Coupled Fe(II)-Fe(III) electron and atom exchange as a mechanism for Fe isotope fractionation during dissimilatory iron oxide reduction

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 delta Fe-56 values that are 0.5-2.0 parts per thousand lower than 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 Fe-56/Fe-54 isotopic fractionation between aqueous Fe(II) and the outermost layers of Fe(III) on the oxide surface is similar to -3 parts per thousand 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-delta Fe-56 values for aqueous Fe(II) during DIR.