Methylmercury (MeHg) is the form of mercury that enters the food web in the aquatic environment. Anaerobic microorganisms such as sulfate- and iron-reducing bacteria are primarily responsible for converting inorganic forms of Hg to MeHg. Thus, the production of MeHg by anaerobic bacteria and the bioavailability of inorganic mercury to these microbes are critical steps towards MeHg bioaccumulation in fish and other high trophic-level species.
In this research we are investigating the relationship between Hg geochemistry in contaminated sediments and microbial methylation potential, a relationship that remains poorly understood. The ‘age’ of mercury in sediments is a factor that reduces bioavailability over time. A known aging process for mercury in sediments is the precipitation of sulfide minerals (i.e., HgS(s) metacinnabar). Our previous research has shown that nanoparticles of HgS can form as stable intermediates of slow precipitation reactions. These nanoparticles, especially amorphous phases, are inherently more soluble than bulk minerals and can be more bioavailable than larger particles.
The objective of this work is to investigate geochemical processes that control the bioavailability of mercury and identify methods to assess the methylation potential of mercury in contaminated sediments. In particular, we are quantifying the microbial methylation potential of nanoparticulate HgS in relation to bulk scale HgS and dissolved HgS species. We hypothesize that nanoscale HgS is more bioavailable than bulk scale HgS(s) due to differences in solubility and also due to the ability for nanoparticles to collect near the cell surfaces of methylating bacteria. The project seeks to establish a premise that links the "age" and chemical form of mercury in sediment pore water to the rate of MeHg formation. This information will be used to improve our understanding of mercury methylation potential in aquatic ecosystems and to develop remediation strategies for contaminated settings.
Mercury methylation potential: ESPI (2015)
The research is currently supported through grants from the Department of Energy and the Superfund Research Program with the National Institute of Environmental Health Sciences.