OXIDATIVE STRESS AND ANTIOXIDANT CONTENT IN CHLORELLA-VULGARIS AFTER EXPOSURE TO ULTRAVIOLET-B RADIATION

Growth of Chlorella vulgaris was measured in cultures irradiated with 0, 0.8, 2.0 and 4.4 kJ m(-2) UV-B. Growth expressed as chlorophyll con tent, declined significantly with increased UV-B dose. Ultraviolet-B i rradiated cultures in log phase of growth showed a 284% increase in ox ygen radical generation and a 145% increase in lipid peroxidation comp ared with unirradiated cultures, whereas cultures in the stationary gr owth phase showed no significant changes in these parameters. The acti vities of superoxide dismutase and catalase increased by 40 and 500%, respectively, after exposure to a UV-B dose of 4.4 kJ m(-2). Contents of the lipophilic antioxidants a-tocopherol and beta-carotene increase d by 180 and 63 amol cell(-1) respectively, between log and stationary phases in unirradiated cultures; but in UV-B-irradiated cultures thes e increases were significantly depressed. Photoreducing capacities of chloroplasts were decreased following UV-B irradiation of both isolate d chloroplasts and those isolated from irradiated algae. Cells exposed to UV-B exhibited increased size and starch accumulation. These resul ts suggest that oxidative stress conditions related to UV-B exposure t rigger an antioxidant response that includes an increase in the activi ty of the antioxidant enzymes (superoxide dismutase and catalase).