Geochemical and microbiological controls on dissimilatory iron reduction

Recent experimental studies permit development of conceptual and quantitative models of microbial Fe(III) oxide reduction at circumneutral pH that can be compared to and contrasted with established models of abiotic mineral dissolution. The findings collectively support a model for controls on enzymatic reduction that differs fundamentally from those applied to abiotic reductive dissolution as a result of two basic phenomena: (1) the relatively minor influence of oxide mineralogical and thermodynamic properties on rates of enzymatic reduction compared to abiotic reductive dissolution, and (2) the major limitation which sorption and/or surface precipitation of biogenic Fe(II) on residual oxide and Fe((III))-reducing bacterial cell surfaces poses to enzymatic electron transfer in the presence of excess electron donor. Parallel studies with two well-characterized Fe((III))-reducing organisms (Shewanella putrefaciens and Geobacter sulfurreducens) lead to common conclusions regarding the importance of these phenomena in regulating the rate and long-term extent of Fe((III)) oxide reduction. Models in which rates of enzymatic reduction are limited by Fe((III))-reducing bacterial cell density together with the abundance of 'available' oxide surface sites (as controlled by oxide surface area and the accumulation of surface-bound biogenic Fe((II)) provide an adequate macroscopic description of controls on the initial rate and long-term extent of oxide reduction.