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Evidence for equilibrium iron isotope fractionation by nitrate-reducing iron(II)-oxidizing bacteria

Kappler, A. and Johnson, C. M. and Crosby, H. A. and Beard, B. L. and Newman, D. K. (2010) Evidence for equilibrium iron isotope fractionation by nitrate-reducing iron(II)-oxidizing bacteria. Geochimica et Cosmochimica Acta, 74 (10). pp. 2826-2842. ISSN 0016-7037. PMCID PMC2873596.

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Iron isotope fractionations produced during chemical and biological Fe(II) oxidation are sensitive to the proportions and nature of dissolved and solid-phase Fe species present, as well as the extent of isotopic exchange between precipitates and aqueous Fe. Iron isotopes therefore potentially constrain the mechanisms and pathways of Fe redox transformations in modern and ancient environments. In the present study, we followed in batch experiments Fe isotope fractionations between Fe(II)_(aq) and Fe(III) oxide/hydroxide precipitates produced by the Fe(III) mineral encrusting, nitrate-reducing, Fe(II)-oxidizing Acidovorax sp. strain BoFeN1. Isotopic fractionation in ^(56)Fe/^(54)Fe approached that expected for equilibrium conditions, assuming an equilibrium Δ^(56)Fe_(Fe(OH)3–Fe(II)aq) fractionation factor of +3.0‰. Previous studies have shown that Fe(II) oxidation by this Acidovorax strain occurs in the periplasm, and we propose that Fe isotope equilibrium is maintained through redox cycling via coupled electron and atom exchange between Fe(II)_(aq) and Fe(III) precipitates in the contained environment of the periplasm. In addition to the apparent equilibrium isotopic fractionation, these experiments also record the kinetic effects of initial rapid oxidation, and possible phase transformations of the Fe(III) precipitates. Attainment of Fe isotope equilibrium between Fe(III) oxide/hydroxide precipitates and Fe(II)_(aq) by neutrophilic, Fe(II)-oxidizing bacteria or through abiologic Fe(II)_(aq) oxidation is generally not expected or observed, because the poor solubility of their metabolic product, i.e. Fe(III), usually leads to rapid precipitation of Fe(III) minerals, and hence expression of a kinetic fractionation upon precipitation; in the absence of redox cycling between Fe(II)_(aq) and precipitate, kinetic isotope fractionations are likely to be retained. These results highlight the distinct Fe isotope fractionations that are produced by different pathways of biological and abiological Fe(II) oxidation.

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URLURL TypeDescription DOIArticle CentralArticle
Newman, D. K.0000-0003-1647-1918
Additional Information:© 2010 Elsevier Ltd. Received 23 June 2009; accepted 10 February 2010; available online 23 February 2010. The research was supported by a post-doc fellowship and an Emmy-Noether fellowship from the German Research Foundation (DFG) to A.K. and a grant from the Packard Foundation to D.K.N. Additional funding from the NASA Astrobiology Institute supported C.M.J. and B.L.B., and funding from the National Science Foundation supported B.L.B. and H.A.C. We would like to thank Ma Chi (Caltech) for help with the XRD. Sebastian Schaedler (University of Tuebingen) and Claus Burkhardt (NMI Reutlingen) are acknowledged for providing scanning electron micrographs. D.K.N. is an Investigator of the Howard Hughes Medical Institute. We thank AE Stephan Kraemer, Thomas Bullen, and an anonymous reviewer, whose comments helped improve the manuscript.
Funding AgencyGrant Number
Deutsche Forschungsgemeinschaft (DFG)UNSPECIFIED
David and Lucille Packard FoundationUNSPECIFIED
Howard Hughes Medical Institute (HHMI)UNSPECIFIED
Issue or Number:10
PubMed Central ID:PMC2873596
Record Number:CaltechAUTHORS:20100528-114059025
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:18495
Deposited By: Tony Diaz
Deposited On:01 Jun 2010 04:26
Last Modified:03 Oct 2019 01:43

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