Microbes in the open ocean convert inorganic mercury into methylmercury, a toxic form that can be taken up by fish, reports a paper published online this week in Nature Geoscience. The findings suggest that open-ocean microbes, and not just microbes found in coastal sediments, contribute to the mercury contamination of the marine food web.

Joel Blum and colleagues examined the isotopic signature of mercury in nine species of fish sampled from the North Pacific Ocean, which feed at different depths. They find that isotopic patterns found in these fish can only be explained if 60-80% of the methylmercury consumed is produced by microbes below the ocean surface mixed layer, rather than sourced from surface waters.