Geochemical and Biological Controls over Methylmercury Production and Degradation in Aquatic Ecosystems

It is the goal of this paper to discuss the more salient recent advances in the understanding of the controls of net CH3Hg formation in natural systems. The discussion highlights the gaps in knowledge and the areas where progress in understanding has occurred. In particular, this chapter focuses on recent developments in Hg bioavailability and uptake by methylating bacteria, on the competing roles of sulfate and sulfide in the control of methylation, and in pathways for demethylation. The role of sulfide in influencing methylation is discussed in detail. In addition, the impact of other environmental variables such as pH, dissolved organic carbon and temperature on mercury methylation are discussed. Lastly, we provide a synthesis of the variability in the methylation response to Hg inputs across ecosystems. We suggest that although methylation is a function of Hg concentration, the range of methylation rates across ecosystems is larger than the range in Hg deposition rates. Overall, we conclude that factors in addition to the amount Hg deposition play a large role in controlling CH3Hg production and bioaccumulation in aquatic ecosystems.