Effects of free Cu2+ and Zn2+ ions on growth and metal accumulation in freshwater algae

Five species of unicellular green algae were exposed to a broad range of Cu2+ and Zn2+ concentrations to examine the relationship between the free Cu2+ and Zn2+ ion concentrations and algal growth at metal concentrations relevant for freshwater phytoplankton. We estimated extra- and intracellular metal concentrations and characterized the adsorption of copper and zinc on algal surfaces. The optimal growth rate of Scenedesmus subspicatus occurred in a broad range of Cu2+ and Zn2+ concentrations (from 10(-15) to 10(-7) and from 10(-12) to 10(-55) M, respectively). Chlamydomonas reinhardtii reacted more sensitively toward copper, optimal growth was achieved only at a pCu (= -log[Cu2+]) around 11, whereas growth was optimal over a broad range of free Zn2+ concentrations (from 10(-12) to 10(-6) M). The optimal range for growth of Chlorella fusca occurred over about three orders of magnitude of the free Cu2+ concentration (from 10(-13) to 10(-10) M). Chlamydomonas cultures isolated from Lake Constance tolerated Cu2+ concentrations over seven orders of magnitude (pCu = 7 to 14). The growth of algae showed a high tolerance toward high intracellular copper and zinc concentrations. This suggests that the cells may immobilize the metals intracellularly. The affinity of copper for algal surfaces is higher than that of zinc in the experimental concentration range: adsorption constants log K-Ca (11.06) > log K-Zn (6.49) (pH = 7.9). These freshwater algae tolerate higher Cu2+ and Zn2+ concentrations than marine algal species. In comparison to our lake data, the results obtained from the culture algae indicate a possible role for copper as a limiting factor for certain algal species in eutrophic lakes.