A chemical equilibrium model for Hg complexation in sediments with sulfidic pore waters was developed to explain observed relationships between sulfide, dissolved inorganic Hg (HgD), and bulk methylmercury (MeHg). The model was constructed to test the hypothesis that the availability of Hg for methylation is a function of the concentration of the neutral dissolved Hg complex, HgS(aq). The model included sorption of Hg onto solids, and it was able to reproduce observed HgD trends in two ecosystems. The model was consistent with HgS(aq) as the dominant neutral Hg complex accumulated by sulfate reducing bacteria (SRB) prior to methylation.

To support the model results, changes in the octanol-water partitioning (Dow) of HgD across a sulfide gradient were investigated. Dow decreased with increasing sulfide, which is consistent with the model-predicted decline in HgS(aq). This neutral complex is replaced by charge disulfide complexes at high sulfide concentrations, hence the permeability of Hg to cell membranes declines. These results suggest that decreased passive uptake of Hg at high sulfide concentrations is the cause of the inverse relationship between sulfide and MeHg observed in aquatic sediments.

A competitive ligand approach was applied to quantify conditional stability constants for Hg complexes with DOC isolates collected from Florida Everglades surface waters. Measured constants were similar to those previously published for Hg binding to low molecular weight thiols. Speciation calculations indicate that at the DOC concentrations frequently measured in Everglades, sub-nanomolar sulfide concentrations would be expected to out-compete DOC for Hg binding.

A number of methylation assays using the SRB Desulfobulbus propionicus were carried out in order to better understand the dynamics of Hg methylation by pure cultures. These experiments utilized a dissolved spike as the Hg(II) source, and it was determined that this type of Hg addition does not adequately reflect physico-chemical conditions of aquatic sediments. Finally, the effect of sulfide on Hg methylation in D. propionicus cultures was investigated. A solid-phase source of Hg was used to better simulate the controls on Hg partitioning between solid and dissolved phases found in natural sediments. Results were consistent with HgS(aq) as the predominant complex available for methylation. Taken together, all of these results support the hypothesis that SRB take up Hg by passive diffusion and that the sulfide speciation of HgD controls uptake and subsequent methylation of Hg.