Ferrous iron removal promotes microbial reduction of crystalline iron(III) oxides

Semicontinuous cultures were used to assess the effect of aqueous Fe(II) removal on the dissimilatory reduction of crystalline Fe(III) oxides by Shewanella alga strain BrY. Aqueous phase replacement in semicontinuous cultures (average residence time of 9 or 18 days) resulted in a 2-3-fold increase in the cumulative amount of Fe(II) produced from synthetic goethite reduction over a 2-month incubation period, compared to parallel batch cultures. A more modest (maximum 30%) but significant stimulation of natural subsoil Fe(III) oxide reduction was observed. The extended Fe(III) reduction resulted from enhanced generation of aqueous Fe(II) which was periodically removed from the cultures. A concomitant stimulation of bacterial protein production was detected, which suggested that Fe(II) removal also promoted bacterial growth. A simulation model in which Fe(II) sorption to the solid-phase resulted in blockage of surface reduction sites captured the contrasting behavior of the batch vs semicontinuous Gt reduction systems. Our findings indicate that elimination of Fe(II) via advective transport could play a significant role in governing the rate and extent of microbial Fe(III) oxide reduction in sedimentary environments.