Relative contributions of mercury bioavailability and microbial growth rate on net methylmercury production by anaerobic mixed cultures

Abstract

Monomethylmercury (MeHg) is produced in many aquatic environments by anaerobic microorganisms that take up and methylate inorganic forms of Hg(II). Net methylation of Hg(II) appears to be correlated with factors that affect the activity of the anaerobic microbial community and factors that increase the bioavailability of Hg(II) to these organisms. However, the relative importance of one versus the other is difficult to elucidate even though this information can greatly assist remediation efforts and risk assessments. Here, we investigated the effects of Hg speciation (dissolved Hg and nanoparticulate HgS) and microbial activity on the net production of MeHg using two mixed microbial cultures that were enriched from marine sediments under sulfate reducing conditions. The cultures were amended with dissolved Hg (added as a dissolved nitrate salt) and nanoparticulate HgS, and grown under different carbon substrate concentrations. The results indicated that net mercury methylation was the highest for cultures incubated in the greatest carbon substrate concentration (60 mM) compared to incubations with less carbon (0.6 and 6 mM), regardless of the form of mercury amended. Net MeHg production in cultures exposed to HgS nanoparticles was significantly slower than in cultures exposed to dissolved Hg; however, the difference diminished with slower growing cultures with low carbon addition (0.6 mM). The net Hg methylation rate was found to correlate with sulfate reduction rate in cultures exposed to dissolved Hg, while methylation rate was roughly constant for cultures exposed to nanoparticulate HgS. These results indicated a potential threshold of microbial productivity: below this point net MeHg production was limited by microbial activity, regardless of Hg bioavailability. Above this threshold of productivity, Hg speciation became a contributing factor towards net MeHg production.

Fig. 1

Growth of microorganism, as shown by optical density (OD) at 660 nm in mixed anaerobic cultures extracted from MS-1 (A, B) and MS-2 (C, D) sediments, grown under different C-substrate concentrations (0.6 mM, 6 mM and 60 mM), and amended with either dissolved Hg (A, C) or nanoparticulate HgS (B, D). The error bars represent 1 s.d. for triplicate biological samples.

Fig. 2

Net production of methylmercury (filled symbols) and reduction of sulfate (open symbols) in mixed anaerobic cultures from MS-1 (A, B) and MS-2 (C, D) sediments, grown under different C-substrate concentrations, and amended with either 5 nM dissolved Hg (A, C) or 5 nM nanoparticulate HgS (B, D). The error bars represent 1 s.d. for triplicate biological samples. Linear regressions were performed for the 10–64 h data points; the regression parameters are listed in Table S2.

Fig. 3

Relationship between methylmercury concentration and cell density of mixed microbial cultures enriched from MS-1 (A, B) and MS-2 (C, D) sediments grown under different C-substrate concentrations, and amended with either dissolved Hg (A, C) or nanoparticulate HgS (B, D). The error bars represent 1 s.d. for triplicate biological samples. Linear regression parameters are listed in Table S3.

Fig. 4

Net MeHg production rate as a function of sulfate reduction rate calculated between 10 and 64 hours for mixed microbial cultures grown at different C-substrate concentrations. Mixed cultures were enriched from MS-1 (A, B) and MS-2 (C, D) sediments and were amended with either dissolved Hg (A, C) or nanoparticulate HgS (B, D). The error bars represent 1 s.d. for triplicate samples in the test groups. Some errors bars are smaller than the data points. Solid black lines correspond to linear least squares regression of the data.

Fig. 5

Net methylation of mercury (as % of total Hg) as a function of dsrA gene relative abundance for the 10 to 64 h incubation time points for mixed microbial cultures grown at different C-substrate concentrations. Mixed cultures were inoculated from enrichments of MS-1 (A, B) and MS-2 (C, D) sediments and were amended with either dissolved Hg (A, C) or nanoparticulate HgS (B, D). Solid black lines correspond to linear least squares regression of the data.