Metal reduction by spores of <i>Desulfotomaculum reducens</i>

The bioremediation of uranium-contaminated sites is designed to stimulate the activity of microorganisms able to catalyze the reduction of soluble U(VI) to the less soluble mineral UO₂. U(VI) reduction does not necessarily support growth in previously studied bacteria, but it typically involves viable vegetative cells and the presence of an appropriate electron donor. We characterized U(VI) reduction by the sulfate-reducing bacterium <i>Desulfotomaculum reducens</i> strain MI-1 grown fermentatively on pyruvate and observed that spores were capable of U(VI) reduction. Hydrogen gas – a product of pyruvate fermentation – rather than pyruvate, served as the electron donor. The presence of spent growth medium was required for the process, suggesting that an unknown factor produced by the cells was necessary for reduction. Ultrafiltration of the spent medium followed by U(VI) reduction assays revealed that the factor's molecular size was below 3 kDa. Pre-reduced spent medium displayed short-term U(VI) reduction activity, suggesting that the missing factor may be an electron shuttle, but neither anthraquinone-2,6-disulfonic acid nor riboflavin rescued spore activity in fresh medium. Spores of <i>D. reducens</i> also reduced Fe(III)-citrate under experimental conditions similar to those for U(VI) reduction. This is the first report of a bacterium able to reduce metals while in a sporulated state and underscores the novel nature of the mechanism of metal reduction by strain MI-1.