In most oceanic and in many fresh-water systems, aqueous (aq) carbon dioxide (CO2) constitutes a small fraction of the total inorganic carbon (C1). Although substantial research has focused on CO2 limitation, and adaptive C1 uptake mechanisms in a few test organisms, the ecological importance of CO2 limitation on dominant phytoplankton in their natural environment has received relatively little attention. The above statement also holds true for Lake Kinneret (Israel). Particularly intriguing is the possible role of CO2 limitation in determining species succession in the lake, especially during the annual bloom of the dinoflagellate, PERIDINIUM GATUNENSE, responsible for over 60% overall primary production in Lake Kinneret. The objectives of this study were: (1) to characterize the C1 uptake mechanisms in P.GATUNENSE; (2) to follow changes in carbonate chemistry of the lake during the P.GATUNENSE bloom, and to examine the role of CO2 availability as a factor influencing the bloom dynamics and species composition.

The response of natural populations of P. GATUNENSE to changes with time in the concentration of C1 in Lake Kinneret subsurface water was recorded by examining seasonal fluctuations in the activity of CA and in photosynthetic parameters. Distinct fluctuations of both external and cytoplasmic CA activity were observed in P. GATUNENSE throughout the annual bloom. Higher levels of activity were triggered by the decline of C1 below 1.8 milliM and more specifically by low concentrations of dissolved CO2 (aq) (1-10 microM) found during the bloom decline in May-June. Predicted rates of photosynthesis from uncatalysed dehydration of HCO3 and CO2 diffusion, showed that only at the beginning of the bloom can CO2 (aq) concentrations support photosynthesis based on diffusion only. Decreased CO2 concentrations caused P. GATUNENSE to also activate a carbon-concentrating mechanism (CCM). The fractionation of delta 13C decreased with the progression of the bloom from ~ -23 parts per thousand to -17 parts per thousand. Eventually, 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. 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. Analysis of multi-year data from the lake suggests that CO2 (aq) concentrations, in addition to other limiting parameters, are influential in determining in situ growth rates and eventually biomass accumulation. This study provides an example of a medium-sized natural lake ecosystem in which C1 availability is one of the factors limiting phytoplankton photosynthetic rates and, indirectly influencing algal species composition. The questions of whether oceanic primary production is CO2-limited and the effects of increasing atmospheric CO2 on phytoplankton remain controversial. Our findings suggest that in systems similar to Lake Kinneret increasing atmospheric CO2 concentrations may enhance primary productivity and growth rates, stimulating further sequestering of atmospheric CO2.