Effect of pH on tolerance of Hormidium rivulare to zinc and copper

The toxicity of zinc to a population of Hormidium rivulare isolated from an acid mine drainage was shown to be least at the optimum pH range for the growth of the alga, pH 3.5-4.0; toxicity increases markedly at higher pH values. Calcium clearly antagonizes the toxicity of zinc. Populations of H. rivulare isolated from higher pH values and which are resistant to zinc, are also especially resistant to low pH values, although they are unlikely ever to encounter such values naturally. Nevertheless raised levels of calcium bring about only a slight improvement of growth at very low pH values in the absence of zinc, so the mechanisms of pH and zinc tolerance are not the same. Although the acid stream population grows in the field in an environment with rather similar levels of zinc and copper, copper is less toxic than zinc at pH 3.5, but much more toxic than zinc at pH 6.0.