Molecular Characterization of a Dual Domain Carbonic Anhydrase From the Ctenidium of the Giant Clam, , and Its Expression Levels After Light Exposure, Cellular Localization, and Possible Role in the Uptake of Exogenous Inorganic Carbon.

A () had been sequenced and characterized from the ctenidia (gills) of the giant clam, , which lives in symbiosis with zooxanthellae. was expressed predominantly in the ctenidium. The complete cDNA coding sequence of from comprised 1,803 bp, encoding a protein of 601 amino acids and 66.7 kDa. The deduced DDCA sequence contained two distinct α-CA domains, each with a specific catalytic site. It had a high sequence similarity with tgCA from . In , the DDCA was localized apically in certain epithelial cells near the base of the ctenidial filament and the epithelial cells surrounding the tertiary water channels. Due to the presence of two transmembrane regions in the DDCA, one of the Zn-containing active sites could be located externally and the other one inside the cell. These results denote that the ctenidial DDCA was positioned to dehydrate [Formula: see text] to CO in seawater, and to hydrate the CO that had permeated the apical membrane back to [Formula: see text] in the cytoplasm. During insolation, the host clam needs to increase the uptake of inorganic carbon from the ambient seawater to benefit the symbiotic zooxanthellae; only then, can the symbionts conduct photosynthesis and share the photosynthates with the host. Indeed, the transcript and protein levels of /DDCA in the ctenidium of increased significantly after 6 and 12 h of exposure to light, respectively, denoting that DDCA could participate in the light-enhanced uptake and assimilation of exogenous inorganic carbon.