Extracellular reduction of uranium via Geobacter conductive pili as a protective cellular mechanism

Abstract

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.

Harnessing the power of microbial nanowires

“…Genetic studies in the model representative Geobacter sulfurreducens (GS) have provided insights into how the cells assemble the GS pilin into a conductive fibre. However, little is known about how respiratory electrons are discharged through the T4P, partly because mutations that inactivate key electron carriers of the cell envelope, whether the T4P or c‐Cyts, are often pleiotropic (Kim et al ., 2005, 2006; Kim and Lovley, 2008; Cologgi et al ., 2011; Richter et al ., 2012; Steidl et al ., 2016). As a result, compensatory effects are often observed that can mask the true phenotype of the mutants.…”

“…The conductive T4P are also the primary mechanism for the reduction of the uranyl cation (UO 2 2+ ) (Cologgi et al ., 2011). The fibres bind the soluble cation extracellularly and discharge respiratory electrons to reduce the U(VI) to a mononuclear U(IV) mineral phase (Cologgi et al ., 2011).…”

“…The fibres bind the soluble cation extracellularly and discharge respiratory electrons to reduce the U(VI) to a mononuclear U(IV) mineral phase (Cologgi et al ., 2011). This reductive reaction effectively mineralizes the radionuclide outside the cell and prevents its permeation and nonspecific reduction in the cell envelope (Cologgi et al ., 2011). Thus, the protein nanowires provide a dual strategy for energy conservation and cellular protection from the toxic uranyl cation.…”

“…These c‐Cyts localize to the inner membrane, periplasmic space and outer membrane, providing a potential path for the transfer of electrons to the cell surface. Micrographs of uranium‐reducing cells show some spots of uranium mineralization on the cell surface, which could correspond to reductive foci of outer membrane c‐Cyts (Cologgi et al ., 2011). However, in contrast to the positive correlation between piliation and extracellular uranium mineralization (Cologgi et al ., 2011), a reverse correlation exists between the outer membrane haem content and the amount of uranium that is mineralized outside the cell and prevented from traversing the outer membrane (Reguera, 2012).…”

“…The ability of T4 pilins to polymerize spontaneously via hydrophobic interactions has inspired ‘bottom‐up’ fabrication protocols for the synthesis of protein nanotubes (Audette et al ., 2004a,b) and biocoatings that prevent the corrosion of metallic implants (Muruve et al ., 2016). The discovery that metal‐reducing bacteria in the genus Geobacter produce conductive T4P for the reduction of ferric (Fe[III]) oxide minerals (Reguera et al ., 2005) and the uranyl cation (Cologgi et al ., 2011) suggests novel applications are to be harnessed from the unique electronic and metal binding properties of these protein filaments and their peptide building blocks.…”