraemer, Janet G. Hering, Stephan J. Hug
    2020 - Environ. Sci. Technol., 54: 768-777


    Dissolution of Fe(III) phases is a key process in making iron available to biota and in the mobilization ofassociated trace elements. Recently, we have demonstrated that submicromolar concentrations of Fe(II) significantly acceleraterates of ligand-controlled dissolution of Fe(III) (hydr)oxides at circumneutral pH. Here, we extend this work by studyingisotope exchange and dissolution with lepidocrocite (Lp) and goethite (Gt) in the presence of 20 or 50μM desferrioxamine-B(DFOB). Experiments with Lp at pH 7.0 were conducted in carbonate-buffered suspensions to mimic environmentalconditions. We applied a simple empirical model to determine dissolution rates and a more complex kinetic model thataccounts for the observed isotope exchange and catalytic effect of Fe(II). The fate of added tracer57Fe(II) was stronglydependent on the order of addition of57Fe(II) and ligand. When DFOB was addedfirst, tracer57Fe remained in solution. When57Fe(II) was addedfirst, isotope exchange between surface and solution could be observed at pH 6.0 but not at pH 7.0 and 8.5where57Fe(II) was almost completely adsorbed. During dissolution of Lp with DFOB, ratios of released56Fe and57Fe werelargely independent of DFOB concentrations. In the absence of DFOB, addition of phenanthroline 30 min after tracer57Fedesorbed predominantly56Fe(II), indicating that electron transfer from adsorbed57Fe to56Fe of the Lp surface occurs on a timescale of minutes to hours. In contrast, comparable experiments with Gt desorbed predominantly57Fe(II), suggesting a longertime scale for electron transfer on the Gt surface. Our results show that addition of 15μM Fe(II) leads to dynamic chargetransfer between dissolved and adsorbed species and to isotope exchange at the surface, with the dissolution of Lp by ligandsaccelerated by up to 60-fold.

  • Mercury Isotope Fractionation in the Subsurface of a Hg(II) Chloride-Contaminated Industrial Legacy Site

    Flora M. Brocza, Harald Biester, Jan-Helge Richard, Stephan M. Kraemer, Jan G. Wiederhold
    2019 - Environmental Science & Technology, 13: 7296-7305


    To understand the transformations of mercury (Hg) species in the subsurface of a HgCl2-contaminated former industrial site in southwest Germany, Hg isotope analysis was combined with an investigation of Hg forms by a four-step sequential extraction protocol (SEP) and pyrolytic thermodesorption. Data from two soil cores revealed that the initial HgCl2was partly reduced to metallic Hg(0) and that Hg forms of different mobility and oxidation state coexist in the subsurface. The most contaminated sample (K2-8, 802 mg kg–1 Hg) had a bulk δ202Hg value of around −0.43 ± 0.06‰ (2SD), similar to published average values for industrial Hg sources. Other sample signatures varied significantly with depth and between SEP pools. The most Hg-rich samples contained mixtures of Hg(0) and Hg(II) phases, and the water-extractable, mobile Hg pool exhibited heavy δ202Hg values of up to +0.18‰. Sequential water extracts revealed slow dissolution kinetics of mobile Hg pools, continuously releasing isotopically heavy Hg into solution. This was further corroborated by heavy δ202Hg values of groundwater samples. Our results demonstrate that the Hg isotope signature of an industrial contamination source can be significantly altered during the transformations of Hg species in the subsurface, which complicates source tracing applications but offers the possibility of using Hg isotopes as process tracers in contaminated subsurface systems.

  • The Effect of pH and biogenic ligands on the weathering of chrysotile asbestos: The pivotal role of tetrahedral Fe in dissolution kinetics and radical formation

    Martin Walter, Walter D. C. Schenkeveld, Michael Reissner, Lars Gille, Stephan M. Kraemer
    2019 - Chemistry A European Journal, 13: 3286-3300


    Chrysotile asbestos is a soil pollutant in many countries. It is a carcinogenic mineral, partly due to its surface chemistry. In chrysotile, FeII and FeIII substitute Mg octahedra (Fe[6]), and FeIII substitutes Si tetrahedra (Fe[4]). Fe on fiber surfaces can generate hydroxyl radicals (HO.) in Fenton reactions, which damage biomolecules. To better understand chrysotile weathering in soils, net Mg and Si dissolution rates over the pH range 3.0–11.5 were determined in the presence and absence of biogenic ligands. Also, HO. generation and Fe bulk speciation of pristine and weathered fibers were examined by EPR and Mössbauer spectroscopy. Dissolution rates were increased by ligands and inversely related to pH with complete inhibition at cement pH (11.5). Surface‐exposed Mg layers readily dissolved at low pH, but only after days at neutral pH. On longer timescales, the slow dissolution of Si layers became rate‐determining. In the absence of ligands, Fe[6] precipitated as Fenton‐inactive Fe phases, whereas Fe[4] (7 % of bulk Fe) remained redox‐active throughout two‐week experiments and at pH 7.5 generated 50±10 % of the HO. yield of Fe[6] at pristine fiber surfaces. Ligand‐promoted dissolution of Fe[4] (and potentially Al[4]) labilized exposed Si layers. This increased Si and Mg dissolution rates and lowered HO. generation to near‐background level. It is concluded that Fe[4] surface species control long‐term HO. generation and dissolution rates of chrysotile at natural soil pH.

  • Constraints to Synergistic Fe Mobilization from Calcareous Soil by a Phytosiderophore and a Reductant

    Walter Schenkeveld, Stephan M. Kraemer
    2018 - Soil Systems, 4: 67


    Synergistic effects between ligand- and reductant-based Fe acquisition strategies can enhance the mobilization of Fe, but also of competing metals from soil. For phytosiderophores, this may alter the time and concentration window of Fe uptake during which plants can benefit from elevated Fe concentrations. We examined how the size of this window is affected by the ligand and reductant concentration and by non-simultaneous addition. To this end, a series of kinetic batch experiments was conducted with a calcareous clay soil to which the phytosiderophore 2′-deoxymugineic acid (DMA) and the reductant ascorbate were added at various concentrations, either simultaneously or with a one- or two-day lag time. Both simultaneous and non-simultaneous addition of the reductant and the phytosiderophore induced synergistic Fe mobilization. Furthermore, initial Fe mobilization rates increased with increasing reductant and phytosiderophore concentrations. However, the duration of the synergistic effect and the window of Fe uptake decreased with increasing reductant concentration due to enhanced competitive mobilization of other metals. Rate laws accurately describing synergistic mobilization of Fe and other metals from soil were parameterized. Synergistic Fe mobilization may be vital for the survival of plants and microorganisms in soils of low Fe availability. However, in order to optimally benefit from these synergistic effects, exudation of ligands and reductants in the rhizosphere need to be carefully matched.

  • Fe(II)-Catalyzed Ligand-Controlled Dissolution of Iron(hydr)oxides

    Jagannath Biswakarma, Kyounglim Kang, Susan C. Borowski, Walter D.C. Schenkeveld, Stephan M. Kraemer, Janet G. Hering, Stephan J. Hug
    2018 - Environmental Science & Technology, 1: 88-97


    Dissolution of iron(III)phases is a key process in soils, surface waters, and the ocean. Previous studies found that traces of Fe(II) can greatly increase ligand controlled dissolution rates at acidic pH, but the extent that this also occurs at circumneutral pH and what mechanisms are involved are not known. We addressed these questions with infrared spectroscopy and 57Fe isotope exchange experiments with lepidocrocite (Lp) and 50 μM ethylenediaminetetraacetate (EDTA) at pH 6 and 7. Addition of 0.2–10 μM Fe(II) led to an acceleration of the dissolution rates by factors of 7–31. Similar effects were observed after irradiation with 365 nm UV light. The catalytic effect persisted under anoxic conditions, but decreased as soon as air or phenanthroline was introduced. Isotope exchange experiments showed that added 57Fe remained in solution, or quickly reappeared in solution when EDTA was added after 57Fe(II), suggesting that catalyzed dissolution occurred at or near the site of 57Fe incorporation at the mineral surface. Infrared spectra indicated no change in the bulk, but changes in the spectra of adsorbed EDTA after addition of Fe(II) were observed. A kinetic model shows that the catalytic effect can be explained by electron transfer to surface Fe(III) sites and rapid detachment of Fe(III)EDTA due to the weaker bonds to reduced sites. We conclude that the catalytic effect of Fe(II) on dissolution of Fe(III)(hydr)oxides is likely important under circumneutral anoxic conditions and in sunlit environments.

  • Low Fe(II) Concentrations Catalyze the Dissolution of Various Fe(III) (hydr)oxide Minerals in the Presence of Diverse Ligands and over a Broad pH Range

    Kyounglim Kang, Walter D. C. Schenkeveld, Jagannath Biswakarma, Susan C. Borowski, Stephan J. Hug, Janet G. Hering, Stephan M. Kraemer
    2018 - Environmental Science & Technology, 1: 98-107


    Dissolution of Fe(III) (hydr)oxide minerals by siderophores (i.e., Fe-specific, biogenic ligands) is an important step in Fe acquisition in environments where Fe availability is low. The observed coexudation of reductants and ligands has raised the question of how redox reactions might affect ligand-controlled (hydr)oxide dissolution and Fe acquisition. We examined this effect in batch dissolution experiments using two structurally distinct ligands (desferrioxamine B (DFOB) and N,N′-di(2-hydroxybenzyl)ethylene-diamine-N,N′-diacetic acid (HBED)) and four Fe(III) (hydr)oxide minerals (lepidocrocite, 2-line ferrihydrite, goethite and hematite) over an environmentally relevant pH range (4–8.5). The experiments were conducted under anaerobic conditions with varying concentrations of (adsorbed) Fe(II) as the reductant. We observed a catalytic effect of Fe(II) on ligand-controlled dissolution even at submicromolar Fe(II) concentrations with up to a 13-fold increase in dissolution rate. The effect was larger for HBED than for DFOB. It was observed for all four Fe(III) (hydr)oxide minerals, but it was most pronounced for goethite in the presence of HBED. It was observed over the entire pH range with the largest effect at pH 7 and 8.5, where Fe deficiency typically occurs. The occurrence of this catalytic effect over a range of environmentally relevant conditions and at very low Fe(II) concentrations suggests that redox-catalyzed, ligand-controlled dissolution may be significant in biological Fe acquisition and in redox transition zones.

  • Structure and reactivity of oxalate surface complexes on lepidocrocite derived from infrared spectroscopy, DFT-calculations, adsorption, dissolution and photochemical experiments

    Susan C. Borowski, Jagannath Biswakarma, Kyounglim Kang, Walter D.C. Schenkeveld, Janet G. Hering, James D. Kubicki, Stephan M. Kraemer, Stephan J. Hug
    2018 - Geochimica et Cosmochimica Acta, 244-262


    Oxalate, together with other ligands, plays an important role in the dissolution of iron(hdyr)oxides and the bio-availability of iron. The formation and properties of oxalate surface complexes on lepidocrocite were studied with a combination of infrared spectroscopy (IR), density functional theory (DFT) calculations, dissolution, and photochemical experiments. IR spectra measured as a function of time, concentration, and pH (50–200 µM oxalate, pH 3–7) showed that several surface complexes are formed at different rates and in different proportions. Measured spectra could be separated into three contributions described by Gaussian line shapes, with frequencies that agreed well with the theoretical frequencies of three different surface complexes: an outer-sphere complex (OS), an inner-sphere monodentate mononuclear complex (MM), and a bidentate mononuclear complex (BM) involving one O atom from each carboxylate group. At pH 6, OS was formed at the highest rate. The contribution of BM increased with decreasing pH. In dissolution experiments, lepidocrocite was dissolved at rates proportional to the surface concentration of BM, rather than to the total adsorbed concentration. Under UV-light (365 nm), BM was photolyzed at a higher rate than MM and OS. Although the comparison of measured spectra with calculated frequencies cannot exclude additional possible structures, the combined results allowed the assignment of three main structures with different reactivities consistent with experiments. The results illustrate the importance of the surface speciation of adsorbed ligands in dissolution and photochemical reactions.

  • A density functional theory investigation of oxalate and Fe(II) adsorption onto the (010) goethite surface with implications for ligand- and reduction-promoted dissolution

    James D. Kubicki, Daniel Tunega, Stephan M. Kraemer
    2017 - Chemical Geology, 14-22


    Oxalic acid is an important, biologically-produced species in the natural environment. The deprotonated form, oxalate, is dominant in aqueous solutions under circumneutral pH conditions and is a strong ligand for Fe(III). The high affinity of oxalate for Fe(III) means that Fe(III)-oxalate surface and aqueous complexes are common and can lead to ligand-enhanced dissolution. Fe(II) adsorption onto goethite (α-FeOOH) has been shown to enhance dissolution-recrystallization reactions. The goethite (010) face is one of the more common and reactive surfaces on this environmentally critical Fe-hydroxide phase. Hence, this study models both separate and coordinated adsorption of oxalate and Fe(II) onto the (010) face of goethite in order to test for synergistic effects of ligand-promoted and reductive dissolution. Periodic and cluster density functional theory (DFT) energy minimizations were performed to determine the structure, vibrational frequencies and energies of various configurations. The adsorption mechanism of oxalate is verified via comparison to observed IR spectra. The potential roles of oxalate and Fe(II) in ligand-enhanced reductive dissolution of goethite are discussed.

  • Experimental considerations in metal mobilization from soil by chelating ligands: The influence of soil-solution ratio and pre-equilibration A case study on Fe acquisition by phytosiderophores

    W.D.C. Schenkeveld, R.L. Kimber, M. Walter, E. Oburger, M. Puschenreiter, Stephan M. Kraemer
    2017 - Science of The Total Environment, 1831-1842


    The efficiency of chelating ligands in mobilizing metals from soils and sediments is generally examined under conditions remote from those under which they are exuded or applied in the field. This may lead to incorrect estimations of the mobilizing efficiency. The aim of this study was to establish the influence of the soil solution ratio (SSR) and pre-equilibration with electrolyte solution on metal mobilization and metal displacement. For this purpose a series of interaction experiments with a calcareous clay soil and a biogenic chelating agent, the phytosiderophore 2′-deoxymugineic acid (DMA) were carried out.

    For a fixed ligand concentration, the SSR had a strong influence on metal mobilization and displacement. Metal complexation was faster at higher SSR. Reactive pools of metals that were predominantly mobilized at SSR 6 (in this case Cu), became depleted at SSR 0.1, whereas metals that were marginally mobilized at SSR 6, were dominantly mobilized at SSR 0.1 (in this case Fe), because of large soil reactive pools. For a fixed “amount of ligand”-to-“amount of soil”-ratio, metal complexation scaled linearly with the SSR. The efficiency of ligands in mobilizing metals under field conditions can be predicted with batch experiments, as long as the ligand-to-soil-ratio is matched. In most previously reported studies this criterion was not met. Equivalent metal-complex concentrations under field conditions can be back-calculated using adsorption isotherms for the respective metal-complexes.

    Drying and dry storage created labile pools of Fe, Cu and Zn, which were rapidly mobilized upon addition of DMA solution to dry soil. Pre-equilibration decreased these labile pools, leading to smaller concentrations of these metals during initial mobilization, but did not reduce the lag time between ligand addition and onset of microbial degradation of the metal-complexes. Hence SSR and pre-equilibration should be carefully considered when testing the metal mobilizing efficiency of chelating ligands.

  • Magnitude and Mechanism of Siderophore-Mediated Competition at Low Iron Solubility in the Pseudomonas aeruginosa Pyochelin System

    Konstanze T. Schiessl, Elisabeth M.-L. Janssen, Stephan M. Kraemer, Kristopher McNeill, Martin Ackermann
    2017 - Frontiers in microbiology, 8: 1964


    A central question in microbial ecology is whether microbial interactions are predominantly cooperative or competitive. The secretion of siderophores, microbial iron chelators, is a model system for cooperative interactions. However, siderophores have also been shown to mediate competition by sequestering available iron and making it unavailable to competitors. The details of how siderophores mediate competition are not well understood, especially considering the complex distribution of iron phases in the environment. One pertinent question is whether sequestering iron through siderophores can indeed be effective in natural conditions; many natural environments are characterized by large pools of precipitated iron, and it is conceivable that any soluble iron that is sequestered by siderophores is replenished by the dissolution of these precipitated iron sources. Our goal here was to address this issue, and investigate the magnitude and mechanism of siderophore-mediated competition in the presence of precipitated iron. We combined experimental work with thermodynamic modeling, using Pseudomonas aeruginosa as a model system and ferrihydrite precipitates as the iron source with low solubility. Our experiments show that competitive growth inhibition by the siderophore pyochelin is indeed efficient, and that inhibition of a competitor can even have a stronger growth-promoting effect than solubilization of precipitated iron. Based on the results of our thermodynamic models we conclude that the observed inhibition of a competitor is effective because sequestered iron is only very slowly replenished by the dissolution of precipitated iron. Our research highlights the importance of competitive benefits mediated by siderophores, and underlines that the dynamics of siderophore production and uptake in environmental communities could be a signature of competitive, not just cooperative, dynamics.

  • Phytosiderophore-induced mobilization and uptake of Cd, Cu, Fe, Ni, Pb and Zn by wheat plants grown on metal-enriched soils

    Markus Puschenreiter, Barbara Gruber, Walter W. Wenzel, Yvonne Schindlegger, Stephan Hann, Bernhard Spangl, Walter D.C. Schenkeveld, Stephan M. Kraemer, Eva Oburger
    2017 - Environmental and Experimental Botany, 67-76


    We investigated to which extent phytosiderophores (PS), released by grasses for the acquisition of iron, solubilize other metals in contaminated soils, and how this affects metal mobilization and uptake in wheat plants. A plant-based bioassay (‘RHIZOtest’) and batch extraction scheme were carried out for assessing metal mobilisation in soil, PS exudation and metal accumulation in wheat. Increased PS exudation was observed in Fe-deficient wheat, leading to enhanced Zn, Cu, Mn and Ni concentrations in wheat shoots on some soils. In contrast, plant Cd and Pb concentrations were not affected. Likewise, in the batch experiment, strongly increased extractable Cu, Ni and Zn concentrations were observed, in particular when 100 or 1000 μM PS were added. Our results suggest that Fe deficiency can enhance the accumulation of some metals in shoots of grass species. Although our results indicate that the risk of enhanced accumulation of Cd and Pb in Fe deficient wheat shoots is rather small, further experiments conducted on soil for the complete vegetation period would be needed to confirm this observation.

  • The effect of pH, electrolytes and temperature on the rhizosphere geochemistry of phytosiderophores

    M. Walter, Stephan M. Kraemer, W. D. C. Schenkeveld
    2017 - Plant and Soil, 1: 5-23


    Background and aims

    Graminaceous plants are grown worldwide as staple crops under a variety of climatic and soil conditions. They release phytosiderophores for Fe acquisition (Strategy II). Aim of the present study was to uncover how the rhizosphere pH, background electrolyte and temperature affect the mobilization of Fe and other metals from soil by phytosiderophores.


    For this purpose a series of kinetic batch interaction experiments with the phytosiderophore 2′-deoxymugineic acid (DMA), a calcareous clay soil and a mildly acidic sandy soil were performed. The temperature, electrolyte concentration and applied electrolyte cation were varied. The effect of pH was examined by applying two levels of lime and Cu to the acidic soil.


    Fe mobilization by DMA increased by lime application, and was negatively affected by Cu amendment. Mobilization of Fe and other metals decreased with increasing ionic strength, and was lower for divalent than for monovalent electrolyte cations at equal ionic strength, due to higher adsorption of metal-DMA complexes to the soil. Metal mobilization rates increased with increasing temperature leading to a faster onset of competition; Fe was mobilized faster, but also became depleted faster at higher temperature. Temperature also affected biodegradation rates of metal-DMA complexes.


    Rhizosphere pH, electrolyte type and concentration and temperature can have a pronounced effect on Strategy II Fe acquisition by affecting the time and concentration ‘window of Fe uptake’ in which plants can benefit from phytosiderophore-mediated Fe uptake.

  • Microbial decomposition of 13C- labeled phytosiderophores in the rhizosphere of wheat: Mineralization dynamics and key microbial groups involved

    Eva Oburger, Barbara Gruber, Wolfgang Wanek, Andrea Watzinger, Christian Stanetty, Yvonne Schindlegger, Stephan Hann, Walter D.C. Schenkeveld, Stephan M. Kraemer, Markus Puschenreiter
    2016 - Soil Biology and Biochemistry, 196-207


    Being low molecular weight carbon (LMW-C) compounds, phytosiderophores (PS) released by strategy II plants are highly susceptible to microbial decomposition. However, to date very little is known about the fate of PS in soil. Using in-house synthesized 13C4-2′-deoxymugineic acid (DMA), the main PS released by wheat, we investigated DMA mineralization dynamics, including microbial incorporation into phospholipid fatty acids (PLFA), in the wheat rhizosphere and bulk soil of two alkaline and one acidic soil. Half-lives of the intact DMA molecule (3–8 h) as well as of DMA-derived C-compounds (8–38 days) were in the same order of magnitude as those published for other LMW-C compounds like sugars, amino acids and organic acids. Combining mineralization with PLFA data showed that between 40 and 65% of the added DMA was either respired or incorporated into soil microbial biomass after 24 h, with the largest part of total incorporated DMA-13C being recovered in gram negative bacteria. Considering root growth dynamics and that PS are mainly exuded from root tips, the significantly slower mineralization of DMA in bulk soil is of high ecological importance to enhance the Fe scavenging efficiency of PS released into the soil.

  • Retention of phytosiderophores by the soil solid phase adsorption and desorption

    M. Walter, E. Oburger, Y. Schindlegger, S. Hann, M. Puschenreiter, Stephan M. Kraemer, W. D. C. Schenkeveld
    2016 - Plant and Soil, 1: 85-97


    Background and aims

    Graminaceous plants exude phytosiderophores (PS) for acquiring Fe. Adsorption of PS and its metal complexes to the soil solid phase reduces the FePS solution concentration and hence Fe uptake. In this study we aimed to quantify adsorption, and to determine to what extent adsorption depends on the complexed metal and on soil properties. Furthermore, we examined if adsorption is a reversible process.


    Adsorption and desorption of PS and metal-PS complexes were examined in batch experiments in which the PS 2′-deoxymugineic acid (DMA) and its metal-complexes (FeDMA, CuDMA, NiDMA and ZnDMA) interacted with several calcareous soils.


    Adsorption of DMA ligand (0–1000 μM) and metal-DMA complexes (0–100 μM) was linear in the concentration range examined. Adsorption varied by a factor ≈2 depending on the complexed metal and by up to a factor 3.5 depending on the soil. Under field-like conditions (50 % water holding capacity), 50–84 % of the DMA was predicted to be retained to the soil solid phase. Alike adsorption, desorption of metal-DMA complexes is fast (approximate equilibrium within 1 hour). However, only a small fraction of the adsorbed FeDMA (28–35 %) could be desorbed.


    Despite this small fraction, the desorbed FeDMA still exceeded the amount in solution, indicating that desorption of FeDMA from soil reactive compounds can be an important process buffering the solution concentration.

  • Synergistic Effects between Biogenic Ligands and a Reductant in Fe Acquisition from Calcareous Soil

    Walter D. C. Schenkeveld, Zimeng Wang, Daniel E. Giammar, Stephan M. Kraemer
    2016 - Environmental Science & Technology, 1: 6381-6388


    Organisms have developed different strategies to cope with environmental conditions of low Fe availability based on the exudation of reducing, ligating, and acidifying compounds. In the context of Fe acquisition from soil, the effects of these reactive compounds have generally been considered independent and additive. However, highly efficient Fe acquisition strategies may rely on synergistic effects between reactive exudates. In the present study, we demonstrate that synergistic effects between biogenic ligands and a reductant (ascorbate) can occur in Fe mobilization from soil. Synergistic Fe mobilization was found for all ligands examined (desferrioxamine B (DFOB), 2′-deoxymugineic acid (DMA), esculetin, and citrate). The size and duration of the synergistic effect on Fe mobilization varied with ligand: larger effects were observed for the sideorphores compared to esculetin and citrate. For DFOB, the synergistic effect lasted for the 168 h duration of the experiment; for DMA, an initial synergistic effect turned into an antagonistic effect after 4 h because of enhanced mobilization of competing metals; and for esculetin and citrate, the synergistic effect was temporary (less than 24 h). Our results demonstrate that synergistic effects greatly enhance the reactivity of mixtures of compounds known to be exuded in response to Fe limitation. These synergistic effects could be decisive for the survival of plants and microorganisms under conditions of low Fe availability.

  • Constraining silica diagenesis in methane-seep deposits

    D. Smrzka, Stephan M. Kraemer, J. Zwicker, D. Birgel, D. Fischer, S. Kasten, J.L. Goedert, J. Peckmann
    2015 - Palaeogeography Palaeoclimatology Palaeoecology, 420: 13-26


    Silicified fossils and authigenic silica are common in ancient seep limestones. Silicification of calcareous fossils facilitates the preservation of even fine details and is therefore of great interest to paleontologists, permitting a reliable taxonomic identification of the chemosynthesis-based taxa that lived at hydrocarbon seeps. Four methane-seep limestones of Paleozoic, Mesozoic, and Cenozoic age with abundant silica phases are compared in this study; one, an Eocene seep deposit on the north shore of the Columbia River at Knappton, western Washington State, USA, is described for the first time. Its lithology and fabrics, negative δ13Ccarbonate values as low as − 27.6‰, and 13C-depleted biomarkers of archaea involved in the anaerobic oxidation of methane (AOM) reveal that the carbonate rock formed at a methane seep. The background sediments of the studied Phanerozoic seep limestones contain abundant siliceous microfossils, radiolarian tests in case of the Carboniferous Dwyka Group deposits from Namibia and the Triassic Graylock Butte deposits from Oregon (USA), as well as diatom frustules in case of the Eocene Knappton limestone and an Oligocene seep deposit from the Lincoln Creek Formation (Washington State, USA). These microfossils are regarded as the source of dissolved silica, causing silicification and silica precipitation. Silica cements formed after AOM-derived cements ceased to precipitate but before equant calcite formed. Numerical experiments using the computer code PHREEQC confirmed that (1) AOM increases the pH of pore waters and that (2) this pH increase subsequently mobilizes biogenic silica, (3) followed by the re-precipitation of the dissolved silica in the periphery of the AOM hotspot. The experiments revealed that degassing of carbon dioxide has the potential to significantly increase the local pH of pore waters, exerting an even stronger control on the local pH and silica dissolution than the rate of AOM alone.

  • Synergistic Effect of Reductive and Ligand-Promoted Dissolution of Goethite

    Zimeng Wang, Walter D. C. Schenkeveld, Stephan M. Kraemer, Daniel E. Giammar
    2015 - Environmental Science & Technology, 1: 7236-7244


    Ligand-promoted dissolution and reductive dissolution of iron (hydr)oxide minerals control the bioavailability of iron in many environmental systems and have been recognized as biological iron acquisition strategies. This study investigated the potential synergism between ligands (desferrioxamine B (DFOB) or N,N′-Di(2-hydroxybenzyl)ethylenediamine-N,N′-diacetic acid (HBED)) and a reductant (ascorbate) in goethite dissolution. Batch experiments were performed at pH 6 with ligand or reductant alone and in combination, and under both oxic and anoxic conditions. Goethite dissolution in the presence of reductant or ligand alone followed classic surface-controlled dissolution kinetics. Ascorbate alone does not promote goethite dissolution under oxic conditions due to rapid reoxidation of Fe(II). The rate coefficients for goethite dissolution by ligands are closely correlated with the stability constants of the aqueous Fe(III)–ligand complexes. A synergistic effect of DFOB and ascorbate on the rate of goethite dissolution was observed (total rates greater than the sum of the individual rates), and this effect was most pronounced under oxic conditions. For HBED, macroscopically the synergistic effect was hidden due to the inhibitory effect of ascorbate on HBED adsorption. After accounting for the concentrations of adsorbed ascorbate and HBED, a synergistic effect could still be identified. The potential synergism between ligand and reductant for iron (hydr)oxide dissolution may have important implications for iron bioavailability in soil environments.

  • Accurate LC-ESI-MS/MS quantification of 2-deoxymugineic acid in soil and root related samples employing porous graphitic carbon as stationary phase and a13C4-labeled internal standard

    Yvonne Schindlegger, Eva Oburger, Barbara Gruber, Walter D. C. Schenkeveld, Stephan M. Kraemer, Markus Puschenreiter, Gunda Koellensperger, Stephan Hann
    2014 - ELECTROPHORESIS, 9: 1375-1385


    For the first time the phytosiderophore 2′‐deoxymugineic acid (DMA) could be accurately quantified by LC‐MS/MS in plant and soil related samples. For this purpose a novel chromatographic method employing porous graphitic carbon as stationary phase combined with ESI‐MS/MS detection in selected reaction monitoring was developed. Isotope dilution was implemented by using in‐house synthesized DMA as external calibrant and 13C4‐labeled DMA as internal standard (concentration levels of standards 0.1–80 μM, determination coefficient of linear regression R2 > 0.9995). Sample preparation involved acidification of the samples in order to obtain complete dissociation of metal‐DMA complexes. Excellent matrix related LOD and LOQ depending on different experimental setups were obtained in the range of 3–34 nM and 11–113 nM, respectively. Standard addition experiments and the implementation of the internal 13C4‐DMA standard proved the accuracy of the quantification strategy even in complex matrices such as soil solution. The repeatability of the method, including sample preparation, expressed as short‐ and long term precision was below 4 and 5% RSD, respectively. Finally, application in the context of plant and soil research to samples from rhizosphere sampling via micro suction cups, from soil solutions and soil adsorption/extraction studies revealed a DMA concentration range from 0.1 to 235 μM.

  • Equilibrium and kinetic modelling of the dynamic rhizosphere

    W. D. C. Schenkeveld, Stephan M. Kraemer
    2014 - Plant and Soil, 1: 395-397
  • Geochemical Processes Constraining Iron Uptake in Strategy II Fe Acquisition

    W. D. C. Schenkeveld, Y. Schindlegger, E. Oburger, M. Puschenreiter, S. Hann, Stephan M. Kraemer
    2014 - Environmental Science & Technology, 2: 12662-12670


    Phytosiderophores (PS) are natural chelating agents, exuded by graminaceous plants (grasses) for the purpose of Fe acquisition (Strategy II). They can form soluble Fe complexes with soil-Fe that can be readily taken up. PS are exuded in a diurnal pulse release, and with the start of PS release a “window of iron uptake” opens. In the present study we examined how this window is constrained in time and concentration by biogeochemical processes. For this purpose, a series of interaction experiments was done with a calcareous clay soil and the phytosiderophore 2′-deoxymugineic acid (DMA), in which metal and DMA speciation were examined as a function of time and DMA concentration. Various kinetically and thermodynamically controlled processes affected the size of the window of Fe uptake. Adsorption lowered, but did not prevent Fe mobilization by DMA. Microbial activity depleted DMA from solution, but not on time scales jeopardizing Strategy II Fe acquisition. Complexation of competing metals played an important role in constraining the window of Fe uptake, particularly at environmentally relevant PS concentrations. Our study provides a conceptual model that takes into account the chemical kinetics involved with PS-mediated Fe acquisition. The model can help to explain how success or failure of PS-mediated Fe acquisition depends on environmental conditions.

  • Metal mobilization from soils by phytosiderophores - experiment and equilibrium modeling

    W. D. C. Schenkeveld, E. Oburger, B. Gruber, Y. Schindlegger, S. Hann, M. Puschenreiter, Stephan M. Kraemer
    2014 - Plant and Soil, 1: 59-71



    To test if multi–surface models can provide a soil-specific prediction of metal mobilization by phytosiderophores (PS) based on the characteristics of individual soils.


    Mechanistic multi-surface chemical equilibrium modeling was applied for obtaining soil-specific predictions of metal and PS speciation upon interaction of the PS 2’-deoxymugineic acid (DMA) with 6 soils differing in availability of Fe and other metals. Results from multi-surface modeling were compared with empirical data from soil interaction experiments.


    For soils in which equilibrium was reached during the interaction experiment, multi-surface models could well predict PS equilibrium speciation. However, in uncontaminated calcareous soils, equilibrium was not reached within a week, and experimental and modeled DMA speciation differed considerably. In soils with circum-neutral pH, on which Fe deficiency is likely to occur, no substantial Fe mobilization by DMA was predicted. However, in all but the contaminated soils, Fe mobilization by DMA was observed experimentally. Cu and Ni were the quantitatively most important metals competing with Fe for complexation and mobilization by DMA.


    Thermodynamics are unable to explain the role of PS as Fe carrier in calcareous soils, and the kinetic aspects of metal mobilization by PS need to be closer examined in order to understand the mechanisms underlying strategy II Fe acquisition.

  • Metallophores and Trace Metal Biogeochemistry

    Stephan M. Kraemer, Owen W. Duckworth, James M. Harrington, Walter D. C. Schenkeveld
    2014 - Aquatic Geochemistry, 2: 159-195


    Trace metal limitation not only affects the biological function of organisms, but also the health of ecosystems and the global cycling of elements. The enzymatic machinery of microbes helps to drive critical biogeochemical cycles at the macroscale, and in many cases, the function of metalloenzyme-mediated processes may be limited by the scarcity of essential trace metals. In response to these nutrient limitations, some organisms employ a strategy of exuding metallophores, biogenic ligands that facilitate the uptake of metal ions. For example, bacterial, fungal, and graminaceous plant species are known to use Fe(III)-binding siderophores for nutrient acquisition, providing the best known and most thoroughly studied example of metallophores. However, recent breakthroughs have suggested or established the role of metallophores in the uptake of several other metallic nutrients. Furthermore, these metallophores may influence environmental trace metal fate and transport beyond nutrient acquisition. These discoveries have resulted in a deeper understanding of trace metal geochemistry and its relationship to the cycling of carbon and nitrogen in natural systems. In this review, we provide an overview of the current state of knowledge on the biogeochemistry of metallophores in trace metal acquisition, and explore established and potential metallophore systems.

  • Root exudation of phytosiderophores from soil-grown wheat

    Eva Oburger, Barbara Gruber, Yvonne Schindlegger, Walter D. C. Schenkeveld, Stephan Hann, Stephan M. Kraemer, Walter W. Wenzel, Markus Puschenreiter
    2014 - New Phytologist, 4: 1161-1174


    For the first time, phytosiderophore (PS) release of wheat (Triticum aestivum cv Tamaro) grown on a calcareous soil was repeatedly and nondestructively sampled using rhizoboxes combined with a recently developed root exudate collecting tool. As in nutrient solution culture, we observed a distinct diurnal release rhythm; however, the measured PS efflux was c. 50 times lower than PS exudation from the same cultivar grown in zero iron (Fe)‐hydroponic culture.

    Phytosiderophore rhizosphere soil solution concentrations and PS release of the Tamaro cultivar were soil‐dependent, suggesting complex interactions of soil characteristics (salinity, trace metal availability) and the physiological status of the plant and the related regulation (amount and timing) of PS release.

    Our results demonstrate that carbon and energy investment into Fe acquisition under natural growth conditions is significantly smaller than previously derived from zero Fe‐hydroponic studies. Based on experimental data, we calculated that during the investigated period (21–47 d after germination), PS release initially exceeded Fe plant uptake 10‐fold, but significantly declined after c. 5 wk after germination.

    Phytosiderophore exudation observed under natural growth conditions is a prerequisite for a more accurate and realistic assessment of Fe mobilization processes in the rhizosphere using both experimental and modeling approaches.

  • Competitive ligand exchange between Cu-humic acid complexes and methanobactin

    Pesch ML, Hoffmann M, Christl I, Stephan M. Kraemer, Kretzschmar R
    2013 - Geobiology, 1: 44-54


    Copper has been found to play a key role in the physiology of methanotrophic micro‐organisms, and methane oxidation may critically depend on the availability of Cu. In natural environments, such as soils, sediments, peat bogs, and surface waters, the presence of natural organic matter (NOM) can control the bioavailability of Cu by forming strong metal complexes. To promote Cu acquisition, methanotrophs exude methanobactin, a ligand known to have a high affinity for Cu. In this study, the capability of methanobactin for Cu acquisition from NOM was investigated using humic acid (HA) as a model substance. The kinetics of ligand exchange between Cu–HA and methanobactin was observed by UV–vis spectroscopy, and the speciation of Cu bound to methanobactin was determined by size‐exclusion chromatography coupled to an ICP‐MS. The results showed that Cu was mobilized from HA by a fast ligand exchange reaction following a second‐order rate law with first‐order kinetics for both methanobactin and Cu–HA complexes. The reaction rates decreased with decreasing temperature. Equilibrium experiments indicated that methanobactin was not sorbed to HA and proved that methanobactin is competitive with HA for Cu binding by forming strong 1:1 Cu–methanobactin complexes. Consequently, our results demonstrate that methanobactin can efficiently acquire Cu in organic‐rich environments.

  • The influence of pH on iron speciation in podzol extracts: Iron complexes with natural organic matter, and iron mineral nanoparticles

    Elisabeth Neubauer, Walter D.C. Schenkeveld, Kelly L. Plathe, Christian Rentenberger, Frank von der Kammer, Stephan M. Kraemer, Thilo Hofmann
    2013 - Science of The Total Environment, 108-116


    The quantities of natural organic matter (NOM) and associated iron (Fe) in soil extracts are known to increase with increasing extractant pH. However, it was unclear how the extraction pH affects Fe speciation for particles below 30 nm. We used flow field-flow fractionation (FlowFFF) and transmission electron microscopy (TEM) to investigate the association of Fe and trace elements with NOM and nanoparticulate iron (oxy)hydroxides in podzol extracts.

    For extracts prepared at the native soil pH (~ 4), and within a 1–30 nm size range, Fe was associated with NOM. In extracts with a pH ≥ 7 from the E and B soil horizons, Fe was associated with NOM as well as with iron (oxy)hydroxide nanoparticles with a size of approximately 10 nm. The iron (oxy)hydroxide nanoparticles may have either formed within the soil extracts in response to the increase in pH, or they were mobilized from the soil. Additionally, pH shift experiments showed that iron (oxy)hydroxides formed when the native soil pH (~ 4) was increased to 9 following the extraction. The iron (oxy)hydroxide nanoparticles aggregated if the pH was decreased from 9 to 4.

    The speciation of Fe also influenced trace element speciation: lead was partly associated with the iron (oxy)hydroxides (when present), while copper binding to NOM remained unaffected by the presence of iron (oxy)hydroxide nanoparticles. The results of this study are important for interpreting the representativeness of soil extracts prepared at a pH other than the native soil pH, and for understanding the changes in Fe speciation that occur along a pH gradient.

  • Analysis of iron-phytosiderophore complexes in soil related samples: LC-ESI-MS/MS versus CE-MS

    Madeleine Dell'mour, Walter Schenkeveld, Eva Oburger, Lisa Fischer, Stephan M. Kraemer, Markus Puschenreiter, Michael Lämmerhofer, Gunda Koellensperger, Stephan Hann
    2012 - ELECTROPHORESIS, 4: 726-733


    Phytosiderophores (PS) form stable complexes with various transition metals. These ligands are exuded by the roots of graminacous plants as a mechanism for mobilizing and acquiring soil iron. To investigate iron mobilization and transport, a novel LC method in combination with ESI‐MS/MS for the determination of three Fe(III)‐complexes with mugineic acid (MA), 2′‐epi‐MA and 2′‐deoxymugineic acid (DMA) has been developed. Liquid chromatographic separation was realized using a silica‐based mixed‐mode reversed phase/weak‐anion exchange type stationary phase and a 50 mM ammonium acetate buffer, pH 6.5. Baseline separation of the two complex diastereomers Fe(III)‐MA and Fe(III)‐epi‐MA could be achieved. ESI‐MS/MS detection allowed for simultaneous quantification of the complexes and the free ligands. Limits of detection were determined to be 0.001 and 0.05 μM for DMA and Fe(III)‐DMA, respectively. The analytical figures of merit of the novel method were evaluated and compared with a CE‐ESI‐MS method that we had published earlier. The LC‐ESI‐MS/MS method has been successfully applied to real samples derived from preliminary extraction experiments.

  • Competitive ligand exchange between Cu-humic acid complexes and methanobactin

    M.-L. Pesch, M. Hoffmann, I. Christl, Stephan M. Kraemer, R. Kretzschmar
    2012 - Geobiology, 1: 44-54


    Copper has been found to play a key role in the physiology of methanotrophic micro‐organisms, and methane oxidation may critically depend on the availability of Cu. In natural environments, such as soils, sediments, peat bogs, and surface waters, the presence of natural organic matter (NOM) can control the bioavailability of Cu by forming strong metal complexes. To promote Cu acquisition, methanotrophs exude methanobactin, a ligand known to have a high affinity for Cu. In this study, the capability of methanobactin for Cu acquisition from NOM was investigated using humic acid (HA) as a model substance. The kinetics of ligand exchange between Cu–HA and methanobactin was observed by UV–vis spectroscopy, and the speciation of Cu bound to methanobactin was determined by size‐exclusion chromatography coupled to an ICP‐MS. The results showed that Cu was mobilized from HA by a fast ligand exchange reaction following a second‐order rate law with first‐order kinetics for both methanobactin and Cu–HA complexes. The reaction rates decreased with decreasing temperature. Equilibrium experiments indicated that methanobactin was not sorbed to HA and proved that methanobactin is competitive with HA for Cu binding by forming strong 1:1 Cu–methanobactin complexes. Consequently, our results demonstrate that methanobactin can efficiently acquire Cu in organic‐rich environments.

  • Copper complexation of methanobactin isolated from Methylosinus trichosporium OB3b: pH-dependent speciation and modeling

    Marie-Laure Pesch, Iso Christl, Martin Hoffmann, Stephan M. Kraemer, Ruben Kretzschmar
    2012 - Journal of Inorganic Biochemistry, 55-62


    Methanobactins are copper-binding ligands produced by aerobic methanotrophic microorganisms. A quantitative understanding of their potential role in methanotrophic copper acquisition requires the investigation of their copper complexes under relevant pH conditions. In this study, a chemical speciation model describing the pH-dependence of copper binding and the formation of the different complexes by methanobactin (mb) is released by Methylosinus trichosporium OB3b was developed. Potentiometric and spectrophotometric titrations of the free ligand indicated the presence of four protonation sites consistent with the molecular structure of methanobactin. Metal titrations revealed a distinct pH-dependence of copper binding to methanobactin between pH 5 and 8. Based on evidence from size-exclusion chromatography coupled to inductively coupled plasma mass spectrometry (ICP-MS), the copper binding was quantitatively described with three different types of copper–methanobactin complexes which can additionally undergo protonation reactions. The high affinity observed upon initial copper additions resulted from the predominant occurrence of copper–methanobactin dimer complexes, mb2H4Cu and mb2H3Cu with log K values of 58 and 52, respectively. With increasing copper to methanobactin ratios, methanobactin bound copper as monomers, mbHCu (log K = 25) and mbCu (log K = 18), whereas at elevated copper activities methanobactin was able to bind two copper ions (mbHCu2 and mbCu2). Model calculations based on the fitted complexation constants suggest that in natural systems, copper–methanobactin complexes are mostly present as monomers.

  • A simple assay for screening microorganisms for chalkophore production

    Yoon S, Dispirito AA, Stephan M. Kraemer, Semrau JD
    2011 - Methods in enzimology, 495: 247-258


    Recently, methanotrophs were found to exude a chalkophore, that is, a metal ligand with great affinity and specificity to copper. A rapid screening method for chalkophore expression was developed by adopting the chrome azurol S (CAS) assay originally used for detecting siderophore production in diverse groups of bacteria and fungi. In this assay, iron(III) chloride was replaced with copper(II) chloride. Both liquid and agar plate versions of the Cu–CAS assay can be used to examine the activity of either isolated methanobactin or to screen organisms for production of a chalkophore. Although here we describe the use of this assay to screen methanotrophs for chalkophore production, it can be modified as necessary to screen other organisms for chalkophore production as well. Many siderophores can also bind copper in the presence of CAS. Therefore, this assay should be done in conjunction with the original iron–CAS assay to determine if any positive Cu–CAS assay results are due to nonspecific binding of copper by a siderophore. This inexpensive assay may also aid in analyses of the genetics of chalkophore synthesis.

  • Copper

    2011 - Encyclopedia of Geobiology, 290-291
  • Iron speciation and isotope fractionation during silicate weathering and soil formation in an alpine glacier forefield chronosequence

    Mirjam Kiczka, Jan G. Wiederhold, Jakob Frommer, Andreas Voegelin, Stephan M. Kraemer, Bernard Bourdon, Ruben Kretzschmar
    2011 - Geochimica et Cosmochimica Acta, 1: 5559-5573


    The chemical weathering of primary Fe-bearing minerals, such as biotite and chlorite, is a key step of soil formation and an important nutrient source for the establishment of plant and microbial life. The understanding of the relevant processes and the associated Fe isotope fractionation is therefore of major importance for the further development of stable Fe isotopes as a tracer of the biogeochemical Fe cycle in terrestrial environments. We investigated the Fe mineral transformations and associated Fe isotope fractionation in a soil chronosequence of the Swiss Alps covering 150 years of soil formation on granite. For this purpose, we combined for the first time stable Fe isotope analyses with synchrotron-based Fe-EXAFS spectroscopy, which allowed us to interpret changes in Fe isotopic composition of bulk soils, size fractions, and chemically separated Fe pools over time in terms of weathering processes. Bulk soils and rocks exhibited constant isotopic compositions along the chronosequence, whereas soil Fe pools in grain size fractions spanned a range of 0.4‰ in δ56Fe. The clay fractions (<2 μm), in which newly formed Fe(III)-(hydr)oxides contributed up to 50% of the total Fe, were significantly enriched in light Fe isotopes, whereas the isotopic composition of silt and sand fractions, containing most of the soil Fe, remained in the range described by biotite/chlorite samples and bulk soils. Iron pools separated by a sequential extraction procedure covered a range of 0.8‰ in δ56Fe. For all soils the lightest isotopic composition was observed in a 1 M NH2OH–HCl–25% acetic acid extract, targeting poorly-crystalline Fe(III)-(hydr)oxides, compared with easily leachable Fe in primary phyllosilicates (0.5 M HCl extract) and Fe in residual silicates. The combination of the Fe isotope measurements with the speciation data obtained by Fe-EXAFS spectroscopy permitted to quantitatively relate the different isotope pools forming in the soils to the mineral weathering reactions which have taken place at the field site. A kinetic isotope effect during the Fe detachment from the phyllosilicates was identified as the dominant fractionation mechanism in young weathering environments, controlling not only the light isotope signature of secondary Fe(III)-(hydr)oxides but also significantly contributing to the isotope signature of plants. The present study further revealed that this kinetic fractionation effect can persist over considerable reaction advance during chemical weathering in field systems and is not only an initial transient phenomenon.

  • Iron(III) Reduction in Anaerobically Incubated Suspensions of Highly Calcareous Agricultural Soils

    Inmaculada Sánchez-Alcalá, M. C. del Campillo, J. Torrent, K. L. Straub, Stephan M. Kraemer
    2011 - Soil Science Society of America Journal, 6: 2136


    The frequent presence of Fe chlorosis in plants grown on calcareous soils is influenced by the forms of soil Fe present and their contents. Previous studies suggest that temporary soil flooding may increase Fe phytoavailability. To study flooding effects in relation to microbial activity in greater depth, we incubated soil slurries in the laboratory under anoxic conditions and monitored changes in Fe mineralogy using wet chemical extractions and diffuse reflectance spectroscopy. Twenty-four calcareous soils from southern Spain ranging widely in their properties were chosen for this purpose. Slurries of sterilized and native soils were compared with those of native soils with different amendments. In contrast to the sterilized controls, most of the slurries containing native soils released substantially increased amounts of Fe(II) to the solution after 6 wk of incubation; also, the extent of Fe(II) production correlated well with native contents of dissolved organic C. Indeed, the addition of organic acids typically found in root exudates resulted in a pronounced increase in Fe(II) production, and a similar effect was observed in soil slurries additionally inoculated with Geobacter sulfurreducens, a well-known Fe(III)-reducing bacterium. Microbial Fe(III) reduction mobilized poorly crystalline and crystalline Fe oxides. The critical extractable Fe value required for Fe nutrition of tolerant plants was reached in 18 of the slurries of native soils and in 22 of the native soils amended with organic acids. Temporary flooding seems to stimulate microbial Fe(III) reduction (especially in the presence of readily available organic C), thereby effectively increasing Fe phytoavailability in calcareous soils.

  • Isolation and purification of Cu-free methanobactin from Methylosinus trichosporium OB3b

    Marie-Laure Pesch, Iso Christl, Kurt Barmettler, Stephan M. Kraemer, Ruben Kretzschmar
    2011 - Geochemical Transactions, 1: 2



    The isolation of highly pure copper-free methanobactin is a prerequisite for the investigation of the biogeochemical functions of this chalkophore molecule produced by methane oxidizing bacteria. Here, we report a purification method for methanobactin from Methylosinus trichosporium OB3b cultures based on reversed-phase HPLC fractionation used in combination with a previously reported resin extraction. HPLC eluent fractions of the resin extracted product were collected and characterized with UV-vis, FT-IR, and C-1s NEXAFS spectroscopy, as well as with elemental analysis and ESI-MS. 


    The results showed that numerous compounds other than methanobactin were present in the isolate obtained with resin extraction. Molar C/N ratios, mass spectrometry measurements, and UV-vis spectra indicated that methanobactin was only present in one of the HPLC fractions. On a mass basis, methanobactin carbon contributed only 32% to the total organic carbon isolated with resin extraction. Our spectroscopic results implied that besides methanobactin, the organic compounds in the resin extract comprised breakdown products of methanobactin as well as polysaccharide-like substances.


    Our results demonstrate that a purification step is indispensable in addition to resin extraction in order to obtain pure methanobactin. The proposed HPLC purification procedure is suitable for semi-preparative work and provides copper-free methanobactin.

  • Siderophores

    2011 - Encyclopedia of Geobiology, 793-796


    Iron is a nutrient to almost all known organisms. Even though iron is the fourth most abundant element on earth, the acquisition of this nutrient poses a serious challenge to organisms in many natural environments. A particularly iron-depleted system is the photic zone of the marine water column (Kraemer et al., 2005). Here, soluble iron is removed by biological uptake and subsequent sinking of the biomass below the mixed zone. In ocean areas with particularly low iron concentrations relative to other nutrients in the photic zone, the primary productivity is limited by the low bioavailability of iron. This marine iron limitation may affect the global climate by limiting the efficiency of the marine carbon pump. However, even in soils that contain abundant iron-bearing mineral phases, iron acquisition...

  • The biogeochemistry of phytosiderophores in the rhizosphere in relation to Fe uptake

    Stephan M. Kraemer, Walter Schenkeveld, Eva Oburger, Madeleine Dell'mour, Markus Puschenreiter, Martin Walter, Stephan Hann, Christian Stanetty
    2011 - Mineralogical Magazine, 75: 1812-1812
  • An assay for screening microbial cultures for chalkophore production

    Yoon S, Stephan M. Kraemer, Dispirito AA, Semrau JD
    2010 - Environmental Microbiology Reports, 2: 295-303


    Methanotrophs, bacteria that utilize methane as their sole carbon and energy source, are known to have high requirements for copper. These bacteria have recently been found to synthesize a copper‐chelating agent, or chalkophore, termed methanobactin. To aid in screening methanobactin production by methanotrophs, a plate assay developed from the chrome azurol S (CAS) assay for siderophore production, was modified. In the typical CAS assay, a colour change from blue to orange in iron–CAS plates is observed as iron (III) ion weakly bound to CAS is sequestered by siderophores with higher affinities. In our modified assay, iron (III) chloride of the original CAS solution was substituted with copper (II) chloride, and removal of copper from CAS caused a colour change from blue to yellow. Assay results indicated that of the four tested methanotrophs (Methylosinus trichosporium OB3b, Methylococcus capsulatus Bath, Methylomicrobium album BG8 and Methylocystis parvus OBBP), only M. trichosporium OB3b, M. capsulatus Bath and M. album BG8 produced chalkophores capable of competing with CAS for copper, while M. parvus OBBP did not or did not export sufficient concentrations of methanobactin for detection by this assay. It was also found using Fe–CAS plates that at least M. trichosporium OB3b and M. album BG8 produce siderophores. These results may be expanded for the detection of chalkophores in other microorganisms as well as for screening of putative mutants of chalkophore synthesis.

  • Iron Isotope Fractionation during Fe Uptake and Translocation in Alpine Plants

    Mirjam Kiczka, Jan G. Wiederhold, Stephan M. Kraemer, Bernard Bourdon, Ruben Kretzschmar
    2010 - Environmental Science & Technology, 1: 6144-6150


    The potential of stable Fe isotopes as a tracer for the biogeochemical Fe cycle depends on the understanding and quantification of the fractionation processes involved. Iron uptake and cycling by plants may influence Fe speciation in soils. Here, we determined the Fe isotopic composition of different plant parts including the complete root system of three alpine plant species (Oxyria digynaRumex scutatusAgrostis gigantea) in a granitic glacier forefield, which allowed us, for the first time, to distinguish between uptake and in-plant fractionation processes. The overall range of fractionation was 4.5‰ in δ56Fe. Mass balance calculations demonstrated that fractionation toward lighter Fe isotopic composition occurred in two steps during uptake: (1) before active uptake, probably during mineral dissolution and (2) during selective uptake of Fe at the plasma membrane with an enrichment factor of −1.0 to −1.7‰ for all three species. Iron isotopes were further fractionated during remobilization from old into new plant tissue, which changed the isotopic composition of leaves and flowers over the season. This study demonstrates the potential of Fe isotopes as a new tool in plant nutrition studies but also reveals challenges for the future application of Fe isotope signatures in soil−plant environments.

  • Iron isotope fractionation during proton- and ligand-promoted dissolution of primary phyllosilicates

    Mirjam Kiczka, Jan G. Wiederhold, Jakob Frommer, Stephan M. Kraemer, Bernard Bourdon, Ruben Kretzschmar
    2010 - Geochimica et Cosmochimica Acta, 1: 3112-3128


    We studied stable iron isotope fractionation during dissolution of a biotite and chlorite enriched mineral fraction from granite by HCl and 5 mM oxalic acid in a pH range of 4–5.9. Batch experiments covered a time period from 2 h to 100 days and were performed at initial potassium concentrations of 0, 0.5, and 5 mM to induce different levels of biotite exfoliation. All experiments were kept anoxic to investigate solely the dissolution step without the influence of oxidation and precipitation of secondary Fe oxyhydroxides. Oxalic acid increased the release of Fe by a factor of ∼15 compared with the HCl experiments. Addition of 0.5 mM K to initial solutions in proton-promoted dissolution decreased the release of Fe by 30–65% depending on the dissolution stage. In ligand-controlled dissolution, K reduced the Fe release only to a minor extent. All solutions of the early dissolution stages were enriched in light Fe isotopes by up to −1.4‰ in δ56Fe compared with the isotopic composition of biotite and chlorite mineral separates, which we explained by a kinetic isotope effect. In proton-promoted dissolution, early released fractions of K-enriched experiments were significantly lighter (−0.7‰ to −0.9‰) than in the initially K-free experiments. The evolution of Fe isotope ratios in solution was modeled by a linear combination of kinetic isotope effects during two independent dissolution processes attacking different crystallographic sites. In ligand-controlled dissolution, K did not influence the kinetic isotope effect and the Fe isotope composition in solution in the late dissolution stages remained slightly lighter than the bulk composition of the biotite/chlorite enriched mineral fraction. This study demonstrates that the initial Fe weathering flux should be enriched in light Fe isotopes and that Fe isotope data in combination with dissolution kinetics and stoichiometry provide new insights into dissolution mechanisms.

  • Adsorption of hydroxamate siderophores and EDTA on goethite in the presence of the surfactant sodium dodecyl sulfate

    Naraya Carrasco, Ruben Kretzschmar, Jide Xu, Stephan M. Kraemer
    2009 - Geochemical Transactions, 1: 5


    Siderophore-promoted iron acquisition by microorganisms usually occurs in the presence of other organic molecules, including biosurfactants. We have investigated the influence of the anionic surfactant sodium dodecyl sulfate (SDS) on the adsorption of the siderophores DFOB (cationic) and DFOD (neutral) and the ligand EDTA (anionic) onto goethite (α-FeOOH) at pH 6. We also studied the adsorption of the corresponding 1:1 Fe(III)-ligand complexes, which are products of the dissolution process. Adsorption of the two free siderophores increased in a similar fashion with increasing SDS concentration, despite their difference in molecule charge. In contrast, SDS had little effect on the adsorption of EDTA. Adsorption of the Fe-DFOB and Fe-DFOD complexes also increased with increasing SDS concentrations, while adsorption of Fe-EDTA decreased. Our results suggest that hydrophobic interactions between adsorbed surfactants and siderophores are more important than electrostatic interactions. However, for strongly hydrophilic molecules, such as EDTA and its iron complex, the influence of SDS on their adsorption seems to depend on their tendency to form inner-sphere or outer-sphere surface complexes. Our results demonstrate that surfactants have a strong influence on the adsorption of siderophores to Fe oxides, which has important implications for siderophore-promoted dissolution of iron oxides and biological iron acquisition.

  • ATR-FTIR spectroscopic study of the adsorption of desferrioxamine B and aerobactin to the surface of lepidocrocite (γ-FeOOH)

    Paul Borer, Stephan J. Hug, Barbara Sulzberger, Stephan M. Kraemer, Ruben Kretzschmar
    2009 - Geochimica et Cosmochimica Acta, 16: 4661-4672


    The adsorption of two model siderophores, desferrioxamine B (DFOB) and aerobactin, to lepidocrocite (γ-FeOOH) was investigated by attenuated total reflection infrared spectroscopy (ATR-FTIR). The adsorption of DFOB was investigated between pH 4.0 and 10.6. The spectra of adsorbed DFOB indicated that two to three hydroxamic acid groups of adsorbed DFOB were deprotonated in the pH range 4.0–8.2. Deprotonation of hydroxamic acid groups of adsorbed DFOB at pH values well below the first acid dissociation constant of solution DFOB species (pKa = 8.3) and well below the point of zero charge of lepidocrocite (pHPZC = 7.4) suggested that the surface speciation at the lower end of this pH range (pH 4) is dominated by a surface DFOB species with inner-sphere coordination of two to three hydroxamic acids groups to the surface. Maximum adsorption of DFOB occurred at approximately pH 8.6, close to the first pKa value of the hydroxamic acid groups, and decreased at lower and higher pH values.

    The spectra of adsorbed aerobactin in the pH range 3–9 indicated at least three different surface species. Due to the small spectral contributions of the hydroxamic acid groups of aerobactin, the interactions of these functional groups with the surface could not be resolved. At high pH, the spectral similarity of adsorbed aerobactin with free aerobactin deprotonated at the carboxylic acid groups indicated outer-sphere complexation of the carboxylate groups. With decreasing pH, a significant peak shift of the asymmetric carboxylate stretch vibration was observed. This finding suggested that the (lateral) carboxylic acid groups are coordinated to the surface either as inner-sphere complexes or as outer-sphere complexes that are strongly stabilized at the surface by hydrogen bonding at low pH.

  • Fe isotope fractionation during phyllosilicate dissolution: Effect of protons, ligands and K concentration

    Stephan M. Kraemer, Mirjam Kiczka, Ruben Kretzschmar, Bernard Bourdon, Jan Wiederhold
    2009 - Geochimica et Cosmochimica Acta, 73: in press
  • Iron dynamics in the rhizosphere as a case study for analyzing interactions between soils, plants and microbes

    Philippe Lemanceau, Petra Bauer, Stephan M. Kraemer, Jean-Francois Briat
    2009 - Plant and Soil, 1: 513-535


    Iron is an essential element for plants and microbes. However, in most cultivated soils, the concentration of iron available for these living organisms is very low because its solubility is controlled by stable hydroxides, oxyhydroxides and oxides. In the rhizosphere, there is a high demand of iron because of the iron uptake by plants, and microorganisms which density and activity are promoted by the release of root exudates. Plants and microbes have evolved active strategies of iron uptake. Iron incorporation by these organisms lead to complex interactions ranging from competition to mutualism. These complex interactions are under the control of physico-chemical properties of the soils in which they occur, and reciprocally iron uptake strategies of plants and microbes impact these soil properties. These iron-mediated interactions between soils, plants and microbes impact the plant growth and health and their analysis, together with that of the resulting iron dynamics, is of a major agronomic interest. Analysis of the complex interactions soils, plants and microbes represent also a unique opportunity to progress in our knowledge of the rhizosphere ecology. This progression requires merging complementary expertises and study strategies in soil science, plant biology and microbiology. This review provides information on (i) iron status in soil and rhizosphere, iron uptake by plants and microbes, and on (ii) the corresponding study strategies. Finally, illustrations of how integration of these approaches allows gaining knowledge in the complex interactions occurring in the rhizosphere are given.

  • Photodissolution of lepidocrocite (γ-FeOOH) in the presence of desferrioxamine B and aerobactin

    Paul Borer, Stephan M. Kraemer, Barbara Sulzberger, Stephan J. Hug, Ruben Kretzschmar
    2009 - Geochimica et Cosmochimica Acta, 16: 4673-4687


    Batch adsorption and dissolution experiments with lepidocrocite (γ-FeOOH) and two siderophores, desferrioxamine B (DFOB) and aerobactin, were performed between pH 3 and 8 in the dark and under irradiation with UV–visible light. The increase in surface concentrations of adsorbed DFOB with increasing pH was explained in terms of electrostatic interactions between the protonated and charged terminal amine group of DFOB surface complexes and the charged lepidocrocite surface. The adsorption of aerobactin was consistent with the typical anion-like adsorption behavior of low molecular weight organic acids and indicated that the adsorption properties are strongly determined by the carboxylic acid groups. The adsorption experiments revealed furthermore that the Fe(III)–DFOB solution complex has a very low affinity for the surface, in contrast to Fe(III)–aerobactin solution complexes. In accordance with a surface-controlled mechanism of ligand-promoted dissolution, we found a linear correlation between dissolution rates of lepidocrocite and the surface concentrations of adsorbed DFOB. In the dark, 6- to 8-fold lower dissolution rate coefficients were determined for aerobactin in comparison to DFOB. These results suggested that aerobactin forms surface complexes that are less dissolution-active, characterized by a higher degree of multinuclear surface complexation and/or by less dissolution-active coordination modes of the involved iron-binding groups. For both DFOB and aerobactin, dissolution rate coefficients increased significantly under irradiation with UV–visible light. This increase was interpreted in terms of light-induced reduction of surface Fe(III), primarily by intrinsic photochemical processes of the lepidocrocite bulk phase, based on the observed photoreductive dissolution in the absence of organic ligands between pH 3 and 6. We hypothesize that the α-hydroxycarboxylate group of aerobactin may form a surface complex that additionally promotes photoreductive dissolution by a ligand-to-metal charge-transfer (LMCT) reaction, similar to citrate. However, LMCT reactions involving the α-hydroxycarboxylate group of aerobactin are rather ineffective, based on the comparison of light-induced dissolution rate coefficients determined in the presence of aerobactin and citrate.

  • Photoreductive Dissolution of Iron(III) (Hydr)oxides in the Absence and Presence of Organic Ligands: Experimental Studies and Kinetic Modeling

    Paul Borer, Barbara Sulzberger, Stephan J. Hug, Stephan M. Kraemer, Ruben Kretzschmar
    2009 - Environmental Science & Technology, 6: 1864-1870


    This study investigated the kinetics of the photoreductive dissolution of various iron(III) (hydr)oxide phases, lepidocrocite (γ-FeOOH), ferrihydrite, and hydrous ferric oxide, in the absence of organic ligands as a function of pH in deaerated and aerated suspensions. Photoreductive dissolution of lepidocrocite and ferrihydrite only occurred below pH 6. Under oxic conditions, we observed both the formation of aqueous Fe(II) and H2O2 during photoreductive dissolution of lepidocrocite and ferrihydrite at pH 3. These experimental findings are consistent with the light-induced reduction of surface Fe(III) at the (hydr)oxide surface and the concomitant oxidation of surface-coordinated water or hydroxyl groups, leading to surface Fe(II) and OH radicals and subsequently to H2O2. The formation of OH radicals at the surface was confirmed by photodissolution experiments conducted in the presence of OH radical scavengers. Kinetic modeling of the experimental data suggests that the relevant pathway for the formation of H2O2 is the reoxidation of surface lattice Fe(II) by molecular oxygen. This study furthermore shows that in the presence of strong iron binding ligands such as siderophores, specifically desferrioxamine B, the photoreductive dissolution of lepidocrocite, ferrihydrite, and to a lesser extent hydrous ferric oxide may also proceed at seawater pH.

  • Photoreductive dissolution of lepidocrocite in the presence and absence of siderophores

    Stephan M. Kraemer, Paul Borer, Ruben Kretzschmar, Stephan J. Hug, Barbara Sulzberger
    2009 - Geochimica et Cosmochimica Acta, 73: in press
  • Wavelength-Dependence of Photoreductive Dissolution of Lepidocrocite (γ-FeOOH) in the Absence and Presence of the Siderophore DFOB

    Paul Borer, Barbara Sulzberger, Stephan J. Hug, Stephan M. Kraemer, Ruben Kretzschmar
    2009 - Environmental Science & Technology, 6: 1871-1876


    Photoreductive dissolution of lepidocrocite (γ-FeOOH) in the presence/absence of the siderophore desferrioxamine B (DFOB) was investigated at different wavelengths. At pH 3 in the absence of DFOB, Fe(II) formation rates normalized to the photon flux increased with decreasing wavelengths below 515 nm, consistent with enhanced Fe(II) formation at lower wavelengths by photolysis of surface Fe(III)−hydroxo groups or by surface scavenging of photoelectrons generated in the semiconducting bulk. In the presence of DFOB at pH 3, photoreductive dissolution rates, normalized to the photon flux, increased more strongly with decreasing wavelengths below 440 nm. We hypothesize that acid-catalyzed hydrolysis of DFOB generates degradation products that form photoreactive surface complexes leading to an increase in photodissolution rates at low pH. At pH 8 in the presence of DFOB, normalized photodissolution rates had a maximum in the spectral window 395−435 nm and were significantly smaller at lower wavelengths, suggesting that adsorbed DFOB is directly involved in the reduction of surface Fe(III) by a light-induced ligand-to-metal charge-transfer reaction within the surface Fe(III)−DFOB complex. The strong response in the visible light suggests that photoreductive dissolution of iron (hydr)oxides promoted by siderophores with hydroxamic acid groups may occur deep into in the euphotic zone of oceans.

  • Effects of anionic surfactants on ligand-promoted dissolution of iron and aluminum hydroxides

    Naraya Carrasco, Ruben Kretzschmar, Marie-Laure Pesch, Stephan M. Kraemer
    2008 - Journal of Colloid and Interface Science, 2: 279-287


    We investigated the influence of the surfactants sodium dodecyl sulfate (SDS) and rhamnolipid (RhL) on ligand-promoted dissolution of goethite (α-FeOOH) and boehmite (γ-AlOOH) at pH 6. The siderophore desferrioxamine B (DFOB), its derivate desferrioxamine D (DFOD), ethylenediaminetetraacetic acid (EDTA), and 8-hydroxyquinoline-5-sulfonic acid (HQS) were used as ligands. The rates of ligand-promoted dissolution of goethite were significantly increased in the presence of low concentrations of anionic surfactants (<80 μM SDS; <6 mg/L RhL). At higher surfactant concentrations, however, the effects of surfactants were negligible. The dissolution rates in the presence of surfactants were not correlated with adsorbed amounts of ligands. Three possible factors contributing to these observations were further investigated and discussed: (i) adsorbed surfactants may influence ligand adsorption by changes in the ligand's surface speciation, (ii) re-adsorption of Fe–DFOB or Fe–DFOD complexes may lead to an underestimation of siderophore-promoted dissolution rates at high surfactant concentrations, and (iii) co-adsorption of protons to goethite with SDS may influence the dissolution rates. However, our results show that none of these three factors can satisfactorily explain the observed effects of anionic surfactants on ligand-promoted dissolution rates of iron and aluminum hydroxides.

  • Iron oxide photodissolution in the presence of siderophores

    Stephan M. Kraemer, Paul Borer, Ruben Kretzschmar, Stephan J. Hug, Barbara Sulzberger
    2008 - Geochimica et Cosmochimica Acta, 72: in press
  • Dissolution mechanisms of goethite in the presence of siderophores and organic acids

    P.U. Reichard, R. Kretzschmar, Stephan M. Kraemer
    2007 - Geochimica et Cosmochimica Acta, 2: 5635-5650


    In dynamic natural systems such as soils and surface waters, transient biogeochemical processes can induce strong chemical non-steady-state conditions. In this paper, we investigate the effects of non-steady-state conditions on ligand-controlled iron oxide dissolution. The rates of goethite dissolution at pH 6 in the presence of low molecular weight organic acids (oxalate, citrate or malonate) were observed. Non-steady-state conditions were induced by rapid additions of fungal, bacterial or plant siderophores. In the presence of the low molecular weight organic acids, dissolved iron concentrations are below detection limit as predicted by equilibrium solubility calculations. The rapid addition of the siderophores triggered reproducible, fast dissolution of kinetically labile iron from the iron oxide surface. The same effect was observed upon rapid additions of high citrate concentrations to goethite-oxalate suspensions. The concentration of the labile iron pool at the mineral surface was a function of the surface concentration of the low molecular weight organic acids and of the reaction time before addition of the siderophores. Isotopic exchange with 59Fe independently confirmed the existence of the labile iron pool before addition of the siderophore. A dissolution mechanism was elucidated that is consistent with these observations and with accepted models of ligand-controlled dissolution. We conclude that the fast dissolution reaction observed here is an important process in biological iron acquisition and that it is based on a general geochemical mechanism.

  • Indirect microbial ferric iron reduction via sulfur cycling

    Kristina Lotte Straub, Bernhard Schink, Stephan M. Kraemer
    2007 - Geophysical Research Abstracts, 9: in press
  • Iron Isotope Fractionation during Pedogenesis in Redoximorphic Soils

    Jan G. Wiederhold, Nadya Teutsch, Stephan M. Kraemer, Alex N. Halliday, Ruben Kretzschmar
    2007 - Soil Science Society of America Journal, 6: 1840


    Stable Fe isotopes provide a potential new tool for tracing the biogeochemical cycle of Fe in soils. Iron isotope ratios in two redoximorphic soils were measured by multicollector inductively coupled plasma mass spectrometry to study the relationships between pedogenic Fe transformation and redistribution processes, and mass-dependent fractionation of Fe isotopes. Redoximorphic Fe depletion and enrichment zones were sampled in addition to the bulk soil samples. A three-step sequential extraction procedure was used to separate different Fe pools, which were examined in addition to total soil digests. Significant enrichments of heavy Fe isotopes of about 0.3‰ in δ57Fe were found in total soil digests of Fe-depleted zones compared with bulk soil samples and were explained by the preferential removal of light isotopes, presumably during microbially mediated Fe oxide dissolution under anoxic conditions. Accordingly, pedogenic Fe enrichment zones were found to be slightly enriched in light Fe isotopes. Distinct Fe isotope variations of >1‰ in δ57Fe were found between different Fe pools within soil samples, specifically enrichments of light isotopes in pedogenic oxides contrasting with heavy isotope signatures of residual silicate-bound Fe. Our data demonstrate that pedogenic Fe transformations in redoximorphic soils are linked to isotope fractionation, revealing greater mobility of lighter Fe isotopes compared with heavier isotopes during pedogenesis. No simple quantitative relationship between Fe depletion and isotope fractionation could be inferred, however. Our findings provide new insights into the behavior of Fe isotopes in soils and contribute to the development of Fe isotopes as a tracer for the biogeochemical Fe cycle.

  • Iron isotope fractionation in oxic soils by mineral weathering and podzolization

    Jan G. Wiederhold, Nadya Teutsch, Stephan M. Kraemer, Alex N. Halliday, Ruben Kretzschmar
    2007 - Geochimica et Cosmochimica Acta, 2: 5821-5833


    Stable iron isotope ratios in three soils (two Podzols and one Cambisol) were measured by MC-ICPMS to investigate iron isotope fractionation during pedogenic iron transformation and translocation processes under oxic conditions. Podzolization is a soil forming process in which iron oxides are dissolved and iron is translocated and enriched in the subsoil under the influence of organic ligands. The Cambisol was studied for comparison, representing a soil formed by chemical weathering without significant translocation of iron. A three-step sequential extraction procedure was used to separate operationally-defined iron mineral pools (i.e., poorly-crystalline iron oxides, crystalline iron oxides, silicate-bound iron) from the soil samples. Iron isotope ratios of total soil digests were compared with those of the separated iron mineral pools. Mass balance calculations demonstrated excellent agreement between results of sequential extractions and total soil digestions. Systematic variations in the iron isotope signature were found in the Podzol profiles. An enrichment of light iron isotopes of about 0.6‰ in δ57Fe was found in total soil digests of the illuvial Bh horizons which can be explained by preferential translocation of light iron isotopes. The separated iron mineral pools revealed a wide range of δ57Fe values spanning more than 3‰ in the Podzol profiles. Strong enrichments of heavy iron isotopes in silicate-bound iron constituting the residue of weathering processes, indicated the preferential transformation of light iron isotopes during weathering. Iron isotope fractionation during podzolization is probably linked to the ligand-controlled iron translocation processes. Comparison of iron isotope data from eluvial and illuvial horizons of the Podzol profiles revealed that some iron must have been leached out of the profile. However, uncertainties in the initial iron content and iron isotopic composition of the parent materials prevented thorough mass balance calculations of iron fluxes within the profiles. In contrast to the Podzol profiles, the Cambisol profile displayed uniform δ57Fe values across soil depth and showed only a small enrichment of light iron isotopes of about 0.4‰ in the poorly-crystalline iron oxide pool extracted by 0.5 M HCl. This work demonstrates that significant iron isotope fractionations can occur during pedogenesis in oxic environments under the influence of organic ligands. Our findings provide new insights into fractionation mechanisms of iron isotopes and will help in the development of stable iron isotopes as tracers for biogeochemical iron cycling in nature.

  • Low Concentrations of Surfactants Enhance Siderophore-Promoted Dissolution of Goethite

    Naraya Carrasco, Ruben Kretzschmar, Marie-Laure Pesch, Stephan M. Kraemer
    2007 - Environmental Science & Technology, 1: 3633-3638


    Surface-active agents (surfactants) are released by many soil bacteria and plant roots and are also important as environmental contaminants. Their presence at interfaces could influence important biogeochemical processes in soils such as ligand-controlled dissolution, an important process in biological iron acquisition. To investigate their potential influence on ligand-controlled dissolution of iron oxides, we studied the dissolution kinetics of goethite (α-FeOOH) at pH 6 in the presence of the bacterial siderophore desferrioxamine B (DFOB) and the anionic surfactant sodium dodecyl sulfate (SDS). The adsorption isotherm of SDS on goethite showed an increase in the slope at concentrations ranging between 300 and 400 μM SDS in solution. This increase in slope suggested the onset of admicelle formation. Adsorption of DFOB onto goethite increased strongly with increasing concentrations of adsorbed SDS. Small concentrations of SDS (5 μM) resulted in a 3-fold acceleration of DFOB-controlled goethite dissolution in the presence of 80 μM DFOB, compared to the suspensions without SDS. The effects of SDS on the goethite dissolution rates were less pronounced at higher SDS concentrations, and became negligible above 600 μM total SDS. The dissolution rates of goethite were not proportional to the adsorbed DFOB concentrations, as would be expected for ligand-controlled dissolution. We speculate that increasing concentrations of adsorbed SDS result in a change in DFOB surface speciation from inner-sphere to outer-sphere complexes and, consequently, the ligand-controlled dissolution rates are not linearly related to the adsorbed DFOB concentration. Our results provide the first evidence for an important role of biosurfactants in biological iron acquisition involving siderophores.

  • Photolysis of Citrate on the Surface of Lepidocrocite: An in situ Attenuated Total Reflection Infrared Spectroscopy Study

    Paul Borer, Stephan J. Hug, Barbara Sulzberger, Stephan M. Kraemer, Ruben Kretzschmar
    2007 - The Journal of Physical Chemistry C, 28: 10560-10569


    The photodecomposition of citrate adsorbed to γ-FeOOH (lepidocrocite) was investigated by batch photodissolution experiments and by in situ attenuated total reflection infrared spectroscopy (ATR-FTIR). Batch photodissolution experiments in suspensions of 125 mg/L γ-FeOOH and 100 μM 14C radio-labeled citrate revealed that the α-hydroxycarboxylic acid functional group of citrate was selectively photooxidized at pH 4 and pH 6. ATR-FTIR spectra recorded during the irradiation of γ-FeOOH-layers with adsorbed citrate showed that the primary photoproduct of citrate was acetonedicarboxylic acid. In the presence of excess citrate, the adsorbed photoproduct was exchanged in a ligand-exchange reaction indicating that citrate forms stronger surface complexes than acetonedicarboxylic acid. The primary photooxidation reaction was resolved from the subsequent ligand-exchange reaction by the application of a relatively high photon flux (5−10 W/cm2, 300−500 nm). Despite consecutive ligand-exchange reactions, the photoconversion of adsorbed citrate to acetonedicarboxylic acid was almost complete at pH 4 within 22 min. At pH 6, only a small photodecomposition was observed. This result was interpreted in terms of (i) different fractions of inner- and outer-sphere citrate surface complexes at pH 4 and pH 6 and (ii) different photoreactivity of different inner-sphere complexes. Furthermore, both batch photodissolution experiments and ATR-FTIR spectroscopy revealed that adsorbed acetonedicarboxylic acid was further decomposed to acetoacetate at pH 4 but not at pH 6. This study shows that the photooxidation of adsorbed citrate leads to the same products as the photodecomposition of dissolved ferric−citrate complexes. Moreover, it highlights the potential of ATR-FTIR spectroscopy for investigating photoreactions at iron oxide surfaces at the molecular level.

  • Physical chemistry of soils and aquifers: A special issue in honor of Garrison Sposito

    Jon Chorover, Stephan M. Kraemer, Javiera Cervini-Silva, Patricia Maurice
    2007 - Geochimica et Cosmochimica Acta, 71: 5578-5582
  • Rate laws of steady-state and non-steady-state ligand-controlled dissolution of goethite

    Petra U. Reichard, Ruben Kretzschmar, Stephan M. Kraemer
    2007 - Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1: 22-28


    Rapid changes of chemical conditions are rather common in natural systems including soils and aquatic environments. Non-steady-state conditions can be induced by biological activity, precipitation events, and other transient processes causing energy or concentration gradients. The quantitative description of biogeochemical processes under such non-steady-state conditions requires kinetic modeling, particularly if slow processes such as mineral dissolution are involved. One example for such a process is plant iron acquisition by diurnal exudation of siderophores into the rhizosphere, as observed for many grasses, leading to enhanced mobilization of iron by dissolution of iron oxides.

    In this study, we investigated ligand-controlled dissolution of goethite under steady-state and non-steady-state conditions and developed rate laws for these processes. The ligands used in this work included plant and microbial siderophores and the low molecular weight organic acid oxalate. Non-steady-state conditions were experimentally induced by pulse-additions of siderophores to goethite suspensions in the presence of oxalate to mimic the diurnal siderophore exudation pattern observed for grasses. We presume that before the addition of the siderophore, a slow oxalate-promoted surface reaction created a pool of kinetically labile iron species at the mineral surface. The subsequent addition of a siderophore increased the iron solubility and triggered a fast dissolution reaction until the labile iron pool was depleted. We concluded that the rate of accumulation of kinetically labile iron is controlled by the same rate determining step as ligand-controlled dissolution under steady-state conditions and that it can be described with the same rate laws. We parameterized the rate laws and successfully modeled goethite dissolution under steady-state and non-steady-state conditions. Thus, we show that general rate laws describe ligand-controlled iron oxide dissolution under both steady-state and non-steady-state conditions. The capability to model such processes has important implications for understanding of weathering processes and nutrient uptake in natural systems.

  • The impact of Fe isotope fractionation by plants on the isotopic signature of soils

    Stephan M. Kraemer, Jan Wiederhold, Mirjam Kiczka, Bernard Bourdon, Ruben Kretzschmar
    2007 - Geochimica et Cosmochimica Acta, 71: in press
  • Geochemical Aspects of Phytosiderophore‐Promoted Iron Acquisition by Plants

    Stephan M. Kraemer, D.E. Crowley, R. Kretzschmar
    2006 - Advances in Agronomy, 91: 1-46


    Iron is an essential trace nutrient for all plants. The acquisition of iron is limited by low solubilities and slow dissolution rates of iron‐bearing minerals in many soils. Therefore, iron limitation can be an important nutritional disorder in crop plants, leading to decreased yields or significant costs for iron fertilization. However, some species among the group of graminaceous plants (including wheat and barley) exhibit a rather low susceptibility to iron deficiency. These species respond to iron‐limiting conditions by the exudation of ligands with a high affinity and specificity for iron complexation, the so‐called phytosiderophores. Soluble iron–phytosiderophore complexes are recognized and transported across the root plasma membrane by specific transport proteins. This chapter focuses on geochemical aspects of this so‐called “strategy II” iron acquisition mechanism. The coordination chemistry of phytosiderophores and their iron complexes in the soil solution are discussed and compared to other organic ligands including low‐molecular weight organic acids and microbial siderophores. The properties of iron complexes and iron‐bearing minerals in the rhizosphere are discussed and compared with regard to their potential as sources of plant available iron. An important focus of this chapter is the elucidation of the thermodynamics, mechanisms, and rates of iron acquisition from these sources by phytosiderophores. Thus, we hope to contribute to the understanding of iron acquisition by strategy II plants in particular and of iron cycling in the rhizosphere in general. © 2006, Elsevier Inc.

  • Iron Isotope Fractionation during Proton-Promoted, Ligand-Controlled, and Reductive Dissolution of Goethite

    Jan G. Wiederhold, Stephan M. Kraemer, Nadya Teutsch, Paul M. Borer, Alex N. Halliday, Ruben Kretzschmar
    2006 - Environmental Science & Technology, 1: 3787-3793


    Iron isotope fractionation during dissolution of goethite (α-FeOOH) was studied in laboratory batch experiments. Proton-promoted (HCl), ligand-controlled (oxalate dark), and reductive (oxalate light) dissolution mechanisms were compared in order to understand the behavior of iron isotopes during natural weathering reactions. Multicollector ICP-MS was used to measure iron isotope ratios of dissolved iron in solution. The influence of kinetic and equilibrium isotope fractionation during different time scales of dissolution was investigated. Proton-promoted dissolution did not cause iron isotope fractionation, concurrently demonstrating the isotopic homogeneity of the goethite substrate. In contrast, both ligand-controlled and reductive dissolution of goethite resulted in significant iron isotope fractionation. The kinetic isotope effect, which caused an enrichment of light isotopes in the early dissolved fractions, was modeled with an enrichment factor for the 57Fe/54Fe ratio of −2.6‰ between reactive surface sites and solution. Later dissolved fractions of the ligand-controlled experiments exhibit a reverse trend with a depletion of light isotopes of ∼0.5‰ in solution. We interpret this as an equilibrium isotope effect between Fe(III)−oxalate complexes in solution and the goethite surface. In conclusion, different dissolution mechanisms cause diverse iron isotope fractionation effects and likely influence the iron isotope signature of natural soil and weathering environments.

  • Bacterial Siderophores Promote Dissolution of UO2 under Reducing Conditions

    Scott W. Frazier, Ruben Kretzschmar, Stephan M. Kraemer
    2005 - Environmental Science & Technology, 1: 5709-5715


    Tetravalent actinides are often considered environmentally immobile due to their strong hydrolysis and formation of sparingly soluble oxide phases. However, biogenic ligands commonly found in the soil environment may increase their solubility and mobility. We studied the adsorption and dissolution kinetics of UO2 in the presence of a microbial siderophore, desferrioxamine-B (DFO-B), under reducing conditions. Using batch and continuous flow stirred tank reactors (CFSTR), we found that DFO-B increases the solubility of UIV and accelerates UO2 dissolution rates through a ligand-promoted dissolution mechanism. DFO-B adsorption to UO2followed a Langmuir-type isotherm. The maximum adsorbed DFO-B concentrations were 3.3 μmol m-2 between pH 3 and 8 and declined above pH 8. DFO-B dissolved UO2 at a DFO-B surface-saturated net rate of 64 nmol h-1 m-2 (pH 7.5, I = 0.01 M) according to the first-order rate equation kL[Lads], with a rate coefficient kL of 0.019 h-1. Even at very low siderophore concentrations (e.g. 1 μM), net dissolution rates (16 nmol h-1 m-2, pH 7.5, I = 0.01 M) were substantially greater than net proton-promoted dissolution rates (3 nmol h-1 m-2, pH 7−7.5, I = 0.01 M). Interestingly, adding dissolved FeIII had negligible effects on DFO-B-promoted UO2dissolution rates, despite its potential as a competitor for DFO-B and as an oxidant of UIV. Our results suggest that strong organic ligands could influence the environmental mobility of tetravalent actinides and should be considered in predictions for nuclear waste storage and remediation strategies.

  • Effect of siderophores on the light-induced dissolution of colloidal iron(III) (hydr)oxides

    Paul M. Borer, Barbara Sulzberger, Petra Reichard, Stephan M. Kraemer
    2005 - Marine Chemistry, 2: 179-193


    Siderophores play an important role in biological iron acquisition in iron-limited aquatic systems. While it is widely accepted that the solubilization of iron-bearing mineral phases is a key function of siderophores, the mechanism of siderophore-promoted mineral dissolution in aquatic systems is largely unknown. In this study, we investigated the effect of siderophores (desferrioxamine B (DFOB) and aerobactin) on light-induced dissolution of goethite and lepidocrocite in the presence or absence of oxalate in aerated and deaerated suspensions at pH 6. For the irradiated two-ligand system (oxalate/siderophore), the experimental results suggest that oxalate acts as the electron donor for the formation of surface Fe(II), and the siderophore acts as an efficient shuttle for the transfer of surface Fe(II) into solution. Furthermore, even in the absence of an electron donor such as oxalate, both DFOB and aerobactin accelerated the light-induced dissolution of lepidocrocite as compared to the thermal dissolution. Experiments with dissolved Fe(III)–DFOB and Fe(III)–aerobactin complexes suggest that this enhancing effect is not due to photolysis of corresponding surface complexes but to efficient transfer of reduced surface Fe(II) into solution, where surface Fe(II) may be formed, e.g., through photolysis of surface Fe(III)–hydroxo groups. Based on this study, we conclude that the interplay of light and siderophores may play a key role in the dissolution of colloidal iron(III) (hydr)oxides in marine systems, particularly in the presence of efficient electron donors.

  • Goethite Dissolution in the Presence of Phytosiderophores: Rates, Mechanisms, and the Synergistic Effect of Oxalate

    P.U. Reichard, Stephan M. Kraemer, S.W. Frazier, R. Kretzschmar
    2005 - Plant and Soil, 276: 115-132


    The purpose of this study was the elucidation of the chemical mechanism of an important process in iron acquisition by graminaceous plants: the dissolution of iron oxides in the presence of phytosiderophores. We were particularly interested in the effects of diurnal root exudation of phytosiderophores and of the presence of other organic ligands in the rhizosphere of graminaceous plants on the dissolution mechanism.

    Phytosiderophores of the type 2′-deoxymugineic acid (DMA) were purified from the root exudates of wheat plants (Triticum aestivum L. cv. Tamaro). DMA-promoted dissolution of goethite under steady-state and non-steady-state conditions and its dependence on pH, adsorbed DMA concentration, and the presence of the organic ligand oxalate were studied. We show that dissolution of goethite by phytosiderophores follows a surface controlled ligand promoted dissolution mechanism. We also found that oxalate, an organic ligand commonly found in rhizosphere soils, has a synergistic effect on the steady-state dissolution of goethite by DMA. Under non-steady-state addition of the phytosiderophore, mimicking the diurnal exudation pattern of phytosiderophore release, a fast dissolution of iron is triggered in the presence of oxalate.

    To investigate the efficiency of these mechanisms in plant iron acquisition, wheat plants were grown on a substrate amended with goethite as only iron source. The chlorophyll status of these plants was similar to iron-fertilized plants and significantly higher than in plants grown in iron free nutrient solutions. This demonstrates that wheat can efficiently mobilize iron, even from well crystalline goethite that is usually considered unavailable for plant nutrition.

  • Siderophores and the Dissolution of Iron-Bearing Minerals in Marine Systems

    2005 - Reviews in Mineralogy and Geochemistry, 1: 53-84
  • Biogeochemical controls on the mobility and bioavailability of metals in soils and groundwater

    Stephan M. Kraemer, Janet G. Hering
    2004 - Aquatic Sciences - Research Across Boundaries, 1: 1-2
  • Iron oxide dissolution and solubility in the presence of siderophores

    2004 - Aquatic Sciences - Research Across Boundaries, 1: 3-18


    Iron is an essential trace nutrient for most known organisms. The iron availability is limited by the solubility and the slow dissolution kinetics of iron-bearing mineral phases, particularly in pH neutral or alkaline environments such as carbonatic soils and ocean water. Bacteria, fungi, and plants have evolved iron acquisition systems to increase the bioavailability of iron in such environments. A particularly efficient iron acquisition system involves the solubilization of iron by siderophores. Siderophores are biogenic chelators with high affinity and specificity for iron complexation.

    This review focuses on the geochemical aspects of biological iron acquisition. The significance of iron-bearing minerals as nutrient source for siderophore-promoted iron acquisition has been confirmed in microbial culture studies. Due to the extraordinary thermodynamic stability of soluble siderophore-iron complexes, siderophores have a pronounced effect on the solubility of iron oxides over a wide pH range. Very small concentrations of free siderophores in solution have a large effect on the solution saturation state of iron oxides. This siderophore induced disequilibrium can drive dissolution mechanisms such as proton-promoted or ligand-promoted iron oxide dissolution. The adsorption of siderophores to oxide surfaces also induces a direct siderophore-promoted surface-controlled dissolution mechanism. The efficiency of siderophores for increasing the solubility and dissolution kinetics of iron oxides are compared to other natural and anthropogenic ligands.

  • Steady-state dissolution kinetics of goethite in the presence of desferrioxamine B and oxalate ligands: implications for the microbial acquisition of iron

    Sing-Foong Cheah, Stephan M. Kraemer, Javiera Cervini-Silva, Garrison Sposito
    2003 - Chemical Geology, 1: 63-75


    This paper reports an investigation of the effects of a trihydroxamate siderophore, desferrioxamine B (DFO-B), and a common biological ligand, oxalate, on the steady-state dissolution of goethite at pH 5 and 25 °C. The main goal of our study was to quantify the adsorption of the ligands and the dissolution of goethite they promote in a two-ligand system. In systems with one ligand only, the adsorption of oxalate and DFO-B each followed an L-type isotherm. The surface excess of oxalate was approximately 40 mmol kg−1 at solution concentrations above 80 μM, whereas the surface excess of DFO-B was only 1.2 mmol kg−1 at 80 μM solution concentration. In the two-ligand systems, oxalate decreased DFO-B adsorption quite significantly, but not vice versa. For example, in solutions containing 40 μM DFO-B and 40 μM oxalate, 30% of the DFO-B adsorbed in the absence of oxalate was displaced. The mass-normalized dissolution rate of goethite in the presence of DFO-B alone increased as the surface excess of the ligand increased, suggesting a ligand-promoted dissolution mechanism. In systems containing oxalate only, mass-normalized goethite dissolution rates were very low at concentrations below 200 μM, despite maximal adsorption of the ligand. At higher oxalate concentrations (up to 8 mM), the steady-state dissolution rate continued to increase, even though the surface excess of adsorbed ligand was essentially constant. Chemical affinity calculations and dissolution experiments with variation of the reactor flow rate showed that far-from-equilibrium conditions did not obtain in systems containing oxalate at concentrations below 5 mM. The dissolution rate in the presence of DFO-B at solution concentrations between 1 and 80 μM was approximately doubled when oxalate was also present at 40 μM solution concentration. The dissolution rate in the presence of oxalate at solution concentrations between 0 and 200 μM was increased by more than an order of magnitude when DFO-B was also present at 40 μM solution concentration. Chemical affinity calculations showed that, in systems containing DFO-B, goethite dissolution was always under far-from-equilibrium conditions, irrespective of the presence of oxalate. These results were described quantitatively by a model rate law containing a term proportional to the surface excess of DFO-B and a term proportional to that of oxalate, with both surface excesses being determined in the two-ligand system. The pseudo first-order rate coefficient in the DFO-B term has the same value as measured for goethite dissolution in the presence of DFO-B only, while the rate coefficient in the oxalate term must be measured in the two-ligand system, since it is only in this system that far-from-equilibrium conditions obtain. These latter conditions do not exist in the system containing oxalate only, but they do exist in the DFO-B/oxalate system because the siderophore is able to remove Fe(III) from all Fe–oxalate complexes rapidly, leaving the uncomplexed oxalate ligand in solution free to react again with the goethite surface. This synergy observed in the two-ligand system implies that the production of modest quantities of siderophore in the presence of very low concentrations of oxalate would be an extremely effective mechanism for the microbially induced release of Fe from goethite.

  • Adsorption of Pb(II) and Eu(III) by Oxide Minerals in the Presence of Natural and Synthetic Hydroxamate Siderophores

    Stephan M. Kraemer, Jide Xu, Kenneth N. Raymond, Garrison Sposito
    2002 - Environmental Science & Technology, 6: 1287-1291


    Trihydroxamate siderophores have been proposed for use as mediators of actinide and heavy metal mobility in contaminated subsurface zones. These microbially produced ligands, common in terrestrial and marine environments, recently have been derivatized synthetically to enhance their affinity for transuranic metal cations. However, the interactions between these synthetic derivative and adsorbed trace metals have not been characterized. In this paper we compare a natural siderophore, desferrioxamine-B (DFO-B), with its actinide-specific catecholate derivative, N-(2,3-dihydroxy-4-(methylamido)benzoyl)desferrioxamine-B (DFOMTA), as to their effect on the adsorption of Pb(II) and Eu(III) by goethite and boehmite. In the presence of 240 μM DFO-B, a strongly depleting effect on Eu(III) adsorption by goethite and boehmite occurred above pH 6. By contrast, almost total removal of Eu(III) from solution in the neutral to slightly acidic pH range was observed in the presence of either 10 or 100 μM DFOMTA, due primarily to the formation of metal-DFOMTA precipitates. Addition of DFOMTA caused an increase in Pb(II) adsorption by goethite below pH 5, but a decrease above pH 5, such that the Pb(II) adsorption edge in the presence of DFOMTA strongly resembled the DFOMTA adsorption envelope, which showed a maximum near pH 5 and decreasing adsorption toward lower and higher pH.

  • Fast ligand controlled goethite dissolution kinetics under non-steady state conditions in the presence of siderophores and oxalate

    Stephan M. Kraemer, Ruben Kretzschmar, Petra Reichard
    2002 - Geochimica et Cosmochimica Acta, 66: in press
  • Temperature dependence of goethite dissolution promoted by trihydroxamate siderophores

    Claudio Cocozza, Calvin C.G. Tsao, Sing-Foong Cheah, Stephan M. Kraemer, Kenneth N. Raymond, Teodoro M. Miano, Garrison Sposito
    2002 - Geochimica et Cosmochimica Acta, 3: 431-438


    This article reports an investigation of the temperature dependence of goethite dissolution kinetics in the presence of desferrioxamine B (DFO-B), a trihydroxamate siderophore, and its acetyl derivative, desferrioxamine D1 (DFO-D1). At 25 and 40°C, DFO-D1 dissolved goethite at twice the rate of DFO-B, whereas at 55°C, the behavior of the two ligands was almost the same. Increasing the temperature from 25 to 55°C caused little or no significant change in DFO-B or DFO-D1 adsorption by goethite. A pseudo-first-order rate coefficient for dissolution, calculated as the ratio of the mass-normalized dissolution rate coefficient to the surface excess of siderophore, was approximately the same at 25 and 40°C for both siderophores. At 55°C, however, this rate coefficient for DFO-D1 was about half that for DFO-B. Analysis of the temperature dependence of the mass-normalized dissolution rate coefficient via the Arrhenius equation led to an apparent activation energy that was larger for DFO-B than for DFO-D1, but much smaller than that reported for the proton-promoted dissolution of goethite. A compensation law was found to relate the pre-exponential factor to the apparent activation energy in the Arrhenius equation, in agreement with what has been noted for the proton-promoted dissolution of oxide minerals and for the complexation of Fe3+ by DFO-B and simple hydroxamate ligands in aqueous solution. Analysis of these results suggested that the siderophores adsorb on goethite with a only single hydroxamate group in bidentate ligation with an Fe(III) center.

  • Effect of hydroxamate siderophores on Fe release and Pb(II) adsorption by goethite

    Stephan M. Kraemer, Sing-Foong Cheah, Rita Zapf, Jide Xu, Kenneth N. Raymond, Garrison Sposito
    1999 - Geochimica et Cosmochimica Acta, 1: 3003-3008


    Hydroxamate siderophores are biologically-synthesized, Fe(III)-specific ligands which are common in soil environments. In this paper, we report an investigation of their adsorption by the iron oxyhydroxide, goethite; their influence on goethite dissolution kinetics; and their ability to affect Pb(II) adsorption by the goethite surface. The siderophores used were desferrioxamine B (DFO-B), a fungal siderophore, and desferrioxamine D1, an acetyl derivative of DFO-B (DFO-D1). Siderophore adsorption isotherms yielded maximum surface concentrations of 1.5 (DFO-B) or 3.5 (DFO-D1) μmol/g at pH 6.6, whereas adsorption envelopes showed either cation-like (DFO-B) or ligand-like (DFO-D1) behavior. Above pH 8, the adsorbed concentrations of both siderophores were similar. The dissolution rate of goethite in the presence of 240 μM DFO-B or DFO-D1 was 0.02 or 0.17 μmol/g hr, respectively. Comparison of these results with related literature data on the reactions between goethite and acetohydroxamic acid, a monohydroxamate ligand, suggested that the three hydroxamate groups in DFO-D1 coordinate to Fe(III) surface sites relatively independently. The results also demonstrated a significant depleting effect of 240 μM DFO-B or DFO-D1 on Pb(II) adsorption by goethite at pH > 6.5, but there was no effect of adsorbed Pb(II) on the goethite dissolution rate.

  • Influence of pH and Competitive Adsorption on the Kinetics of Ligand-Promoted Dissolution of Aluminum Oxide

    Stephan M. Kraemer, Van Q. Chiu, Janet G. Hering
    1998 - Environmental Science & Technology, in press
  • Influence of pH and Competitive Adsorption on the Kinetics of Ligand-Promoted Dissolution of Aluminum Oxide

    Stephan M. Kraemer, Van Q. Chiu, Janet G. Hering
    1998 - Environmental Science & Technology, 19: 2876-2882


    The kinetics of δ-Al2O3 dissolution were examined in the presence of 8-hydroxyquinoline-5-sulfonate (HQS) and salicylate over the pH range 3−9. The greatest effects of both of these ligands on δ-Al2O3 dissolution were observed at pH values higher than those corresponding to maximal adsorbed ligand concentrations. Thus, calculated rate constants were pH dependent. For HQS, correlation between the fluorescence of the surface complex and the adsorbed HQS concentration indicates that the pH dependence of the rate constant cannot be explained by a change in the structure of the metal−organic surface complex. Rather, it is proposed that the rate-determining step in the dissolution reaction involves a mixed surface complex in which aluminum is coordinated by both the organic ligand and hydroxide. Similarly, dissolution rates in the presence of the competing adsorbates HQS and fluoride suggest a synergistic action of these two ligands. Dissolution rates predicted from measured adsorbed concentrations of both ligands assuming independent, parallel pathways for HQS- and fluoride-promoted dissolution underpredict observed dissolution rates at some adsorbed ligand concentrations. In contrast, dissolution rates in the presence of the competing adsorbates HQS and arsenate could be predicted simply by accounting for the displace ment of HQS from the oxide surface by arsenate.

  • Influence of solution saturation state on the kinetics of ligand-controlled dissolution of oxide phases

    Stephan M. Kraemer, Janet G. Hering
    1997 - Geochimica et Cosmochimica Acta, 1: 2855-2866


    Ligand-controlled dissolution of δ-Al2O3 and goethite in the presence of 8-hydroxyquinoline-5-sulfonate (HQS) has been observed as a function of the solution saturation state at constant pH and temperature. A continuous flow stirred tank reactor (CFSTR) was designed to conduct the experiments under steady-state conditions. Far from equilibrium, the dissolution kinetics of goethite are independent of the solution saturation state. Net dissolution rates decrease as equilibrium is approached. As the solution becomes oversaturated, precipitation is observed. Two factors may influence the goethite dissolution kinetics as a function of solution saturation state: the increase of the back reaction (precipitation) as equilibrium is approached and changes of the metal-organic surface speciation. Both effects can be accounted for using a general rate law for ligand-controlled oxide dissolution kinetics. The effect of the solution composition on the dissolution of δ-Al2O3 is characterized by a ligand-promoted recrystallization of the unstable parent mineral phase to yield a (meta) stable product.

  • Kinetics of complexation reactions at surfaces and in solution: Implications for enhanced radionuclide migration

    Janet Hering, Stephan M. Kraemer
    1994 - Radiochimica Acta: international journal for chemical aspects of nuclear science and technology, 66: 63-71

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