276 - Influence of Temperature, Salinity, and DOC on Methylmercury Uptake into a Heterotrophic Marine Dinoflagellate

Methylmercury (MeHg) enters the marine food chain through active and passive uptake into plankton. Uptake can be affected by various environmental parameters; notably dissolved organic carbon (DOC), salinity, temperature, and cell size. Past MeHg uptake experiments have been mostly carried out using autotrophs, with few using heterotrophs such as dinoflagellates, which are widely abundant in the marine environment and play a significant role in energy transfer to higher trophic levels. Results of a prior study (Lee and Fisher, 2016) showed higher than expected MeHg uptake by a dinoflagellate Prorocentrum minimum, so we hypothesized that the enhanced MeHg uptake in dinoflagellates is related to their mixo- and heterotrophic energy acquisition modes. While MeHg uptake by autotrophic phytoplankton is usually inhibited by DOC, dinoflagellates have the ability to engulf DOC as an energy source. Three laboratory experiments were carried out to examine how MeHg uptake by the heterotrophic marine dinoflagellate Oxyrrhis marina is influenced by: 1) low DOC (130 µM) vs. high DOC (210 µM) vs. an added trophic step; 2) salinity; and 3) temperature. Through these experiments it was found that volume concentration factors (VCFs) for MeHg uptake were lower at the high DOC concentration (1.9x10^4) compared to the low (3.5x10^4), but an added trophic step increased uptake to VCFs around 6x10^4 regardless of DOC treatment. This suggests that DOC inhibits MeHg uptake for O. marina and the engulfment of DOC-bound MeHg is not a significant pathway. Uptake was higher at medium (17 psu; 1.7x10^5) and high (43 psu; 2.4x10^5) salinity compared to low salinity (11 psu; 6.0x10^4). This follows expected trends seen in the literature from prior uptake experiments. Uptake did not vary significantly between temperatures (12, 15, and 22 °C) (9.7x10^4, 9.7x10^4, and 1.1x10^5 respectively), suggesting dominantly passive MeHg uptake.

Authors: Patricia Myer ,Robert Mason, Zofia Baumann