DIMETHYLMERCURY AND DIMETHYLMERCURY-SULFIDE OF MICROBIAL ORIGIN IN THE BIOGEOCHEMICAL CYCLE OF HG

The transformation of MeHg under anaerobic conditions in axenic cultures of Desulfovibrio desulfuricans strain LS is compared to that in anoxic marine sediments contaminated by Hg of industrial origin. MeHg was added to cultures of D. desulfuricans strain LS and incubated at 28 degrees for two weeks. Significant amounts of dimethylmercury and metacinnabar were produced. These two Hg compounds were formed from the slow decomposition of the intermediate dimethylmercury-sulfide. Other collateral compounds, such as methane and ionic Hg, were also detected during the MeHg degradation process. On the other hand a sample of fresh sediment (1.5 g d.w.) was spiked with 10 mu g of MeHg and 2 mmoles.ml(-1) of pyruvate, as carbon source for sulfate-reducing bacteria. After 9 days of incubation at 28 degrees C, significant amounts of dimethylmercury were produced. A lower content of this volatile species was found in a sub-sample of sediment supplemented with sodium molybdate, which is a strong inhibitor of sulfate-reducing activity. A kinetic study showed the disappearance of monomethylmercury from the sediment and the formation of dimethylmercury over the incubation period. The environmental significance of dimethylmercury and dimethylmercury-sulfide in the natural biogeochemical cycle of Hg is discussed.