Effect of chemical speciation on inorganic and methyl mercury uptake by a freshwater alga (Chlorella ellipsoidea)

Mercury contamination of biota is most severe in aquatic ecosystems. Algae concentrate aqueous mercury to high levels, and control the amount entering the food web. In this study, the factors controlling mercury uptake by algae were investigated in laboratory experiments with an axenic culture of Chlorella ellipsoidea. Techniques were developed for measuring inorganic mercury (InHg) and methylmercury (MeHg) uptake rates using environmentally-realistic mercury levels. An artificial lakewater was formulated to be the base experimental medium. To determine which complexes were being transported by algae, the medium chemistry was manipulated to form different InHg and MeHg complexes.; Uptake of both neutral chloride and neutral hydroxide mercury complexes into algae was demonstrated for the first time. Uptake was consistent with the diffusion mechanism, leading to membrane permeability estimates of 5.2 × 10−5 and 7.7 × 10−5 cm s−1 for Hg(OH)₂ and HgCl ₂, and 2.8 × 10−4 and 3.2 × 10 −4 cm s−1 for MeHgOH and MeHgCl, respectively. Permeability estimates could explain phytoplankton-mercury levels reported in the literature, together with the chemistries of Canadian Shield lakes. Experiments with sulfide and organic ligands had weak results due to a low signal to noise ratio. At low sulfide concentrations, the formation of HgS _(s) was physically demonstrated in abiotic experiments. No InHg uptake was observed at these conditions, giving no evidence for a dissolved HgS _(aq) complex. InHg uptake was observed at high sulfide concentrations, where InHg was calculated to be in the form of HgS₂2− and HgS₂H− complexes, which could only be taken up by facilitated or active transport. The neutral (MeHg) ₂S complex, formed at low sulfide levels, was taken up by algae, while the neutral MeHgSH complex formed at high sulfide levels was not. In experiments with organic compounds, sulfhydryl-rich cysteine and glutathione complexed InHg and MeHg and limited uptake. Sulfhydryl-free EDTA and oxidized glutathione did not limit uptake. Surface adsorption of InHg and MeHg to algae was observed under many but not all conditions, suggesting that surface binding sites were specific to certain complexes. Experiments were not designed to test for facilitated or active transport, but uptake results were also consistent with these mechanisms.