Copper and lead internalisation by freshwater microalgae at different carbonate concentrations

The possible contribution of metal-carbonato complexes to metal uptake or toxicity has been mentioned several times in the literature, often in studies where dissolved inorganic carbon (DIC) concentrations and pH were varied together, but a thorough study of the effect on DIC on metal bioavailability to aquatic organisms has not been done. By using closed systems - allowing changes in DIC concentrations at fixed pH - and ion selective electrodes to determine free metal ion concentrations, we show that lead internalisation by the unicellular alga Chlamydomonas reinhardtii in the presence of high DIC concentrations is higher than predicted by the free Pb2+ ion concentration at bulk [Pb2+] lower than 50 nM, but not at higher [Pb2+]. This effect is not observed for another microalga, Chlorella vulgaris, which shows a lower rate of Pb internalisation. Copper internalisation by C. reinhardtii seems also to be slightly higher than predicted on the basis of free Cu2+ at low (20 nM) bulk Cu2+ concentrations but not at higher ones. The possibility that Pb (and Cu) internalisation by C. reinhardtii is partially limited by diffusion from the bulk solution to the algal surface is identified and discussed as a possible explanation for these results.