Evidence for equilibrium iron isotope fractionation by nitrate-reducing iron(II)-oxidizing bacteria
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1.
California Institute of Technology
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2.
University of Wisconsin–Madison
Abstract
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.
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.Attached Files
Accepted Version - nihms200176.pdf
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Additional details
- PMCID
- PMC2873596
- Eprint ID
- 18495
- Resolver ID
- CaltechAUTHORS:20100528-114059025
- Deutsche Forschungsgemeinschaft (DFG)
- David and Lucille Packard Foundation
- NASA
- NSF
- Howard Hughes Medical Institute (HHMI)
- Created
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2010-06-01Created from EPrint's datestamp field
- Updated
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2021-11-08Created from EPrint's last_modified field
- Caltech groups
- Division of Geological and Planetary Sciences (GPS)