Thiol-Facilitated Cell Export and Desorption of Methylmercury by Anaerobic Bacteria

Abstract

Methylmercury (MeHg) toxin, formed by anaerobic bacteria, is rapidly excreted from cells, but the mechanism of this process is unclear. We studied the factors affecting MeHg export and its distribution in cells, on cell surfaces, and in solution by two known mercury methylators, Geobacter sulfurreducens PCA and Desulfovibrio desulfuricans ND132. Thiols, such as cysteine, were found to greatly facilitate desorption and export of MeHg, particularly by PCA cells. In cysteine-free assays (4 h), less than 10% of the synthesized MeHg was found in solution and greater than 90% was associated with PCA, of which about 73% was sorbed on the cell surface and 19% remained inside the cells. In comparison, 77% of MeHg was in solution, leaving about 13% of MeHg sorbed and about 10% inside the ND132 cells. Our results demonstrate that MeHg export is bacteria specific, time dependent, and influenced by thiols, implicating important roles of ligands, such as natural organic matter, in MeHg production and mobilization in th...