The colorful mantle of the giant clam, Tridacna squamosa, expresses a light-dependent manganese superoxide dismutase to ameliorate oxidative stresses due to its symbiotic association with zooxanthellae

Light-Dependent Phenomena and Related Molecular Mechanisms in Giant Clam-Dinoflagellate Associations: A Review

“…To supply CO 2 effectively to the photosynthesizing symbionts, the outer mantle of the host possesses a light-enhanced carbon concentration mechanism, which involves iridocytes and tubular epithelial cells, with Vacuolar-type H + -ATPase (VHA; Ip et al, 2018) and Carbonic Anhydrase 2 (CA2; Ip et al, 2017b) as the major catalytic mechanisms. Due to O 2 released by the symbionts, the host needs to augment the detoxification of superoxide radicals (O 2 -) by increasing the expression levels of copper zinc superoxide dismutase (CuZnSOD; Hiong et al, 2018) and manganese superoxide dismutase (MnSOD; Chew et al, 2020) in the outer mantle in response to illumination. The host also conducts light-enhanced inorganic phosphate (P i ) absorption through Sodium-dependent Phosphate Transport protein 2a (NPT2a; and supply it to the photosynthesizing symbionts.…”

“…•− , and the anti-oxidative defense must be more vigorous in light than in darkness (Hiong et al, 2018;Chew et al, 2020). In giant clams, ROS leaking from the photosynthesizing symbionts are unlikely to affect the oxidative status of the host cells directly, because the symbionts are located extracellularly inside the tubular system (Figure 1).…”

“…Thus, it can be concluded that the light-dependent increase in expression level of CuZnSOD in the outer mantle is needed to detoxify the ROS produced in the cytoplasm resulting from hyperoxia generated by the photosynthesizing symbionts (Figure 1). Moreover, the complete cDNA coding sequence of the host's MnSOD has been cloned from the outer mantle of T. squamosa (Hiong et al, 2018). In light, the metabolic rate of the outer mantle probably increases because of the increased supply of nutrients to the photosynthesizing symbionts and the receipt of O 2 and photosynthate from them.…”

“…This can lead to increases in the production of ROS in the mitochondria. Thus, exposure to light for 12 h leads to significant increases in the transcript level and protein abundance of MnSOD/MnSOD in the outer mantle of T. squamosa (Table 1; Hiong et al, 2018), in order to detoxify excess mitochondrial O 2…”

“…Two separate studies have demonstrated that the diel changes in expression levels of MnSOD (Hiong et al, 2018) and NCX3 (Boo et al, 2018) in T. squamosa are unrelated to the circadian rhythm. Apparently, T. squamosa has acquired a general signaling mechanism to coordinate the gene and protein expression levels of a variety of enzymes and transporters in multiple organs in relation to various diurnally lightdependent physiological processes (Table 1).…”