Mercury (Hg) is mobilized from the Earth’s crust and accumulates as monomethylmercury (MMeHg) in aquatic food webs, thereby posing a threat to human and wildlife health. This has motivated over 50 years of research on Hg biogeochemistry. Still, our understanding of the processes controlling the amount of MMeHg available for bioaccumulation remains limited. The understanding of Hg cycling in marine environments is particularly incomplete even though marine harvested food is the primary MMeHg exposure route for most human populations. The overall aim of this research is to close two of the most important knowledge gaps regarding Hg cycling in marine systems; by what pathways is dimethylmercury (DMeHg) formed and degraded in marine waters, and how does these processes impact the amount of Hg available for bioaccumulation (i.e. MMeHg). These results will be critical for accurate predictions of how environmental changes may alter the biogeochemical cycle of Hg in marine systems and impact the environmental and health risks of Hg.