Reservoirs provide critical drinking water supply, flood control, and renewable energy. However, they also have significant impacts on biogeochemical cycles, in part due to thermal and redox stratification. In this study, Hg speciation and redox-active compounds were measured throughout a hydroelectric reservoir in Idaho with the goal to identify the location where and redox conditions under which methylmercury is produced. Redox profiles, combined with genome-resolved metagenomics focused on hgcA-carrying microbes, indicated that methylmercury production occurs in this system under nitrate- or manganese-reducing conditions, which were previously thought to preclude Hg-methylation. This work expands the known conditions conducive to producing methylmercury and suggests that the Hg-methylation mitigation efforts by nitrate or manganese amendment may be unsuccessful in some locations.

DOI: 10.1038/s41396-023-01482-1.