The in situ stimulation of Fe(III) oxide reduction by Geobacter bacteria leads to the concomitant precipitation of U(VI) from groundwater. However, the biological mechanism behind this reaction has remained elusive for almost two decades. Because Fe(III) oxide reduction requires the expression of Geobacter's conductive pili, we evaluated their contribution to uranium reduction using pili-defective or hyperpiliated strains. Pili expression significantly enhanced the rate and extent of uranium immobilization per cell. The soluble U(VI) was reduced extracellularly to mononuclear U(IV) complexed by carbon containing ligands of the pili. This prevented the soluble uranium from penetrating inside the cell and preserved the vital respiratory activities of the cell envelope and the cell's viability. These results demonstrate a previously unrecognized role for Geobacter conductive pili in the extracellular reduction of uranium and highlight its essential function as a catalytic and protective cellular mechanism that is of interest for the bioremediation of uranium-contaminated groundwater.

Microorganisms with a taste for nuclear chemistry

243rd National Meeting (2012)

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