Antioxidative protection of Peridinium gatunense in Lake Kinneret: Seasonal and daily variation

A comprehensive antioxidative mechanism was found in the freshwater dinoflagellate Peridinium gatunense Lemm. during the spring bloom in Lake Kinneret. Ascorbate runs present throughout the bloom period and was responsible, together with catalase, for the elimination of photosynthetically produced H2O2. As glutathione conc entrations and ascorbate regenerative enzymes were negligible during mid-spring, ascorbate was presumably biosynthesized during the photosynthetically active period. Antioxidative activity increased overall at the end of the spring in conjunction with elevated ambient stress conditions, for example high light. Under such circumstaces, ascorbate was regenerated. Ascorbate levels doubled when cells were exposed to an increase in irradiance fr om 60 to 600 mu mol photons.m(-2). s(-1), and on addition of H2O2, concentrations increased a further 20-fold. Significant antioxidative activity was also noted in the dark, although this was dependent on the presence of H2O2. Diurnal changes in antioxidants and their regenterative enzymes were observed. The activities of monodehydroascorbate reductase, glutathione reductase, and ascorbate concentrations showed ultraradian periodicity and were completely in phase throughout the day/night period. Dehydroascorbate reductase activity and glutathione concentrations were also in phase but showed aperiodic variation, as did ascorbate peroxidase activity Superoxide dismutase and catalase activities were generally out of phase during the 24-h period but did show ultraradian periodicity. Lake samples entrained under constant light revealed an inate 12-h rhythm for catalase activity, during at least 36 h.