Sulfide Controls on Mercury Speciation and Bioavailability to Methylating Bacteria in Sediment Pore Waters

Abstract

A chemical equilibrium model for Hg complexation in sediments with sulfidic pore waters is presented. The purpose of the model was to explain observed relationships between pore water sulfide, dissolved inorganic Hg (HgD), and bulk methylmercury (MeHg) in surficial sediments of two biogeochemically different ecosystems, the Florida Everglades and Patuxent River, MD. The model was constructed to test the hypothesis that the availability of Hg for methylation in sediments is a function of the concentration of neutral dissolved Hg complexes rather than Hg2+ or total HgD. The model included interaction of mercury with solids containing one or two sulfide groups, and it was able to reproduce observed HgD and bulk MeHg trends in the two ecosystems. The model is consistent with HgS0 as the dominant neutral Hg complex and the form of Hg accumulated by methylating bacteria in sulfidic pore waters. The model-estimated decline in HgS0 with increasing sulfide was consistent with the observed decline in bulk sediments MeHg. Since bacterial Hg uptake rate is one of the factors affecting methylation rate, Hg complexation models such as the one presented are helpful in understanding the factors that control MeHg production and accumulation in aquatic ecosystems.

Geochemical and Biological Controls over Methylmercury Production and Degradation in Aquatic Ecosystems

“…A limited number of experiments with SRB support this idea (38). For this reason, diffusion across the cell membrane has been proposed as the important uptake mechanism (30,38,74). This hypothesis is consistent with studies that have demonstrated diffusion of neutral mercury complexes (chloride complexes) across artificial membranes and into diatom cells (75)(76)(77).…”

“…It has been noted in many field studies that the rate of Hg methylation, and the CH 3 Hg concentration in sediments, decrease as the sediment sulfide concentration increases (28,29,36,37,74,78,79). A number of mercury complexes exist in solution in the presence of dissolved sulfide, including HgSO, Hg(SH)t, Hg(SH)+, HgS;Z· and HgHS z "(80-84) and it is possible that inorganic Hg uptake by SRB occurs via diffusion of the dissolved neutral Hg complexes, such as HgS°.…”

“…If so, then the bioavailability of Hg to the bacteria would be a function of sulfide .levels, as this is the ligand controlling Hg speciation in solution in low oxygen zones where SRB are active. It has been shown through chemical complexation modeling that the speciation of Hg tends to shift toward charged complexes as sulfide levels increase (74,84,85), decreasing the fraction of Hg as uncharged complexes, and, as a result. the bioavailability of Hg to methylating bacteria.…”

“…These experiments showed that HgSO and Hg(SHhO, the neutral complexes present under the experimental conditions, had relatively higb K"w's such that they would be taken up at rates more than sufficient to account for methylation in both pure cultures and in the field, based on the estimated permeability through the cell membrane. which is a function of Kow (30,74,86). There is the potential for polysulfide formation in porewaters and the solubility of Hg in the presence of HgS(s) has been shown to be dramatically increased by the complexation of Hg with polysulfide species (87).…”

“…a higher fraction of the Hg diffusing aerasli the ollfer membrane is transferred to the site within 'thecyto(>lll$m where methylation CWl occurcQUlpaIed to other organisms, beeaus:eof th~kinetics of the intracellular reactions. 74,86) cover thespectitiOn ofHg 10 the presence of sulfide but it is known that Hg binds strongly to (jrganic rnatter~and that dissolved organic carbon (DOC) impacts Hg methylation. The impact of organic content on Hg metbylatic)8 appears to be comple;K (16,57,58 suggests lhat a decrease in pH will lead to a decrew;c in methylation rate in sediments because of changes in the concentration of bioavailab1e Hg in porewaters.…”