Nature of the Inorganic Carbon Species Actively Taken Up by the Cyanobacterium Anabaena variabilis.

The nature of the inorganic carbon (C(i)) species actively taken up by cyanobacteria CO(2) or HCO(3) (-) has been investigated. The kinetics of CO(2) uptake, as well as that of HCO(3) (-) uptake, indicated the involvement of a saturable process. The apparent affinity of the uptake mechanism for CO(2) was higher than that for HCO(3) (-). Though the calculated V(max) was the same in both cases, the maximum rate of uptake actually observed was higher when HCO(3) (-) was supplied. C(i) uptake was far more sensitive to the carbonic anhydrase inhibitor ethoxyzolamide when CO(2) was the species supplied. Observations of photosynthetic rate as a function of intracellular C(i) level (following supply of CO(2) or HCO(3) (-) for 5 seconds) led to the inference that HCO(3) (-) 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 C(i) transport apparatus binds CO(2) and releases HCO(3) (-) at or near a membrane porter. The latter transports HCO(3) (-) ions to the cell interior.