Nature of the inorganic carbon species actively taken up by the cyanobacterium Anabaena variabilis

The nature of the inorganic carbon (C₁) species actively taken up by cyanobacteria CO₂ or HCO₃^- has been investigated. The kinetics of CO₂ uptake, as well as that of HCO3- uptake, indicated the involvement of a saturable process. The apparent affinity of the uptake mechanism for CO₂ was higher than that for HCO₃^-. Though the calculated V. was the same in both cases, the maximum rate of uptake actually observed was higher when HCO₃^- was supplied. C. uptake was far more sensitive to the carbonic anhydrase inhibitor ethoxyzolamide when CO₂ was the species supplied. Observations of photosynthetic rate as a function of intracellular C₁ level (following supply of CO₂ or HCO₃^- for 5 seconds) led to the inference that HCO₃^- is the species which arrives at the inner membrane surface, regardless of the species supplied. When the two species were supplied simultaneously, mutual inhibition of uptake was observed. On the basis of these and other results, a model is proposed postulating that a carboic anhydrase-like subunit of the C₁ transport apparatus binds CO₂ and releases HCO₃^- at or near a membrane porter. The latter transports HCO₃^- ions to the cell interior.