Reversible hydration of carbon dioxide. Essential to photosynthetic carbon dioxide fixation, supplies CO(2) to RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) in the carboxysome (Probable). Loss of activity results in limitation of CO(2) availability to RuBisCO located in the cytoplasm (PubMed:24585024). H(+) + hydrogencarbonate = CO2 + H2O Binds 1 zinc ion per monomer. A hexamer formed by a trimer of dimers (By similarity). Purified from carboxysomes with the both RuBisCO subunits and the full-length form of CcmM, probably interacts with the N-terminus of CcmM (PubMed:17675289). This cyanobacterium makes beta-type carboxysomes (PubMed:22928045). Associates with the shell portion of carboxysomes (By similarity). Carboxysome size and components vary with growth conditions. When grown in ambient air at medium light (50 uE meter(-2) second(-1)) there are 14 units of this protein per carboxysome, the numbers decrease slightly under low light, and increase under high light and high CO(2) (at protein level). Essential for photosynthetic CO(2) fixation, cells do not grow in normal air, but will grow in 5% CO(2), called a high-CO(2) requiring phenotype, HCR (PubMed:1584776). Cells do not grow in 2% CO(2), and grow more slowly than wild-type at 5% CO(2). 70% reduction in the efficiency of RuBisCO carboxylation activity from isolated carboxysomes; there is probably considerable oxygenation of the substrate (PubMed:24585024). Heterologous expression of 12 carboxysomal genes in E.coli (ccaA, ccmK2, ccmK3, ccmK4, ccmL, ccmM, ccmN, ccmO, ccmP, rbcL, rbcS, rbcX) leads to the formation of bodies that resemble carboxysomes, have densely packed paracrystalline arrays and RuBisCO activity. These structures open the door to generating carboxysomes in plant cells to increase their photosynthesis and productivity, as well as tailoring bacterial microcompartments to specific metabolic needs and molecule delivery. The absence of ccaA, ccmK3, ccmK4, ccmP and rbcX leads to less active RuBisCO. Belongs to the beta-class carbonic anhydrase family.