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

Abstract

The nature of the inorganic C (C1) species actively taken up by cyanobacteria CO2 or HCO3- was 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 Vmax was the same in both cases, the maximum rate of uptake actually observed was higher when HCO3- was supplied. C1 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 s) led to the inference that HCO3- is the species which arrives at the inner membrane surface, regardless of the species supplied. When the 2 species were supplied simultaneously, mutual inhibition of uptake was observed. Based on these and other results, a model is proposed postulating that a carbonic 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.