Both reduction and surface adsorption of mercuric mercury [Hg(II)] are found to occur simultaneously on G. sulfurreducens 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 10(11) 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.