原文摘要:Extracellular reduction of uranium via Geobacter conductive pili as a protective cellular mechanism
The in situ stimulation of Fe(III) oxide reduction by Geobacter
bacteria leads to the concomitant precipitation of hexavalent uranium
[U(VI)] from groundwater. Despite its promise for the bioremediation of
uranium contaminants, the biological mechanism behind this reaction
remains elusive. Because Fe(III) oxide reduction requires the expression
of Geobacter's conductive pili, we evaluated their contribution to
uranium reduction in Geobacter sulfurreducens grown under pili-inducing
or noninducing conditions. A pilin-deficient mutant and a genetically
complemented strain with reduced outer membrane c-cytochrome content
were used as controls. Pili expression significantly enhanced the rate
and extent of uranium immobilization per cell and prevented periplasmic
mineralization. As a result, pili expression also preserved the vital
respiratory activities of the cell envelope and the cell's viability.
Uranium preferentially precipitated along the pili and, to a lesser
extent, on outer membrane redox-active foci. In contrast, the
pilus-defective strains had different degrees of periplasmic
mineralization matching well with their outer membrane c-cytochrome
content. X-ray absorption spectroscopy analyses demonstrated the
extracellular reduction of U(VI) by the pili to mononuclear tetravalent
uranium U(IV) complexed by carbon-containing ligands, consistent with a
biological reduction. In contrast, the U(IV) in the pilin-deficient
mutant cells also required an additional phosphorous ligand, in
agreement with the predominantly periplasmic mineralization of uranium
observed in this strain. These findings 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.