A ?CO2 supply? mechanism in zooxanthellate cnidarians: role of carbonic anhydrase

Abstract

Carbonic anhydrase (CA, EC 4.2.1.1) activity was detected in 22 species of tropical cnidarians which contain endosymbiotic dinoflagellates (=zooxanthellae). CA activity was 2 to 3 times higher in animal tissue than in algae and ca. 29 times higher in zooxanthellate than azooxanthellate species. It was also higher in the zooxanthellate tentacle tissue than in the azooxanthellate column tissue of the anemone Condylactis gigantea. CA was therefore significantly related to the presence of endosymbiotic algae. Further results indicated that CA functions in the photosynthetic carbon metabolism of zooxanthellate cnidarians as evidenced by (1) low CA activity in shade-adapted and deep water colonies compared to the more productive shallow water, light-adapted colonies of the coral Stylophora pistillata, and (2) the 56 to 85% reduction in photosynthetic carbon assimilation by zooxanthellae in situ in the presence of Diamox, an inhibitor of CA. Although CA has been proposed to function in calcification, its association with zooxanthellae and photosynthetic activity in both calcifying and non-calcifying associations suggests a role in photosynthetic metabolism of algal/cnidarian symbioses. It is proposed that CA acts as a “CO2 supply” mechanism by releasing CO2 from bicarbonate, and enabling zooxanthellae to maintain high rates of photosynthesis in their intracellular environment.