Characterization and function of carbonic anhydrases in the zooxanthellae-giant clam symbiosis

Carbonic anhydrase (CA) has been purified from the host tissue of Tridacna gigas, a clam that lives in symbiosis with the dinoflagellate alga, Symbiodinium. At least two isoforms of CA were identified in both gill and mantle tissue. The larger (70 kDa) isoform is a glycoprotein with both N- and O-glycans attached and has highest homology to CAII. It is associated with the membrane fraction while the smaller (32 kDa) is present in the aqueous phase in both tissues. The 32 kDa CA has high homology with mammalian CAI at the N-terminus. Both isoforms cross-reacted with antibodies to CAII from chicken. Immunohistology demonstrated that the 70 kDa CA is present within the ciliated branchial filaments and cells lining the tertiary water channels in the gills of T. gigas. This is consistent with a role in the transport of inorganic carbon (C-i) to the haemolymph and therefore supply of C-i to the zooxanthellae. CA was also detected in mantle epithelial cells where it may also contribute to C-i supply to the zooxanthellae. The hyaline body and nerve tissue in the mantle express the 70 kDa CA where it may be involved in light sensing and nervous transmission.