ANTIOXIDATIVE PROTECTION OF PERIDINIUM-GATUNENSE IN LAKE KINNERET - SEASONAL AND DAILY VARIATION

A comprehensive antioxidative mechanism was found in the freshwater di noflagellate Peridinium gatunense Lemm. during the spring bloom in Lak e Kinneret. Ascorbate runs present throughout the bloom period and was responsible, together with catalase, for the elimination of photosynt hetically 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 l evels 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, con centrations increased a further 20-fold. Significant antioxidative act ivity was also noted in the dark, although this was dependent on the p resence of H2O2. Diurnal changes in antioxidants and their regenterati ve enzymes were observed. The activities of monodehydroascorbate reduc tase, glutathione reductase, and ascorbate concentrations showed ultra radian periodicity and were completely in phase throughout the day/nig ht period. Dehydroascorbate reductase activity and glutathione concent rations were also in phase but showed aperiodic variation, as did asco rbate peroxidase activity Superoxide dismutase and catalase activities were generally out of phase during the 24-h period but did show ultra radian periodicity. Lake samples entrained under constant light reveal ed an inate 12-h rhythm for catalase activity, during at least 36 h.