INTRACELLULAR MERCURY OXIDATION AND COMPLEXATION IN ANAEROBIC BACTERIA

Mercury (Hg) methylating bacteria are known to oxidize elemental mercury (Hg[0]) to ionic mercury (Hg[II]) under anoxic conditions. Currently, the mechanism and cellular location of Hg[0] oxidation in anaerobic bacteria are poorly understood. In this study, we performed experiments with the Hg-methylating bacteria Desulfovibrio desulfuricans and Geobacter sulfurreducens to determine if Hg[0] oxidation occurs within the cytoplasm. To examine intracellular Hg[0] oxidation, mercury uptake and Hg L_III edge X-ray absorption fine structure (XAFS) spectroscopy experiments were carried out with spheroplasts (cells with cell wall removed). D. desulfuricans and G. sulfurreducens spheroplasts were prepared using a lysozyme treatment and then exposed to a continuous source of Hg[0]_(aq) under strict anaerobic conditions. At periodic intervals samples collected and purged with N₂ gas to remove all volatile Hg[0] and analyzed for non-purgeable Hg. The results indicate that both D. desulfuricans and G. sulfurreducens spheroplasts rapidly oxidize Hg[0] to Hg[II]. XAFS spectroscopy analysis revealed that cell-associated Hg(II) was covalently bound to intracellular thiol functional groups and that the stoichiometry of Hg-thiol complexes were sensitive to reaction time. Incorporating this new findings in biogeochemical models will improve our ability to predict Hg redox transformations in the environment and estimate the bioavailable pool of Hg[II] for methylation.