Effects Of Microcystin-LR On The Bioavailability Of Trace Metals To A Freshwater Green Alga Chlamydomonas Reinhardtii

The hepatotoxin microcystins (MCs), as cyclic heptapeptides, are produced by various cyanobacteria with intermediate binding affinity to trace metals such as Cu2+ and Zn2+.In the present study, the effects of MC-LR (1000μg/l) on the bioavailability of Cd2+, Cu2+, Zn2+ and Cr2O72- to a green alga Chlamydomonas reinhardtii were investigated. Different types of metal concentrations, including the total dissolved, the cell surface adsorbed and intracellular metal concentrations, were quantified. The free metal ion concentrations were also calculated. Their correlation with the changes of cell growth was then examined to elicit whether the presence of MC-LR could bring about any changes of metal biovailability and further their toxicity. Furthermore, the DGT (Diffusive gradients in thin-films) technique, as a method to operationally quantify the biovailable metals, was adopted to see whether there's any variation of the amount of metals accumulated in the ion-exchange resin of DGT with and without the addition of MC-LR.MC-LR at the concentration used in the present study is not toxic to C. reinhardtii. However, no toxicity difference was observed between the treatments with or without the addition of MC-LR no matter which type of metal concentration was applied. As the MC-LR concentration was kept relatively constant during the experimental period, the non-significant effects of MC-LR may be due to its low concentration as compared with those of metals. Although the concentrations of MC-LR and various metals are of the same order of magnitude when DGT was used to test the metal bioavailability, there was no considerable effects from MC-LR either, suggesting that the intermediate binding affinity of MC-LR is not high enough to significantly influence the metal speciation. According to the free metal ion concentration based median inhibition concentrations, the toxicity of the four trace metals follows the order of Cu2+＞Cd2+＞Cr2O72-＞Zn2+, which changed to Cr2O72-＞Cu+＞Cd2+＞Zn2+ when the intracellular metal concentration based median inhibition concentration was used. The toxicity of Cr2O72- in the anion form may thus be underestimated with the free ion activity model. Although the molecule of MC-LR (about 1000Da) is relatively big, a considerable amount was still found in the algal cells with a higher concentration observed at high metal levels, possibly as a result of the increased membrane permeability by metal exposure.