Abstract
Giant clams Tridacna crocea harbor symbiotic zooxanthellae in tubular systems and display high growth rate when exposed to light. Light/dark alternation plays an important role in growth performance and physiological change of giant clam–zooxanthellae symbiosis in natural ecosystems. In this study, photosynthetic performance, antioxidant enzymes, transcriptome, and expression of eight illumination-related genes were investigated in the mantle of giant clam at different treatment times of 12-h light/12-h dark (15,000 lx). (1) The effective photochemical quantum yield of PSII (Y(II)) was high in the early morning, but significantly decreased to a low level at 6 h of light treatment (P < 0.05), and recovered slightly thereafter during light, then dropped at night; (2) Superoxide dismutase (SOD) and catalase (CAT) activity in the clam mantle increased slightly, and peaked at 6 h of light treatment (201.23 U/mg protein and 20.95 U/mg protein, respectively). This was significantly different (P < 0.05) to the results of SOD and CAT activities, exposed to 12 h of dark treatment; (3) Six paired-end libraries were sequenced in 3 h of light and 12 h of dark treatments. Through the assembling of 310,548,208 of clean reads with lengths of 150 bp, a total of 463,645 unigenes were obtained, which mostly derived from giant clam. Bioinformatics analysis revealed that 14,036 genes were up-regulated and 44,361 genes were down-regulated in 3 h of light treatment; (4) The results of quantitative real-time PCR demonstrated that the expression related to circadian rhythm (Bmal1 and NFIL3), antioxidant enzymes (SOD and CAT) and sodium- and chloride-dependent taurine transporters (SLC6A6) of giant clam, as well as photosynthesis (psbA and rbcl) of symbiotic zooxanthellae peaked at ~ 6 h of light phase, except of psbA at 1 h of light treatment. This study provides information on the light-adaptive mechanisms of giant clam–zooxanthellae symbiosis during daily light and dark cycles.
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