UV-B induced mortality and antioxidant enzyme activities in Daphnia magna at different oxygen concentrations and temperatures

Survival of adult Daphnia magna was assessed after acute (<96h) exposure to UV₃₁₂ under various temperatures (6, 12 and 18°C)or oxygen concentrations (5.6, 8.5 and 14.1 mg O₂ l^–1)in the laboratory. The surviving animals were screened for theenzymes catalase (CAT) and glutathione transferase (GST), whichmay protect against UV-induced oxidative damage. In addition,the same two enzymes were assayed in separate experiments afteracute exposure to UV₃₁₂ (6 h, 0.014 mW cm^–2) and the differentlevels of temperature and oxygen. No differences were observedin either CAT or GST activity after exposure to the three levelsof oxygen, but there was a tendency for decreasing specificactivity with decreasing temperature for both enzymes. CAT activitywas not influenced by UV radiation, whereas GST activity displayeda slight increase. Oxygen concentration did not influence survivalduring UV exposure but, contrary to expectations, survival testsat different temperatures clearly showed that reduced temperatureincreased survival. The results indicate that temperature effectsmust be considered when comparing dose effect relationshipsin situ, and suggest that low temperature is not a major causeof UV susceptibility in cold-adapted alpine and Arctic populationsof Daphnia.