Prey engulfment as the dominant pathway of methylmercury uptake in a heterotrophic dinoflagellate

Methylmercury (MeHg) uptake by the dinoflagellate Oxyrrhis marina was investigated to determine if uptake from the dissolved phase or prey engulfment contribute more to cellular MeHg accumulation during 8-h exposures. Effects of varying environmental factors were examined, including dissolved organic carbon (DOC) concentration, salinity, and temperature. The overarching hypothesis was that dinoflagellates utilizing heterotrophy would accumulate higher MeHg due to prey engulfment than from the uptake of MeHg-DOC complexes. Furthermore, it was hypothesized that MeHg uptake would be highest at the salinity optimal for O. marina and dependent on temperature. Laboratory experiments were carried out to test these hypotheses by varying: 1.) dissolved organic matter concentration and prey presence (DOC: 130 vs. 210 μM vs. Isochrysis galbana as prey at 5 × 10⁵ cells mL⁻¹); 2.) salinity (11, 17, and 34); and 3.) temperature (12, 15, and 22 °C), representing possible conditions in a temperate estuary. Results show that MeHg uptake was highest in the presence of phytoplankton prey, and there was no significant difference in uptake between DOC concentrations. Among salinity treatments, MeHg uptake from water was significantly lower at 11, compared to 17 and 34. Contrary to what was hypothesized, temperature had no effect on MeHg uptake. Overall, these studies support the hypothesis that the consumption of phytoplankton prey by this dinoflagellate increases its MeHg burden, and provide more detail of the contrasting factors influencing MeHg assimilation by heterotrophic protists in the presence of DOC.