Microbial and surface chemistry controls on reduction of synthetic Fe(III) oxide minerals by the dissimilatory iron‐reducing bacteriumShewanella alga

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Abstract
The role of Fe(II) biosorption and the effect of medium components on the rate and long‐term extent of Fe(III) oxide reduction (FeRed) by a dissimilatory Fe(III)‐reducing bacterium (Shewanella alga strain BrY) were examined in batch culture experiments. Introduction of fresh S. alga cells into month‐old cultures in which Fe(III) reduction had ceased resulted in further reduction of synthetic amorphous Fe(III) oxide, hematite, and two forms of goethite (Gt). Fresh S. alga cells were also able to reduce a substantial amount of synthetic Gt that had been partly or completely saturated with sorbed Fe(II). Cells that had been precoated with Fe(II) showed a reduced rate and capacity for FeRed. These results indicated that biosorption of Fe(II) had a major impact on FeRed. S. alga cells were shown to have an Fe(II) sorption capacity of ∼0.1 mmol g−1, compared with ∼0.25 mmol g−1 determined for the synthetic Gt. Sorption experiments with component mixtures indicated that direct interaction between cells and oxide resulted in increased Fe(II)‐binding capacity of the mixed system, possibly through production of exopoly‐meric materials by the cells. Medium constituents that affected Fe(II) speciation were shown to have a significant indirect influence on the extent of oxide reduction. Malaie, which formed soluble complexes with Fe(II), promoted the extent of oxide reduction. In contrast, high (mM) PO4 3− concentrations favored surface/bulk precipitation processes which reduced the extent of oxide reduction. Collectively, our results indicate that Fe(II) sorption by oxide and cell surfaces, together with Fe(II) complexation by or precipita‐tion with medium components, all influence the rate and extent of FeRed. Furthermore, saturation of sorption sites with Fe(II) does not appear to limit the ability of S. alga to reduce Fe(III) oxides, especially if conditions favor growth.