Assessing the Effects of Anthropogenic and Biogenic Ligands on Mercury Bioavailability

Mercury (Hg), released into the environment primarily by the combustion of fossil fuels, is a global pollutant that travels through the atmosphere before depositing into surface waters. When internalized by anaerobic bacteria living in aquatic ecosystems, Hg can be converted into monomethylmercury (MeHg), a potent neurotoxin. The Free Ion Activity Model (FIAM), which predicts trace-metal bioavailability, assumes that the free metal ion is the only bioavailable form. However, environmental Hg is almost always found complexed with a ligand, and thus it is necessary to study the bioavailability of Hg-ligand complexes. In this investigation, an Escherichia coli biosensor (containing a chromosomally inserted merR::luxCDABE fusion) that emits light in proportion to concentration of intracellular Hg was used to directly assess the bioavailability of various Hg-ligand complexes. Thiol-containing, anthropogenic ligands 2-mercaptopropionic acid, 3-mercaptopropionic acid, and thioacetic acid, as well as biogenic ligands cysteine and glutathione were used. Each ligand tested, except cysteine, was shown to decrease Hg bioavailability. The reason for cysteine’s promotion of Hg biouptake is unclear. However, we can generally conclude that Hg-ligand complexation prevents ligand-exchange reactions from occurring and that thiol-containing ligands have a higher affinity for Hg than cell-surface thiol groups