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

Through repeated subculturing of a Cu-sensitive population (X72) of the microalga Scenedesmus acutus f. alternans Hortobagyi (Chlorophyceae) in sublethal concentrations of copper (2.4 muM), a Cu-tolerant population (XCu) was derived. Tolerance to copper, measured as yield over 10 days in batch culture with 0.79 muM Cu, was constitutive. Adsorbed copper as a percentage of total copper was 77, 63, and 40% for a Cu-tolerant field isolate (B-4), XCu, and X72, respectively, during 16 h exposure to 10 muM Cu. These percentages were positively correlated with the 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 surface 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 suggested as the primary tolerance mechanism for the XCu strain. No metal cotolerance was demonstrated by XCu when assayed at 0. 85 muM Co or Ni. Smaller 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 challenge.