Symbiodiniaceae play important roles in the establishment of coral reef ecosystems in oligotrophic marine waters. Symbiodiniaceae Effrenium voratum is probably an exclusively free-living and heterotrophic species. How this species responds and acclimates to warming (a main environenmtal factor causes degradation of coral reef ecosystem) remain largerly unknow. In this study, we experimentally established the phenotypic landscapes related to the photobleaching and recovery processes of Effrenium voratum SCS01 following thermal stress. We found that thermal stress bleached the plastids of E. voratum SCS01 and caused the cells to become lighter in color; thereafter, the bleached cells recovered rapidly when they were returned to the optimal temperature. Accordingly, the dominant life form of E. voratum SCS01 shifted from mastigote cells to coccoid cells and then returned to mastigote cells. Transcriptome analysis revealed that the photobleaching of E. voratum SCS01 was due to increased degradation and decreased biosynthesis of photosynthetic pigments. Furthermore, the thermally induced life form changes were related to the downregulation of genes for cell motility. Our results revealed the mechanism of photobleaching in E. voratum SCS01 and indicated life form transformation as a newly identified survival strategy of Effrenium voratum SCS01 under thermal stress.

Life-history strategies play a critical role in susceptibility to environmental stresses for scleractinia coral. Metabolomics, which is regarded as the ultimate response of biological systems to genetic and environmental changes, is competent for the characterization of species’ biological traits. In this study, we combined untargeted mass spectrometry metabolomics and molecular networking to characterize the differential metabolic pathways between Pocillopora meandrina, one of the dominant species in South China Sea (SCS) with the life-history strategy of “competitive”, and Seriatopora hystrix, one of the nondominant species with the life-history strategy of “weedy” in SCS. The results show that lyso-PAF, DGCC, aromatic amino acids, and sulfhydryl compounds were more enriched in P. meandrina, whereas new phospholipid, de-PG DHC, MAG, FA (C<18), peptides, and guanidine compounds were more enriched in S. hystrix. The metabolic pathways involving immune response, energy metabolism, cellular membrane structure regulation, oxidative stress system, secondary metabolite synthesis, and et al. were speculated to relate to coral’s physiological traits, which finally resulted in the faster growth rate of P. meandrina and more robust environmental adaptability of S. hystrix. This study contributed to understanding of the potential molecular traits underlying life-history strategies of different coral species.