Detailed Assessment of the Kinetics of Hg-Cell Association, Hg Methylation, and Methylmercury Degradation in Several Desulfovibrio Species

Abstract

. We tested the hypothesis that differences in Hg(II) i sorption and/or uptake rates drive observed differences in methylation rates among Desulfovibrio species. Hg(II) i associated rapidly and with high affinity to both methylating and nonmethylating species. MeHg production by Hg-methylating strains was rapid, plateauing after ϳ3 h. All MeHg produced was rapidly exported. We also tested the idea that all Desulfovibrio species are capable of Hg(II) i methylation but that rapid demethylation masks its production, but we found this was not the case. Therefore, the underlying reason why MeHg production capability is not universal in the Desulfovibrio is not differences in Hg affinity for cells nor differences in the ability of strains to degrade MeHg. However, Hg methylation rates varied substantially between Hg-methylating Desulfovibrio species even in these controlled experiments and after normalization to cell density. Thus, biological differences may drive crossspecies differences in Hg methylation rates. As part of this study, Microbial mercury methylation is the main driver of risk associated with Hg pollution. Methylmercury production is an anaerobic process that occurs in saturated soils and wetlands (26,44,45,53), decaying periphyton mats (1, 14, 31), aquatic bottom sediments (16,27,33,36), and anaerobic bottom waters (56). Early investigations, prior to the advent of modern methylmercury (MeHg) analyses, reported a wide variety of aerobic and anaerobic Gram-positive and Gram-negative bacteria (30,49,55,58) and fungi (55) to be capable of MeHg production. However, subsequent studies with pure cultures have conclusively demonstrated a role only for sulfate-reducing bacteria (SRB) (4,8,11,13,20,23,38,50) and iron-reducing bacteria (FeRB; principally Geobacter spp.) (21, 37), all belonging to the Deltaproteobacteria. Many field studies, using selective microbial stimulants (1, 10, 26, 44, 57), inhibitors (1, 16, 24, 26, 59), and biogeochemical correlates (6,39,40,45), have buttressed the paradigm of SRB and FeRB as the dominant Hg methylators in natural aquatic systems (16,24,59), though recent studies have hypothesized that methanogens may be significant in some systems (31).Only a subset of SRB and FeRB are capable of Hg methylation (11,23,37,50), but why this is the case remains unclear. Early work by Choi and Bartha (13) suggested that Hg methylation was a "metabolic mistake" of SRB utilizing the acetyl coenzyme A (acetyl-CoA) pathway for carbon metabolism. Subsequent studies, however, indicated that Hg methylation capability is not restricted to SRB possessing the acetyl-CoA pathway (20). At present, it is not possible to conclusively identify the methyltransferase or methyl donor in SRB (or other Deltaproteobacteria) responsible for in vivo Hg methylation. Hg methylation occurs intracellularly (23), and significant effort has therefore been devoted to elucidating the mechanism(s) of Hg uptake by Hg-methylating bacteria.Passive diffusion of neutral HgS species has been hypothesized to control Hg uptak...

Mercury Methylation by Novel Microorganisms from New Environments

“…Further, Hgmethylating SRB and FeRB appear to sorb lower amounts of Hg than do non-methylating strains. 17 14…”

Stoichiometry of mercury-thiol complexes on bacterial cell envelopes

“…Certain 96 bacterial strains such as Geobacter sulfurreducens PCA can function both as a reductant and as 97 an adsorbent for Hg(II) at different cell biomass to Hg ratios (Hu et al, 2013), with adsorption 98 being the dominant mechanism at low Hg:biomass ratios. Sorption of Hg(II) to cell envelope 99 sites has been thought to serve as a "sink" for Hg(II) that restricts transport into the cytoplasm, 100 thus lowering the bioavailability of Hg(II) (Graham et al, 2012). Recent studies indicate that in 101 addition to gene expression and regulation, cell envelope chemistry is likely an important driver 102 for cross-species differences in Hg methylation rates (Graham et al, 2012).…”

“…Sorption of Hg(II) to cell envelope 99 sites has been thought to serve as a "sink" for Hg(II) that restricts transport into the cytoplasm, 100 thus lowering the bioavailability of Hg(II) (Graham et al, 2012). Recent studies indicate that in 101 addition to gene expression and regulation, cell envelope chemistry is likely an important driver 102 for cross-species differences in Hg methylation rates (Graham et al, 2012). Furthermore, 103 reactivity of thiols towards Hg(0), resulting in thiol mediated passive microbial oxidation of 104 Hg(0), has been recently reported (Colombo et al, 2013; Colombo et al, 2014).…”

Metabolism of Metals and Metalloids by the Sulfate-Reducing Bacteria

“…Gilmour et al [38] indicate that over 50 % of the Desulfovibrio strains tested have the capability of producing methylmercury. From a study involving 59 species of Desulfovibrio, Graham et al [39] determined that mercury methylation is species-specific and is not restricted to a given phylogeny. Recently, it was shown that for the mercury methylation process in D. desulfuricans ND132, genes hgcA and hgcB are required [72].…”