Mercury methylation and demethylation in anoxic lake sediments and by strictly anaerobic bacteria

The impact of methylmercury on human health prompted an examination of the mercury methylation and demethylation processes in anoxic lake sediments and by the strictly anaerobic bacteria active in them. After spiking them with either {dollar}rm 1.0 mu g HgClsb2/ml{dollar} or {dollar}rm 0.1 mu g CHsb3Hgsp+/ml,{dollar} both mercury methylation and demethylation rates were measured in anoxic sediment slurries of three acidic oligotrophic lakes. High mercury methylation potentials were accompanied by high demethylation potentials in the same sediments. These high potentials correlated positively with sediment organic matter, dissolved sulfate and mercury levels in fish. Adjustment of the acidic sediment pH to neutrality failed to influence either the methylation or the demethylation rates of mercury. The opposing methylation and demethylation processes converged to established similar {dollar}rm Hgsp{lcub}2+{rcub}/CHsb3Hgsp+{dollar} equilibria in all three sediments.; Because of their metabolic importance in anoxic sediments, mercury methylation and demethylation were similarly measured in pure cultures of sulfidogenic, methanogenic and acetogenic bacteria. Earlier measurement on such pure cultures were inconclusive. In the present study, sulfidogens were found to both methylate and demethylate mercury. The methanogen tested only catalyzed demethylation and the acetogen neither methylated nor demethylated mercury.; Co-cultures of Desulfovibrio desulfuricans and Methanococcus maripaludis grew on sulfate-free lactate medium while vigorously methylating {dollar}rm Hgsp{lcub}2+{rcub}{dollar} Individually, neither bacterium could grow or methylate mercury in this medium. Similar synergistic growth of sulfidogens and methanogens is likely to create favorable conditions for {dollar}rm Hgsp{lcub}2+{rcub}{dollar} methylation in low-sulfate anoxic freshwater sediments.; As earlier studies on anoxic lake sediments showed that the carbon of {dollar}rmsp{lcub}14{rcub}CHsb3Hgsp+{dollar} in such sediments was released as {dollar}rmsp{lcub}14{rcub}COsb2,{dollar} the decomposition products of {dollar}rm CHsb3HgI{dollar} by pure cultures of D. desulfuricans LS and ND and by M. maripaludis were measured. Only {dollar}rmsp{lcub}14{rcub}CHsb4{dollar} was produced by all three pure cultures, and no {dollar}sp{lcub}14{rcub}COsb2{dollar} was detected. Therefore, the {dollar}rmsp{lcub}14{rcub}CHsb3Hgsp+{dollar}-derived {dollar}rmsp{lcub}14{rcub}COsb2{dollar} identified in earlier sediment studies appears to be a secondary oxidation product of {dollar}rmsp{lcub}14{rcub}CHsb4{dollar} rather than the primary product of {dollar}rm sp{lcub}14{rcub}CHsb3Hgsp+{dollar} demethylation.