LABORATORY SELECTION FOR COPPER TOLERANCE IN SCENEDESMUS-ACUTUS (CHLOROPHYCEAE)

Through repeated subculturing of a Cu-sensitive population (X72) of th e microalga Scenedesmus acutus f. alternans Hortobagyi (Chlorophyceae) in sublethal concentrations of copper (2.4 muM), a Cu-tolerant popula tion (XCu) was derived. Tolerance to copper, measured as yield over 10 days in batch culture with 0.79 muM Cu, was constitutive. Adsorbed co pper as a percentage of total copper was 77, 63, and 40% for a Cu-tole rant field isolate (B-4), XCu, and X72, respectively, during 16 h expo sure to 10 muM Cu. These percentages were positively correlated with t he level of copper tolerance displayed by the algal strains, i.e., B-4 > XCu > X72. Cells of XCu adsorbed twice as much copper per unit surf ace area in comparison with the parent strain (X72 = 13.1 amol . mum-2 ; XCu = 26.0 amol . mum-2). Exclusion of copper from the cell interior due to the high Cu-adsorptive capacity of the cell surface is suggest ed as the primary tolerance mechanism for the XCu strain. No metal cot olerance was demonstrated by XCu when assayed at 0. 85 muM Co or Ni. S maller cell size of the XCu but similar growth rate in comparison with the parent population X72 suggests that selection of a cell type from the original population may have occurred as a result of the copper c hallenge.