Methylmercury Bioaccumulation, Transformation, and Trophic Transfer in Marine Plankton Assemblages

Few studies have quantified the bioconcentration of methylmercury (MeHg) in marine phytoplankton from seawater, even though this is by far the largest bioaccumulation step in aquatic organisms. Aquatic animals acquire MeHg mainly from dietary exposure and it is important to evaluate the bioaccumulation of this compound in planktonic organisms that form the base of marine food webs. We used a gamma-emitting radioisotope, ²⁰³Hg, to assess the rate and extent of MeHg uptake in marine diatoms, dinoflagellates, coccolithophores, cryptophytes chlorophytes, and cyanobacteria held in unialgal cultures under varying temperature, light and nutrient conditions. For experimental conditions in which cells were exposed to MeHg at 300 pM, the uptake rates in all species ranged from 0.001 to 0.034 atto-mol MeHg µm^-2 cell surface h^-1 and reached steady state within 2 d. Volume concentration factors (VCFs) ranged from 0.3 to 40 x 10⁵ for the different species. Temperature, light and nutrient conditions had no direct effect on cellular MeHg uptake but ultimately affected growth of the cells, resulting in greater suspended particulate matter and associated MeHg. VCFs strongly correlated with cell surface area to volume ratios in all species. Nearly 40 % of the MeHg was released into the air from coccolithophore cultures within 4 d, but <10 % from other algal cultures. Assimilation efficiencies of MeHg from different phytoplankton diets in a marine copepod (Acartia tonsa) ranged from 74 to 92%, directly proportional to the cytoplasmic partitioning of MeHg in the phytoplankton cells. MeHg uptake in copepods from the aqueous phase was low and modeling shows that nearly all the MeHg acquired by this zooplankter is from diet. Herbivorous zooplankton can be an important link from phytoplankton at the base of the food web to fish higher in the food chain.