Reversible hydration of carbon dioxide (PubMed:14729686, Ref.3). Essential for photosynthetic carbon dioxide fixation, supplies CO(2) to RuBisCO (ribulose bisphosphate carboxylase, cbbL-cbbS) in the carboxysome (Probable). H(+) + hydrogencarbonate = CO2 + H2O Binds 1 Zn(2+) per monomer. Inhibited by dithiothreitol, partially inhibited by acetatzolamide and cyanide. Homodimer. This cyanobacterium makes alpha-type carboxysomes. Carbonic anhydrase activity of nitrate- and urea-grown cells decreases with rising temperature (from 25 to 28 degrees Celsius, present versus predicted future ocean temperatures); there is probably more than one carbonic anhydrase in this cyanobacteria. Belongs to the beta-class carbonic anhydrase family. CsoSCA subfamily.