Reversible hydration of carbon dioxide (PubMed:14729686). Essential for photosynthetic carbon dioxide fixation, supplies CO(2) to RuBisCO (ribulose bisphosphate carboxylase, cbbL-cbbS) in the carboxysome (Probable). There are estimated to be 29 CsoSCA oligomers per carboxysome (PubMed:22155772). H(+) + hydrogencarbonate = CO2 + H2O Binds 1 Zn(2+) per monomer. kcat is 8700 sec(-1) at pH 8.5, from purified carboxysomes. Homodimer. While the protein is very difficult to identify due to its low abundance, the enzyme activity fractionates with the carboxysome. This cyanobacterium makes alpha-type carboxysomes. Belongs to the beta-class carbonic anhydrase family. CsoSCA subfamily.