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The objective of this study was to determine the effects of 12 h of exposure to light, as compared with 12 h of exposure to darkness (control), on enzymatic activities of transporters involved in the transport of NH +  4  or H + , and activities of enzymes involved in converting NH +  4  to glutamate/glutamine in inner mantle, outer mantle, and ctenidia of the giant clam,  Tridacna squamosa . Exposure to light resulted in a significant increase in the effectiveness of NH +  4  in substitution for K +  to activate Na + /K + -ATPase (NKA), manifested as a significant increase in the Na + /NH +  4 -activated-NKA activity in the inner mantle. However, similar phenomena were not observed in the extensible outer mantle, which contained abundant symbiotic zooxanthellae. Hence, during light-enhanced calcification, H +  released from CaCO 3  deposition could react with NH 3  to form NH +  4  in the extrapallial fluid, and NH +  4  could probably be transported into the shell-facing inner mantle epithelium through NKA. Light also induced an increase in the activity of glutamine synthetase, which converts NH +  4  and glutamate to glutamine, in the inner mantle. Taken together, these results explained observations reported elsewhere that light induced a significant increase in pH and a significant decrease in ammonia concentration in the extrapallial fluid, as well as a significant increase in the glutamine concentration in the inner mantle, of  T. squamosa . Exposure of  T. squamosa  to light also led to a significant decrease in the N-ethylmaleimide (NEM)-sensitive-V-H + -ATPase (VATPase) in the inner mantle, and significant increases in the Na + /K + -activated-NKA, H + /NH +  4 -activated-H + /K + -ATPase, and NEM-sensitive-VATPase activities in ctenidia, indicating that light-enhanced calcification might perturb Na +  homeostasis and acid/base balance in the hemolymph, and might involve the active uptake of NH +  4  from the environment. This is the first report on light having direct enhancing effects on activities of certain transporters/enzymes related to light-enhanced calcification in the inner mantle and ctenidia of  T. squamosa .

frontiers in physiology

Light induces changes in activities of Na+/K+-ATPase, H+/K+-ATPase and glutamine synthetase in tissues involved directly or indirectly in light-enhanced calcification in the giant clam, Tridacna squamosa