THE FLUTED GIANT CLAM Tridacna squamosa POSSESSES A HOST CARBON CONCENTRATION MECHANISM TO DELIVER INORGANIC CARBON TO ITS SYMBIONTS

The giant clam, Tridacna squamosa, can strive in nutrient-deficient tropical marine environments and conduct light-enhanced shell formation (calcification) with the aid of photosynthates donated by the symbiotic zooxanthellae. It harbors symbionts extracellularly in the lumen of zooxanthellal-tubules (z-tubules), which originate from the digestive tract and infiltrate mainly the colorful and extensible outer mantle. During insolation, T. squamosa increases exogenous inorganic carbon (C_i) absorption mainly through its ctenidia to support photosynthesis in the symbionts and light-enhanced calcification of its shell. The absorbed C_i is transferred as HCO₃⁻ in the hemolymph to other parts of clam's body, and it must permeate the basolateral (hemolymph-facing) and the apical (lumen-facing) membranes of the epithelial cells of the z-tubules before it can be absorbed by the symbionts. This is achieved through a host-mediated and light-dependent carbon-concentrating mechanism (CCM), consisting of Carbonic Anhydrase 2-like (CA2-like) and Vacuolar-type H⁺-ATPase (VHA). The transcript and protein expression levels of the VHA subunit A (ATP6V1A/ATP6V1A) increase significantly in the outer mantle of T. squamosa during light exposure. In the outer mantle, the irridocytes, which deflect light of useful wavelengths to the zooxanthellae, are strongly ATP6V1A-immunopositive. They contain immuno-reactive vesicles that can release the acidic vesicular content to the hemolymph sinuses, leading to increased dehydration of HCO₃⁻ to CO₂ around the z-tubules, during light exposure. This can augment the permeation of CO₂ across the basolateral membrane of the tubule epithelial cells, although the specific transporters/channels involved have not been identified. Furthermore, the epithelial cells, which encircle the z-tubules, express CA2-like in the cytoplasm and ATP6V1A in the apical membrane. With CA2-like, CO₂ entering the epithelial cells of the z-tubules can be converted back to HCO₃⁻, and then transported into the lumen via certain apical HCO₃⁻ transporters. As zooxanthellae freshly isolated from giant clams absorb mostly CO₂ to support photosynthesis, the luminal fluid in the z-tubules must have a high level of PCO₂; only then, will the zooxanthellae be able to flourish in vivo. Hence, HCO₃⁻ must undergo dehydration to form CO₂ in the luminal fluid, and there must be an increased supply of H⁺ to the luminal fluid in light to drive this reaction. Indeed, ATP6V1A is located in the apical membrane in the epithelial cells of the z-tubules, indicating that VHA can secrete H⁺ into the lumen. Besides the host CCM, zooxanthellae also possess molecular mechanisms to augment CO₂ uptake to support photosynthesis during insolation. Together, these mechanisms maintain the symbiotic relationship of the giant clam-zooxanthellae association in regard to C_i transfer, and an understanding of these mechanisms may offer insight into how the giant clam-zooxanthellae association can be sustained under adverse environmental conditions.