Facilitated cell export and desorption of methylmercury by anaerobic bacteria

Abstract:

Neurotoxic methylmercury (MeHg), formed by certain anaerobic bacteria, is shown to be rapidly excreted from the cell, but the mechanism of this process is unclear. Using both G. sulfurreducens PCA and D. desulfuricans ND132 strains, we investigated the factors affecting export and distribution of MeHg in mercury [Hg(II)] methylation as well as MeHg sorption and desorption assays. Thiols, such as cysteine, were found to greatly facilitate desorption and export of MeHg, particularly by G. sulfurreducens PCA cells. In short-term cysteine-free assays, we found that >90% of the synthesized MeHg was associated with PCA, among which ~73% was sorbed on the cell surface and 19% remained inside the cells, leaving only a small fraction in the phosphate buffered solution. However, MeHg export by PCA increased with increasing cysteine concentrations (0.05–50 mM), and nearly 100% of the MeHg was in solution in the presence of 50 mM cysteine. In comparison, ND132 cells were much more efficient than PCA in producing and exporting MeHg. In the absence of cysteine, a majority of the MeHg (~70%) was exported in 4 h, leaving about 20% of the MeHg sorbed on the surface and 10% inside the cells. When MeHg was directly added to the cell suspensions, ND132 adsorbed much lower MeHg than PCA cells; however, ND132 cells took up more MeHg (20%) inside cells than PCA did (8%). Taken together, our results demonstrate that MeHg export efficiency is bacteria strain-specific and is influenced by the ligand concentration and complexation, which could be important in facilitating MeHg synthesis and bioavailability in anoxic water and sediments.