Photosynthesis and the intracellular inorganic carbon pool in the bluegreen alga Anabaena variabilis: Response to external CO2 concentration.

The apparent photosynthetic affinity of A. variabilis to CO2 is greatly affected by the CO2 concentration in the medium during growth. Halfmaximal rate of photosynthetic O2 evolution is achieved at 10 μM and 100 μM inorganic carbon (Cinorg) in cells grown at low-CO2 (air) and high CO2 (5% v/v CO2 in air), respectively, whilst the maximum rate of photosynthesis is similar in both cases. Both high- and low-CO2-grown Anabaena accumulate Cinorg within the cell; however, the rate of accumulation and the steady-state internal Cinorg concentration reached is much higher in low as compared with high-CO2-grown cells. It is suggested that Anabaena cells actively accumulate Cinorg. Measurements of the kinetics of Cinorg transport indicate that the affinity of the transport mechanism for Cinorg is similar (Km(Cinorg(≃150 μM) in both high- and low-CO2-grown cells. However, V max is 10-fold higher in the latter case. It is suggested that this higher V max for transport is the basis of the superior capability to accumulate Cinorg and the higher apparent photosynthetic affinity for external Cinorg in low-CO2-grown Anabaena. Carbonic anhydrase activity was not detectable in Anabaena, yet both photosynthetic affinity to Cinorg in the medium (but not V max) and the rate of accumulation of Cinorg were inhibited by the carbonic-anhydrase inhibitor ethoxyzolamide.