Giant clams represent symbiotic associations between a host clam and its extracellular zooxanthellae. They are able to grow in nutrient-deficient tropical marine environments and conduct light-enhanced shell formation (calcification) with the aid of photosynthates donated by the symbiotic zooxanthellae. In light， there is a high demand for inorganic carbon (C) to support photosynthesis in the symbionts and light-enhanced calcification in the host. In this study， we cloned and characterized a host Carbonic Anhydrase 4 homolog (CA4-like) from the whitish inner mantle of the giant clam Tridacna squamosa. The full cDNA coding sequence of CA4-like consisted of 1002 bp， encoding for 334 amino acids of 38.5 ... More
Giant clams represent symbiotic associations between a host clam and its extracellular zooxanthellae. They are able to grow in nutrient-deficient tropical marine environments and conduct light-enhanced shell formation (calcification) with the aid of photosynthates donated by the symbiotic zooxanthellae. In light， there is a high demand for inorganic carbon (C) to support photosynthesis in the symbionts and light-enhanced calcification in the host. In this study， we cloned and characterized a host Carbonic Anhydrase 4 homolog (CA4-like) from the whitish inner mantle of the giant clam Tridacna squamosa. The full cDNA coding sequence of CA4-like consisted of 1002 bp， encoding for 334 amino acids of 38.5 kDa. The host CA4-like was phenogramically distinct from algal CAs. The transcript level of CA4-like in the inner mantle was ~3-fold higher than those in the colorful outer mantle and the ctenidium. In the inner mantle， CA4-like was immunolocalized in the apical membrane of the seawater-facing epithelial cells， but absent from the shell-facing epithelium. Hence， CA4-like was positioned to catalyze the conversion of HCO to CO in the ambient seawater which would facilitate CO uptake. The absorbed CO could be converted back to HCO by the cytoplasmic CA2-like. As the protein abundance of CA4-like increased in the inner mantle after 6 or 12 h of light exposure， there could be an augmentation of the total CA4-like activity to increase C uptake in light. It is plausible that the absorbed C was allocated preferentially for shell formation due to the close proximity of the seawater-facing epithelium to the shell-facing epithelium in the inner mantle that contains only few zooxanthellae.
