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 of important functions which directly rely upon the viability of the water. The annual outbreak of a dinoflagellate bloom strictly governs the nature of the macro and micro food web and ultimately determines water quality. The freshwater dinoflagellate Peridinium gatunense is subject to a wide range of environmental stresses through out the spring bloom period. It was confirmed that SOD played an important antioxidative maintainance role throughout the bloom, especially during periods of relatively high photosynthetic activity (820 mg C m^-2 day^-1), when activity reached ~500 Units SOD mg protein^-1. In addition, high light stress (>300 μmol photons m^-2 s^-1) induced SOD activity, despite the low dissolved inorganic carbon (DIC) concentrations at the end of the bloom (1.3 mM).Catalase activity was only significant at the end of the bloom, peaking at 120 μm O₂ mg protein^-1 min^-1, when induced by photorespiratory activity. A series of experiments with Peridinium cultures showed that 2 x 10^-4 M ascorbate inhibited catalase activity >50% within 15 min incubation, both in vivo and 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 indirectly) the induction of catalase.