Mercury is a toxic that was mined in California’s Coast Range, and then used in the Sierra Nevada foothills for extraction of gold. Weathering of abandoned waste rock piles and mines, plus erosion of contaminated microorganisms, transform it into a more toxic form, methyl-mercury. This enters food chains where it bioaccumulates to concentrations that can cause impaired neurological function in a variety of higher organisms (fish, birds, humans). This toxic conversion has, in the scientific literature, been quite dogmatically attributed to activities of sulfate-reducing bacteria. Importantly, recent unpublished results from our laboratory with freshwater sediments show that iron-reducing bacteria can also convert inorganic mercury into methyl mercury, and do so at rates equivalent to those of sulfate-reducing bacteria. Due to California’s high concentration of iron in coastal sediments, we propose to test the hypothesis that iron-reducing bacteria also contribute significantly to the overall production of methyl mercury in marine sediments. We will do this by exploring the linkage between methyl mercury production and the activity of iron-reducing bacteria. In mercury-contaminated marine sediments, we will measure rates of methyl mercury production along with signature activities of different bacterial metabolic types, i.e. sulfate reduction and iron reduction. A second approach involves culturing evolutionarily diverse iron-reducing bacteria from contaminated marine sediments to compare (vs. sulfate-reducers) their relative abilities to methylate mercury. Understanding, based on potential in pure cultures and activities in contaminated sediments, which bacterial types contribute significantly to mercury methylation in coastal sediments will aid in modeling of marine methyl mercury problems, and in creating remediation strategies for impacted sites. This project also has implications for certain commercial fisheries that are impacted by bioaccumulation of methyl mercury.