CO2 AVAILABILITY, CARBONIC-ANHYDRASE, AND THE ANNUAL DINOFLAGELLATE BLOOM IN LAKE KINNERET

Changes with time in the concentration of inorganic carbon in Lake Kin neret subsurface water were followed throughout two seasonal dinoflage llate blooms. The response of natural populations of the dominant dino flagellate, Peridinium gatunense, to these changes was recorded by exa mining fluctuations over time in the activity of the enzyme carbonic a nhydrase (CA) and in photosynthetic parameters. Our results show disti nct fluctuations of both external and cytoplasmic CA activity in P. ga tunense throughout the annual bloom. Higher levels of activity were tr iggered by the decline of total. dissolved inorganic C below 1.8 mM an d more specifically by low concentrations of dissolved CO2 (1-10 mu M) during the seasonal bloom decline in May-June. Laboratory studies on cultured P. gatunense confirmed our field observations, suggesting tha t supplemental mechanisms are activated in P. gatunense that enhance i norganic C uptake when CO2 is limiting for photosynthesis. Eventually, the cellular adaptations of P. gatunense to the declining CO2 concent rations could not prevent decline of photosynthetic rates contributing to the subsequent decrease in P. gatunense biomass in May-June. In La ke Kinneret, P. gatunense is succeeded by Peridiniopsis spp., the phot osynthetic rates and external CA activities of which were much higher under environmental conditions typical of the end of the bloom.