Effects of microcystin-LR on the metal bioaccumulation and toxicity in Chlamydomonas reinhardtii.

Microcystin-LR (MC-LR) is one of the most notorious toxins liberated from cyanobacteria in eutrophicated freshwater ecosystems. Its effects on the bioaccumulation and toxicity of Cd(2+), CrO(4)(2-), Cu(2+), and Zn(2+) in a green alga Chlamydomonas reinhardtii were investigated in the present study. The metal bioaccumulation in the alga was unaffected by MC-LR. The surface-adsorbed and intracellular metal concentrations in the treatments with and without the addition of MC-LR could be well simulated by a single Freundlich isotherm for each metal with their accumulation ability following the order of Cu(2+) > Cd(2+) > Zn(2+) > CrO(4)(2-). The bioavailable metal concentrations measured by diffusion gradients in thin-films remained unchanged when MC-LR was applied. Accordingly, the growth of C. reinhardtii was similarly inhibited at the same metal concentration regardless of the addition of MC-LR. The metal toxicity could also be well delineated with the classic free ion activity and biotic ligand models. However, the intracellular metal concentration was found to have the best predictability suggesting its more direct relationship with metal toxicity. Metal exposure induced the accumulation of MC-LR in the alga, which was leveled off at high metal levels. The underlying uptake mechanisms need to be further examined.