Mercury (Hg) in seafood is a major food safety concern, as the neurotoxicant monomethylmercury (MMHg) bioaccumulates in fish and shellfish consumed by people worldwide. In the United States, much of the seafood supply is imported from regions with high Hg emissions, increasing the risk of MMHg exposure for consumers. Elevated MMHg intake is linked to health concerns, including cardiovascular disease and developmental impacts in children, with estimated economic losses exceeding $8 billion annually in the United States. To protect food security and public health, it is critical to understand how Hg is transformed into MMHg in the ocean and enters the marine food web. While most studies focus MMHg formation from inorganic Hg, another related compound, dimethylmercury (DMHg), can degrade and contribute to MMHg sources in seafood, but this remains poorly understood. Recent work has shown that DMHg is produced in coastal surface waters during algal blooms, suggesting that marine microbes play a role in this process. This research will investigate how these microbes produce DMHg and how this process affects the supply of MMHg to food webs. By advancing our understanding of the marine Hg cycle, this work will support seafood safety and public health efforts. This project also supports training for a postdoctoral fellow, building future leadership in environmental health and food security research. This project investigates the microbial mechanisms of DMHg formation in marine envi