The fate of proton in the extrapallial fluid and inner mantle of Tridacna squamosa during Light-enhanced calcification

The giant clam Tridacna squamosa harbours symbiotic zooxanthellae which photosynthesizes in the presence of light and releases photosynthates to the host clam. Giant clams undergo light-enhanced calcification, a phenomenon also displayed by corals. Calcification involves the deposition of calcium carbonate according to the reaction: Ca²⁺ + HCO₃⁻ ó CaCO₃ + H⁺. Hence, calcification rate can increase by the removal of H⁺, but how H⁺ is removed from the extrapallial fluid of T. squamosa during light enhanced calcification is unclear at present. In general, H⁺ can be transported by V-type H⁺-ATPase, H⁺/K⁺-ATPase or Na⁺/H⁺ exchanger, but all these three transporters catalyze an efflux of H⁺ from the epithelial cell and therefore cannot facilitate H⁺ removal from the extracellular fluid. Here, we report that the infusion of NH₄Cl into the extrapallial fluid of T. squamosa led to an instantaneous increase in the total ammonia concentration therein, but the total ammonia concentration decreased subsequently and returned to the control level in 1 h. This is indicative of NH₃/NH₄⁺ being transported from the extrapallial fluid to the inner mantle. Additionally, the infusion of HCl into the extrapallial fluid led to an instantaneous decrease in the pH of the extrapallial fluid, but the pH increased significantly within 1 h. During this 1-h period, the increase in pH was accompanied by a significant decrease in the total ammonia concentration in the extrapallial fluid, indicating that H⁺ can combine with NH₃ and be transported as NH₄⁺ into the tissues of the inner mantle.  More importantly, exposure to light for 12 h induced a significant increase in the pH of, and a significant decrease in the total ammonia concentration in, the extrapallial fluid of T. squamosa. Light also induced a significant increase in Na⁺/NH₄⁺-activated-Na⁺/K⁺-ATPase (NKA) activity, with an increase in the effectiveness of NH₄⁺ in substitution for K⁺ to activate NKA, in the inner mantle of T. squamosa. Thus, light-enhanced calcification in T. squamosa was achieved through the transport of H⁺ as NH₄⁺, in substitution for K⁺, through NKA, into the adjacent inner mantle. Inside the inner mantle, NH₄⁺ could act partially as a substrate for glutamine production, as the glutamine concentration increased significantly in response to light. Light also induced a ~4-fold increase in the activity of glutamine synthetase (GS), an enzyme which catalyse the formation of glutamine from glutamate and NH₄⁺, in the inner mantle of T. squamosa. Furthermore, we obtained the complete coding cDNA sequence of a host form of GS from the inner mantle of T. squamosa, and demonstrated that its mRNA expression level in the inner mantle could be up-regulated by light. These results support the hypothesis that light may induce the zooxanthellae, which can act as 'light-sensing' elements for the host clam, to produce some signaling molecules, which, upon release to the animal tissue, activate a cascade of transcriptional, and perhaps also translational and post-translational, events leading to increases in activities of transporters/enzymes essential to light-enhanced calcification.