Investigation of Mercury Methylation Pathways in Biofilm versus Planktonic Cultures of Desulfovibrio desulfuricans

Abstract

Biofilms can methylate mercury (Hg) at higher rates than unattached bacteria and are increasingly recognized as important Hg methylation sites in the environment. Our previous study showed that methylation rates in biofilm cultures were up to 1 order of magnitude greater than those in planktonic cultures of a sulfate-reducing bacterium. To probe whether the differential Hg methylation rates resulted from metabolic differences between these two cultures, Hg methylation assays following molybdate or chloroform inhibition (a specific inhibitor of the acetyl-CoA pathway) were conducted on biofilm and planktonic cultures of Desulfovibrio desulfuricans strains M8 and ND132. Molybdate was as effective in inhibiting Hg methylation as well as growth in both planktonic and biofilm cultures. The addition of chloroform only impacted Hg methylation in biofilm cultures, suggesting that different pathways are used for methylation in biofilm compared to planktonic cultures. To investigate this further, expression of the cooS gene, which encodes for carbon monoxide dehydrogenase, a key enzyme in the acetyl-CoA pathway, was compared in biofilm and planktonic cultures of ND132. Biofilm cultures showed up to 4 times higher expression of cooS than planktonic cultures. On the basis of these results, the acetyl-CoA pathway appears to play an important role in methylation in biofilm cultures of this organism, possibly by supplying the methyl group to Hg methylating enzymes; methylation in planktonic cultures appears to be independent of this pathway. This observation has important implications, particularly in developing reliable models to predict Hg methylation rates in different environments and perhaps eventually in being able to control this undesirable chemical transformation.

Mercury-methylating bacteria are associated with copepods: A proof-of-principle survey in the Baltic Sea

“…Hg-methylating genes have been detected in invertebrate microbiota, including termites, beetles, and oligochaetes [5,16], and in some invertebrates the endogenous MeHg production has been documented [39]. As a life form, intestinal microbiota exists in biofilms, and such communities are increasingly recognized as important sites for environmental Hg methylation [40,41]. Commensal biofilms are present in both planktonic and benthic animals that actively exchange gut and body-surface microbiota with the ambient microbial communities and other animals [42].…”

The Response of <em>Cupriavidus Metallidurans</em> CH34 to Cadmium Involves a Decrease in Intracellular Levels of C-Di-GMP Probably Mediated by a Novel Metal Regulated Phosphodiesterase That Inhibits the Biofilm Lifestyle

“…In contrast to the strong induction by cadmium of the expression of the merA gene (121 fold) in biofilm cells, a lower induction by cadmium of the cadA gene (10 fold) was observed, indicating a differential behavior of both operons in the sessile biofilm lifestyle. Previously, differential mercury resistance mechanisms of Desulfovibrio desulfuricans planktonic cells and biofilms have been reported [75], which resembles the response to cadmium in strain CH34.…”

Progress in the study of mercury methylation and demethylation in aquatic environments

“…It was proposed that these neutral Hg species could be transported to microorganism cells by passive diffusion and then methylated [109][110]. This opinion was supported by the positive relationship between Hg methylation rates and calculated concentrations of neutral Hg-sulfide complex in solution, which was observed both in laboratory pure culture of SRB [101,111] and field investigations [112,113]. Hg 2+ -thiol complexes are another Hg 2+ species that have been proposed to be available for methylation [114,115].…”

“…Neutral Hg-sulfide complexes (e.g. HgS(0) (aq) and Hg(SH) 2 ) have been suggested to be the major species of Hg that are available for methylation since the late 1990s [101,105,[109][110][111][112]. It was proposed that these neutral Hg species could be transported to microorganism cells by passive diffusion and then methylated [109][110].…”