METHYLMERCURY RESISTANCE IN DESULFOVIBRIO-DESULFURICANS STRAINS IN RELATION TO METHYLMERCURY DEGRADATION

Two strains of Desulfovibrio desulfuricans, one known to synthesize mo nomethylmercury from ionic mercury, were grown to determine methylmerc ury toxicity and for comparison with an anaerobic strain of Clostridiu m pasteurianum, a H-2 producer, and with the broad-spectrum mercury-re sistant Pseudomonas putida strain FB-1, capable of degrading 1 mug of methylmercury to methane and elemental mercury in 2 h. The CH3HgCl res istance of D. desulfuricans strains was 10 times that of P. putida FB- 1 and 100 times that of C. pasteurianum. The methylmercury resistance of D. desulfuricans was related to the disappearance of methylmercury from cultures by transformation to dimethylmercury, metacinnabar, meth ane, and traces of ionic mercury. During a 15-day experiment the kinet ics of the two volatile compounds dimethylmercury (CH3)2Hg! and metha ne were monitored in the liquid by a specific new technique with purge -and-trap gas chromatography in line with Fourier transform infrared s pectroscopy and in the headspace by gas chromatography with flame ioni zation detection. Insoluble metacinnabar (cubic HgS) of biological ori gin was detected by X-ray diffractometry in the gray precipitate from the insoluble residue of the pellet of a 1-liter culture spiked with 1 00 mg of CH3HgCl. This was compared with a 1-liter culture of D. desul furicans LS spiked with 100 mg of HgCl2. In a further experiment, it w as demonstrated that insoluble, decomposable, white dimethylmercury su lfide (CH3Hg)2S! formed instantly in the reaction of methylmercury wi th hydrogen sulfide. This organomercurial was extracted with chlorofor m and identified by gas chromatography in tine with mass spectrometry. The D. desulfuricans strains were resistant to high concentrations of methylmercury because they produced insoluble dimethylmercury sulfide , which slowly decomposed under anaerobic conditions to metacinnabar a nd 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 CH3HgCl at a lower rate than by the broad-s pectrum Hg-resistant P. putida strain FB-1.