Award: OCE-2023031

The goal of this project was to gain a greater understanding of the toxic metal mercury (Hg), present at low concentrations in seawater but abundant enough to represent a threat to human health. That threat arises as a result of bioaccumulation of Hg inside of marine organisms and biomagnification of that Hg up through food chains, resulting in Hg concentrations in fish that can be a million or more times higher than in the water. This bioaccumulation/biomagnification would not occur if it were not for the conversion of Hg in seawater into methylmercury as most of the mercury entering the ocean is not methylated. The conversion of mercury into methylmercury is only partially understood and this project was designed to shed light on that conversion.

Methylated mercury can be found in the ocean in two forms: monomethylmercury (MMHg) and dimethylmercury (DMHg). The MMHg form is the one thought to bioaccumulate in seafood and was once thought to comprise the majority of the methylated Hg in the ocean. During this project, we demonstrated that the DMHg form was not being measured correctly and that in fact in our study region (the California Current of southern and central California coastline) it was often the majority. Furthermore, we demonstrated that much of the MMHg present was likely the product of demethylation reactions that can convert DMHg into MMHg.

What then is the source of DMHHg? We still do not know but found that sinking particles in the ocean are clearly a source of methylated Hg in the California Current. Our studies continue including examining the effect that bacteria might play as well as the impact of local oceanographic phenomenon like upwelling.

The broader impacts of this work include learning more about a pollutant to which an alarming number of American women of child-bearing age and children are regularly exposed to by eating seafood. Seafood is otherwise a very healthy food and an important industry for many coastal communities. Therefore, our work is part of a long-term effort to safeguard our food and economy.

 

Last Modified: 01/31/2026
Modified by: Carl Lamborg