SEASONAL AND ENVIRONMENTAL-INFLUENCES ON ANTIOXIDATIVE PROTECTION IN PERIDINIUM GATUNENSE IN LAKE KINNERET

As the only freshwater lake in Israel, Lake Kinneret serves a number o f important functions which directly rely upon the viability of the wa ter. The annual outbreak of a dinoflagellate bloom strictly governs th e nature of the macro and micro food web and ultimately determines wat er quality. The freshwater dinoflagellate Peridinium gatunense is subj ect to a wide range of environmental stresses through-out the spring b loom period. It was confirmed that SOD played an important antioxidati ve maintainance role throughout the bloom, especially during periods o f relatively high photosynthetic activity (820 mg C m(-2) day(-1)), wh en activity reached similar to 500 Units SOD mg protein(-1). In additi on, high light stress (>300 mu mol photons m(-2) s(-1)) induced SOD ac tivity, despite the low dissolved inorganic carbon (DIC) concentration s at the end of the bloom (1.3 mM). Catalase activity was only signifi cant at the end of the bloom, peaking at 120 mu mol O-2 mg protein(-1) min(-1) when induced by photorespiratory activity. A series of experi ments with Peridinium cultures showed that 2 x 10(-4) M ascorbate inhi bited catalase activity >50% within 15 min incubation, both in vivo an d in vitro. It is suggested that the high concentrations of ascorbate, found previously in Peridinium during early and mid-bloom (0.2-1.6 mM ), not only eliminate H2O2 build-up, but also prevent (directly or ind irectly) the induction of catalase.