Zinc and Manganese Inhibition of Biological Hematite Reduction

The effects of zinc and manganese on the reductive dissolution of hematite by the dissimilatory metal-reducing bacterium (DMRB) Shewanella putrefaciens CN32 were studied in batch culture. Experiments were conducted with hematite (2.0 g L 1) in 10 mM PIPES (pH 6.8), and H2 as the electron donor under nongrowth conditions (108 cell mL 1), spiked with zinc (0.02–0.23 mM) or manganese (0.02–1.8 mM) and incubated for 5 days. Zinc inhibition was calculated based on the 5-day extent of hematite bioreduction in the absence and presence of zinc. Zinc inhibition of hematite bioreduction increased with anthraquinone-2,6-disulfonate (AQDS), a soluble electron shuttling agent, and ferrozine, a strong Fe(II) complexant. Both amendments would otherwise stimulate hematite bioreduction. These amendments did not significantly increase zinc sorption, but may have increased zinc toxicity by some unknown mechanism. At equal total Me(II) concentrations, zinc inhibited hematite reduction more than manganese and caused greater cell death. At equal sorbed Me(II) concentrations, manganese inhibited hematite reduction more than zinc and caused greater cell death. Results support the interpretation that Me(II) toxicity was more important than Me(II) sorption/surface blockage in inhibiting hematite reduction.