Coupled Mercury–Cell Sorption, Reduction, and Oxidation on Methylmercury Production by <i>Geobacter sulfurreducens</i> PCA

<i>G. sulfurreducens</i> PCA cells have been shown to reduce, sorb, and methylate Hg­(II) species, but it is unclear whether this organism can oxidize and methylate dissolved elemental Hg(0) as shown for <i>Desulfovibrio desulfuricans</i> ND132. Using Hg­(II) and Hg(0) separately as Hg sources in washed cell assays in phosphate buffered saline (pH 7.4), we report how cell-mediated Hg reduction and oxidation compete or synergize with sorption, thus affecting the production of toxic methylmercury by PCA cells. Methylation is found to be positively correlated to Hg sorption (<i>r</i> = 0.73) but negatively correlated to Hg reduction (<i>r</i> = −0.62). These reactions depend on the Hg and cell concentrations or the ratio of Hg to cellular thiols (−SH). Oxidation and methylation of Hg(0) are favored at relatively low Hg to cell–SH molar ratios (e.g., <1). Increasing Hg to cell ratios from 0.25 × 10^–19 to 25 × 10^–19 moles-Hg/cell (equivalent to Hg/cell–SH of 0.71 to 71) shifts the major reaction from oxidation to reduction. In the absence of five outer membrane <i>c</i>-type cytochromes, mutant Δ<i>omcBESTZ</i> also shows decreases in Hg reduction and increases in methylation. However, the presence of competing thiol-binding ions such as Zn²⁺ leads to increased Hg reduction and decreased methylation. These results suggest that the coupled cell-Hg sorption and redox transformations are important in controlling the rates of Hg uptake and methylation by <i>G. sulfurreducens</i> PCA in anoxic environments

Mercury Reduction and Cell-Surface Adsorption by <i>Geobacter sulfurreducens</i> PCA

Both reduction and surface adsorption of mercuric mercury [Hg­(II)] are found to occur simultaneously on <i>G. sulfurreducens</i> PCA cells under dark, anaerobic conditions. Reduction of Hg­(II) to elemental Hg(0) initially follows a pseudo-first order kinetics with a half-life of <2 h in the presence of 50 nM Hg­(II) and 10¹¹ cells L^–1 in a phosphate buffer (pH 7.4). Multiple gene deletions of the outer membrane cytochromes in this organism resulted in a decrease in reduction rate from ∼0.3 to 0.05 h^–1, and reduction was nearly absent with heat-killed cells or in the cell filtrate. Adsorption of Hg­(II) by cells is found to compete with, and thus inhibit, Hg­(II) reduction. Depending on the Hg to cell ratio, maximum Hg­(II) reduction was observed at about 5 × 10^–19 mol Hg cell^–1, but reduction terminated at a low Hg to cell ratio (<10^–20 mol Hg cell^–1). This inhibitory effect is attributed to bonding between Hg­(II) and the thiol (−SH) functional groups on cells and validated by experiments in which the sorbed Hg­(II) was readily exchanged by thiols (e.g., glutathione) but not by carboxylate compounds such as ethylenediamine-tetraacetate (EDTA). We suggest that coupled Hg­(II)–cell interactions, i.e., reduction and surface binding, could be important in controlling Hg species transformation and bioavailability and should therefore be considered in microbial Hg­(II) uptake and methylation studies