Influence of pH and dissolved Si on Fe isotope fractionation during dissimilatory microbial reduction of hematite

Microbial dissimilatory iron reduction (DIR) has been identified as a mechanism for production of aqueous Fe(II) that has low 56Fe/54Fe ratios in modern and ancient suboxic environments that contain ferric oxides or hydroxides. These studies suggest that DIR could have played an important role in producing distinct Fe isotope compositions in Precambrian banded iron formations or other marine sedimentary rocks. However, the applicability of experimental studies of Fe isotope fractionation produced by DIR in geochemically simple systems to ancient marine environments remains unclear. Here we report Fe isotope fractionations produced during dissimilatory microbial reduction of hematite by Geobacter sulfurreducens in the presence and absence of dissolved Si at neutral and alkaline pH. Hematite reduction was significantly decreased by Si at alkaline (but not neutral) pH, presumably due to Si polymerization at the hematite surface. The presence of Si altered Fe isotope fractionation factors between aqueous Fe(II) or sorbed Fe(II) and reactive Fe(III), reflecting changes in bonding environment of the reactive Fe(III) component at the oxide surface. Despite these changes in isotopic fractionations, our results demonstrate that microbial Fe(III) oxide reduction produces Fe(II) with negative δ56Fe values under conditions of variable pH and dissolved Si, similar to the large inventory of negative δ56Fe in Neoarchean and Paleoproterozoic age marine sedimentary rocks.