Species-specific isotope tracers to study the accumulation and biotransformation of mixtures of inorganic and methyl mercury by the microalga Chlamydomonas reinhardtii

• IHg and CH3Hg uptake by algae can be determined at environmental concentrations.
• Transformation and uptake were determined using species-specific stable isotopes.
• CH3Hg demethylation was detected in axenic cultures of Chlamydomonas reinhardtii.

The present study demonstrates that species-specific isotope tracing is an useful tool to precisely measure Hg accumulation and transformations capabilities of living organisms at concentrations naturally encountered in the environment. To that end, a phytoplanktonic green alga Chlamydomonas reinhardtii Dangeard (Chlamydomonadales, Chlorophyceae) was exposed to mixtures of 199-isotopically enriched inorganic mercury (199IHg) and of 201-isotopically enriched monomethylmercury (201CH3Hg) at a concentration range between less than 1 pM to 4 nM. Additionally, one exposure concentration of both mercury species was also studied separately to evaluate possible interactive effects. No difference in the intracellular contents was observed for algae exposed to 199IHg and 201CH3Hg alone or in their mixture, suggesting similar accumulation capacity for both species at the studied concentrations. Demethylation of 201CH3Hg was observed at the highest exposure concentrations, whereas no methylation was detected.