Phytoplankton could influence the aquatic cycle of mercury directly through uptake and biotic transformations, and indirectly through the excretion of bioligands influencing different abiotic and biotic transformations. Opposite to the substantial work performed with bacteria, the role of phytoplankton in Hg transformations is poorly understood.

The goal of the study is to get insights on the role of phytoplankton natural community in biotic transformations in the freshwater system of an eutrophic lake, Soppensee (Switzerland). Since the biotic transformations are essentially intracellular, the accumulation of inorganic (iHg) and monomethylmercury (MeHg, CH₃Hg⁺), their subcellular distribution, and transformations by the natural community of the lake were studied. Phytoplankton communities were harvested in-situ at 2 different depths with Niskin bottle and concentrated by centrifugation. Then, the re-suspended community is exposed to 1nM of ¹⁹⁹iHg and 100pM of ²⁰¹MeHg for exposure of 24h. The cellular accumulation of Hg species was determined following isolation from the exposure medium by gentle centrifugation. To get information about cellular compartments involved in the possible transformations, the cells were further separated into membranes/organelles and cytosol fractions. Hg analyses were carried out by isotopic dilution analysis gas chromatography coupled to inductively plasma mass spectrometry. The changes in the ratio between the difference of control isotopes concentrations before and after exposure and the measured isotopes concentrations before exposure in the whole cells and the cellular fractions were used to quantify the Hg biotransformations.

Results showed that chemical parameters are similar to the 2 depths and, the phytoplanktonic community is heterogeneous at the first depth and dominated by diatoms at the second depth. Both communities accumulated a significant amount of iHg and MeHg but, the cellular distribution was species- and community-dependant. Under comparable incubation conditions, Hg species were more accumulated in membranes and, results showed that community dominated by diatoms accumulated more Hg in the cytosol and less in membranes than the heterogeneous community. The Hg uptake is almost equal without dependence on Hg species or community. The calculation of biotic transformations did not allow the detection of Hg methylation. The demethylation process was found, higher in cytosol fraction and at the second depth. Statistical analysis showed a dependence on the community with a positive correlation between the demethylation and diatoms group.