Influence of Mn oxides on the reduction of uranium(VI) by the metal-reducing bacterium Shewanella putrefaciens

The potential for Mn oxides to modify the biogeochemical behavior of U during reduction by the subsurface bacterium Shewanella putrefaciens strain CN32 was investigated using synthetic Mn(III/IV) oxides (pyrolusite β-MnO₂], bixbyite Mn₂O₃] and K⁺-birnessite K₄Mn₁₄O₂₇ · 8H₂O]). In the absence of bacteria, pyrolusite and bixbyite oxidized biogenic uraninite (UO₂s]) to soluble U(VI) species, with bixbyite being the most rapid oxidant. The Mn(III/IV) oxides lowered the bioreduction rate of U(VI) relative to rates in their absence or in the presence of gibbsite (AlOH]₃) added as a non-redox-reactive surface. Evolved Mn(II) increased with increasing initial U(VI) concentration in the biotic experiments, indicating that valence cycling of U facilitated the reduction of Mn(III/IV). Despite an excess of the Mn oxide, 43 to 100% of the initial U was bioreduced after extended incubation. Analysis of thin sections of bacterial Mn oxide suspensions revealed that the reduced U resided in the periplasmic space of the bacterial cells. However, in the absence of Mn(III/IV) oxides, UO₂(s) accumulated as copious fine-grained particles external to the cell. These results indicate that the presence of Mn(III/IV) oxides may impede the biological reduction of U(VI) in subsoils and sediments. However, the accumulation of U(IV) in the cell periplasm may physically protect reduced U from oxidation, promoting at least a temporal state of redox disequilibria.