Light and calcification in giant clam

07 Sep 2015 NUS scientists discovered that light induces increases in activities of certain enzymes related to light-enhanced calcification in the giant clam.

Calcification involves the accumulation and deposition of calcium salts in body tissues. A team led by Prof IP Yuen Kwong from the Department of Biological Sciences and Tropical Marine Science Institute in NUS reported information on the mechanisms of light-enhanced calcification in the giant clam, Tridacna squamosa (Ip et al., 2015). In T. squamosa, light-enhanced calcification involves the increased transport of H⁺ as NH₄⁺ from the calcification site into the inner mantle through light-enhanced Na⁺/K⁺(NH₄⁺)-ATPase activity. Light also enhances the activity of glutamine synthetase, which converts NH₄⁺ to glutamine, in the inner mantle. The H⁺/NH₄⁺-ATPase activity in ctenidia is also induced by light, indicating a relationship between light-enhanced calcification and active NH₄⁺ absorption from the environment. This is the first report on light having direct enhancing effects on activities of certain transporters/enzymes in T. squamosa, suggesting that the symbiotic zooxanthellae may act as ‘light-sensing elements’ for the animal host.

Such information would be useful in formulating strategies to achieve faster growth in, and hence stock enhancement of, giant clams in Singapore coastal water. For instance, it is possible that the growth of the giant clam can be enhanced by very low concentrations of ammonia in the ambient water, if indeed light-enhanced calcification involves active NH₄⁺ absorption and a futile glutamine-glutamate cycle. Furthermore, results obtained offer insight into how calcification rate can be enhanced, which may have biomimetic application to enhance bone-healing in patients with bone fracture.

Results reported by Prof Ip (Ip et al., 2015) suggested that light may induce the zooxanthellae in T. squamosa to produce some signaling molecules, which, upon release to the animal tissue, activate the gene and protein expression of some relevant transporters/enzymes essential to light-enhanced calcification. Indeed, Prof IP and his team have obtained preliminary results through next-generation-sequencing to confirm that light exposure led to increases in mRNA expression of Na⁺/K⁺(NH₄⁺)-ATPase and Glutamine synthetase in the inner mantle of the giant clam (see Figure).

The graphs shows the mass-specific activity (µmol Pi min^-1 g^-1 tissue) of glutamine synthetase (GS) from the inner mantle, the outer mantle and ctenidia of Tridacna squamosa exposed to 12 h of darkness (control; closed bar) or 12 h of light (open bar). Values represent means + S.E.M. (N=4). Means not sharing the same letter (a, b) are significantly different from each other (P<0.05). ^*Significantly different from the control (P<0.05). [Image credit: YK Ip]

Reference

Ip YK, Ching B, Hiong KC, Choo CYL, Boo MV, Wong WP, Chew SF. “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”. Front. Physiol. 6 (2015) 68.