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Carbon dioxide
Light-enhanced calcification
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Chew, S. F., Koh, C. Z. Y., Hiong, K. C., Choo, C. Y. L., Wong, W. P., Neo, M. L., & Ip, Y. K. (2019). Light-enhanced expression of Carbonic Anhydrase 4-like supports shell formation in the fluted giant clam Tridacna squamosa. Gene, 683, 101-112.
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 (Ci) 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 1,002 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 HCO3 − to CO2 in the ambient seawater which would facilitate
CO2 uptake. The absorbed CO2 could be converted back to HCO3 − 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 Ci uptake in light. It is plausible that the absorbed Ci 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.
This is the final draft, after peer-review, of a manuscript published in Gene. The published version is available online at
0378-1119 (print)
1879-0038 (online)
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