Coupled Fe(II)−Fe(III) Electron and Atom Exchange as a Mechanism for Fe Isotope Fractionation during Dissimilatory Iron Oxide Reduction

Abstract

Microbial dissimilatory iron reduction (DIR) is an important pathway for carbon oxidation in anoxic sediments, and iron isotopes may distinguish between iron produced by DIR and other sources of aqueous Fe(II). Previous studies have shown that aqueous Fe(II) produced during the earliest stages of DIR has delta56Fe values that are 0.5-2.0%o lowerthan the initial Fe(III) substrate. The new experiments reported here suggest that this fractionation is controlled by coupled electron and Fe atom exchange between Fe(II) and Fe(III) at iron oxide surfaces. In hematite and goethite reduction experiments with Geobacter sulfurreducens, the 56Fe/54Fe isotopic fractionation between aqueous Fe(II) and the outermost layers of Fe(III) on the oxide surface is approximately -3%o and can be explained by equilibrium Fe isotope partitioning between reactive Fe(II) and Fe(III) pools that coexist during DIR. The results indicate that sorption of Fe(II) to Fe(III) substrates cannot account for production of low-delta56Fe values for aqueous Fe(II) during DIR.

Iron isotope fractionation in sediments of an oligotrophic freshwater lake

“…Both biotic and abiotic processes can fractionate Fe isotopes; the largest fractionations are associated with redox transformations between ferric and ferrous Fe (Crosby et al, 2005;Wu et al, 2012). Oxidation of aqueous Fe(II) and reduction of solid Fe(III) phases may both generate isotopically light Fe(II).…”

“…In the former case, the combination of equilibrium fractionation between aqueous Fe(II) and aqueous Fe(III) and kinetic fractionation upon precipitation of Fe(III) yields an overall fractionation of -1 to -2‰, that is, values smaller than would be obtained under exclusively equilibrium conditions (Beard and Johnson, 2004;Wu et al, 2013). Dissimilatory Fe(III) reduction (DIR) has been shown to produce fractionations on the order of -3‰, both in the laboratory and in natural settings (Crosby et al, 2005;Percak-Dennett et al, 2011;Tangalos et al, 2010). (Crosby et al, 2005;Wu et al, 2009;.…”

“…Dissimilatory Fe(III) reduction (DIR) has been shown to produce fractionations on the order of -3‰, both in the laboratory and in natural settings (Crosby et al, 2005;Percak-Dennett et al, 2011;Tangalos et al, 2010). (Crosby et al, 2005;Wu et al, 2009;.…”

Geomicrobial functional groups: A window on the interaction between life and environments

“…In contrast, the iron oxides produced by nitrate-dependent iron-reducing bacteria show a δ 56 Fe value 3‰ greater than aquatic Fe 2+ , but close to the equilibrium fractionation between Fe 3+ and Fe 2+ in aquatic systems at room temperature [144]. An experimental study by Crosby et al [145] indicated that soluble Fe 2+ produced by the DIRP, Geobacter sulfurreducens, shows δ 56 Fe values 3‰ more depleted than reactive Fe(III) on iron oxide surfaces, which was caused by fractionation during atomic and electron exchanges between them.…”

Fe Isotopic Compositions of Modern Seafloor Hydrothermal Systems and Their Influence Factors

“…Crosby et al reported that the Fe dissimilatory reduction (DIR) promoted by Geobacter sulfurreducens and Shewanella putrefaciens strains caused Fe isotopic fractionation of approximately 2.2‰, with final Fe(II) species enriched in light isotopes [76], which demonstrated that the Fe isotope composition of Fe(II) aq is largely controlled by isotopic exchange with a reactive Fe(III) pool that lies in the outer layers of the ferric oxide substrate. The adsorption effect led to a particularly strong heavy iron enrichment onto cyanobacteria cells relative to Fe(II) aq when compared to similar experiments performed with Fe(II) aq [77].…”