Five species of unicellular green algae were exposed to a broad range of Cu 2ϩ and Zn 2ϩ concentrations to examine the relationship between the free Cu 2ϩ and Zn 2ϩ 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 Cu 2ϩ and Zn 2ϩ concentrations (from 10 Ϫ15 to 10 Ϫ7 and from 10 Ϫ12 to 10 Ϫ5.5 M, respectively). Chlamydomonas reinhardtii reacted more sensitively toward copper, optimal growth was achieved only at a pCu (ϭ Ϫlog[Cu 2ϩ ]) around 11, whereas growth was optimal over a broad range of free Zn 2ϩ concentrations (from 10 Ϫ12 to 10 Ϫ6 M). The optimal range for growth of Chlorella fusca occurred over about three orders of magnitude of the free Cu 2ϩ concentration (from 10 Ϫ13 to 10 Ϫ10 M). Chlamydomonas cultures isolated from Lake Constance tolerated Cu 2ϩ concentrations over seven orders of magnitude (pCu ϭ 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 Cu (11.06) Ͼ log K Zn (6.49) (pH ϭ 7.9). These freshwater algae tolerate higher Cu 2ϩ and Zn 2ϩ 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.
Keywords-Copper
Zinc Algae Limitation Freshwater
MATERIALS AND METHODS
Preparation of culture medium
The growth medium used in these experiments was based on the Organisation for Economic and Co-operative Development (OECD) guideline medium for testing chemicals . The medium contained 2.8 ϫ 10 Ϫ4 M NaNO 3 , 1.18 ϫ 10 Ϫ5 M KH 2 PO 4 when using a 1 ϫ 10 Ϫ3 M N-2-hydroxyethylpipera-Effects of copper and zinc on freshwater algae