Insights on the genetic repertoire of the coral Mussismilia braziliensis endosymbiont Symbiodinium

Reef-building corals form a symbiotic association with photosynthetic dinoflagellates of the family Symbiodiniaceae. This symbiosis is crucial for the maintenance of coral reefs. In this work, we evaluate the effect of light conditions on the transcriptomic response of Symbiodinium CCMR0100 (ITS2 type A4), isolated from the Southwestern Atlantic Ocean endemic Mussismilia braziliensis. We obtained a total of 36,224 transcripts (N50 = 1007 bases, mean GC = 55.7%; ~25 Gb of assembled bases). We observed ecologically relevant transcripts encoding i. the complete antioxidant enzymatic system, ii. the recently described algal dimethylsulfoniopropionate (DMSP) lyase, and iii. The Mycosporine-like aminoacids (MAA) biosynthesis pathway. Cultures maintained in dark and light conditions yielded different transcriptomic profiles, and 48 transcripts were differentially expressed between these treatments. Expression of cytochrome P450 was inhibited by light, suggesting that endoplasmic reticulum monooxygenase activity might play a role in light-independent coral bleaching. Light conditions also triggered the induction of transcripts associated to chromatin condensation and mitosis, consistent with the light dependent progression of Symbiodiniaceae cell cycle. The repression of transcripts associated to the phosphatidylinositol (PI) signaling pathwaysuggests this pathway shall be related to light-induced morphological changes in Symbiodiniaceae cell.