Effects of cadmium, cobalt, copper, and nickel on growth of the green alga Chlamydomonas reinhardtii: the influences of the cell wall and pH

Comparative studies of the toxicity of Cd, Co, Cu, and Ni to walled (UTCC 11) and wall-less (UTCC 12) strains of Chlamydomonas reinhardtii were made in order to test the hypothesis that the cell wall affords some protection against metal toxicity. The wall-less strain was consistently more sensitive than the walled strain to all four metals, indicating that the cell wall plays a role in conferring metal tolerance. Between-strain differences were most striking for Cu and for Co. The effect of hydrogen ion concentration (pH 5 and 6.8) on metal toxicity was also determined for the two strains. Having established that both strains grew equally well at pH 5 or 7 in the absence of added metal, it was necessary to correct for the changes in metal speciation due to pH in the medium used for the tests. Speciation of each metal at each pH was determined by mathematical (GEOCHEM) modeling of the medium and the calculated free (ionic) metal concentration was used to express toxicity. In addition, the concentration of ionic metal that reduced final cell density to 30% of that in control solution (EC sub(30)) was used as an indicator of relative metal toxicity. For both strains, all metals were less toxic at pH 5 than at pH 7, supporting previous observations. The results are discussed in terms of the possible mechanisms by which the cell wall could protect the cell from metal toxicity, and the relevance of the results to more general considerations of metal tolerance mechanisms in plants.