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Two strains ofDesulfovibrio desulfuricans , one known to synthesize monomethylmercury from ionic mercury, were grown to determine methylmercury toxicity and for comparison with an anaerobic strain ofClostridium pasteurianum , a H2 producer, and with the broad-spectrum mercury-resistantPseudomonas putida strain FB-1, capable of degrading 1 μg of methylmercury to methane and elemental mercury in 2 h. The CH3 HgCl resistance ofD. desulfuricans strains was 10 times that ofP. putida FB-1 and 100 times that ofC. pasteurianum . The methylmercury resistance ofD. desulfuricans was related to the disappearance of methylmercury from cultures by transformation to dimethylmercury, metacinnabar, methane, and traces of ionic mercury. During a 15-day experiment the kinetics of the two volatile compounds dimethylmercury [(CH3 )2 Hg] and methane were monitored in the liquid by a specific new technique with purge-and-trap gas chromatography in line with Fourier transform infrared spectroscopy and in the headspace by gas chromatography with flame ionization detection. Insoluble metacinnabar (cubic HgS) of biological origin was detected by X-ray diffractometry in the gray precipitate from the insoluble residue of the pellet of a 1-liter culture spiked with 100 mg of CH3 HgCl. This was compared with a 1-liter culture ofD. desulfuricans LS spiked with 100 mg of HgCl2 . In a further experiment, it was demonstrated that insoluble, decomposable, white dimethylmercury sulfide [(CH3 Hg)2 S] formed instantly in the reaction of methylmercury with hydrogen sulfide. This organomercurial was extracted with chloroform and identified by gas chromatography in line with mass spectrometry. TheD. desulfuricans strains were resistant to high concentrations of methylmercury because they produced insoluble dimethylmercury sulfide, which slowly decomposed under anaerobic conditions to metacinnabar and volatilized to dimethylmercury and methane between pHs 6.2 and 6.5 for high (4.5-g · liter-1 ) or low (0.09-g · liter-1 ) sulfate contents. Methane was produced from CH3 HgCl at a lower rate than by the broad-spectrum Hg-resistantP. putida strain FB-1.

Methylmercury Resistance in Desulfovibrio desulfuricans Strains in Relation to Methylmercury Degradation