CO2 availability, carbonic anhydrase, and the annual dinoflagellate bloom in Lake Kinneret

Changes with time in the concentration of inorganic carbon in Lake Kinneret subsurface water were followed throughout two seasonal dinoflagellate blooms. The response of natural populations of the dominant dinoflagellate, Peridinium gatunense, to these changes was recorded by examining fluctuations over time in the activity of the enzyme carbonic anhydrase (CA) and in photosynthetic parameters. Our results show distinct fluctuations of both external and cytoplasmic CA activity in P. gatunense throughout the annual bloom. Higher levels of activity were triggered by the decline of total dissolved inorganic C below 1.8 mM and more specifically by low concentrations of dissolved CO2 (1–10 µM) during the seasonal bloom decline in May–June. Laboratory studies on cultured P. gatunense confirmed our field observations, suggesting that supplemental mechanisms are activated in P. gatunense that enhance inorganic C uptake when CO2 is limiting for photosynthesis. Eventually, the cellular adaptations of P. gatunense to the declining CO2 concentrations could not prevent decline of photosynthetic rates contributing to the subsequent decrease in P. gatunense biomass in May–June. In Lake Kinneret, P. gatunense is succeeded by Peridiniopsis spp., the photosynthetic rates and external CA activities of which were much higher under environmental conditions typical of the end of the bloom.