Cobalamin-mediated mercury methylation by Desulfovibrio desulfuricans LS

Abstract

The prominence of sulfate reducers in mercury biomethylation prompted the examination of the methyl carrier and mercury methylation activity ofDesulfovibrio desulfuricans LS. There was a low degree of mercury tolerance and a high degree of methylation during fermentative growth; the opposite was true during sulfate reduction. During 2 days of fermentative growth, up to 37% of HgCl2 was methylated at 0.1 ,ug/ml, but obly 1.5% was methylated at 10.0 ,ug/ml. Less than 1% of the added HgCl2 was methylated under sulfate-reducing conditions. D. desulfuricans LS radioimmunoassay results were positive for cobalamin. The addition of CoCI2 and benzimidazole to fermentative cultures increased methylation activity. From D. desulfuricans LS grown in the presence of 57CoC12, a corrinoid was extracted and purified. High-performance liquid chromatography analysis of the purified extract yielded a single peak with the retention time of cobalamin, and 97% of the 57Co * Corresponding author. t New Jersey Agricultural Experiment Station publication D-01408-01-92.

Mercury methylation and hydrogen sulfide production among unexpected strains isolated from periphyton of two macrophytes of the Amazon

“…Generally, the degree of methylation increased with the increasing spike concentrations (Table 1). This is in agreement with Gilmour et al (2011) results, but is in contradiction to Choi & Bartha's (1993) findings. However, the concentrations of added Hg(II) used in the second study were three orders of magnitude higher than those used in the present study (Table 1).…”

“…Autoclaved and blank controls showed almost no detectable amounts of MMHg (< 0.05 and 0.2 pg mL À1 respectively), which tend to decline over time. The potential of some of the strains to produce methylmercury is comparable to that of SRB reported by King et al (King et al, 2000) and Choi and Bartha (Choi & Bartha, 1993), but up to two orders of magnitude lower than the rates reported elsewhere (Ekstrom et al, 2003;Ranchou-Peyruse et al, 2009) (Table 1). However, the differences in the magnitude of methylmercury produced can be attributed to factors that are not necessarily related to the significance of these bacteria for Hg methylation in the environment.…”

Mercury Cycling and Effects in Freshwater Wetland Ecosystems

“…Evidence from this experiment suggests that methanogens contributed little to mercury methylation, unlike previous suggestions [66]. Choi and Bartha [67] showed that cobalamin (vitamin Biz) is the intermediate metabolite that methylates mercury. Although sulfate reduction is associated with methylation, fermentation produces the cobalamin and methylation is only observed during fermenta-tion; methylation ceases when fermentation stops, even though sulfate reduction continues.…”

Bacterial mercury resistance from atoms to ecosystems

“…More recent work, combining Hg chemical speciation with methylation rate measurements [59^61], showed that soluble, neutral HgS is the substrate for methylation by SRB. Richard Bartha and his students, studying methylation by crude cell extracts of Desulfovibrio desulfuricans strain LS, showed that the methyl group originated either from serine C3 [62] or from formate via acetyl-CoA [63] via CH 3 -tetrahydrofolate (MeTHF) to methylcobalamin [64] followed by enzymatic methylation of Hg [65]. A positive correlation of Hg(II)-dependent transformation of MeTHF to THF in soil and sediment extracts with in situ MeHg concentrations [66] further supported the notion that enzymatic catalysis, rather than spontaneous transfer of CH 3 from methylcobalamin [53], is the mechanism of microbial MeHg synthesis.…”