Seasonal variations in coral polyps and their skeletal microbiome in the marginal reef area of South China Sea

Coral holobionts are affected by seasonal environmental fluctuations in the marginal reef areas of the South China Sea (SCS). However, few studies have examined the dynamic changes in these coral polyps and their skeletal microbiome in response to seasonal variations and the potential relationship with coral acclimatization. In this study, Coelastrea palauensis corals were collected from a marginal reef area in the SCS, and the associated bacterial and archaeal communities in their polyps and skeletons were analyzed for seasonal variations (January, April, July, and October 2022). Results showed that the alpha diversity and composition of polyp and skeletal microbes differed significantly, and a higher alpha diversity was observed in the skeletal microbes than in the polyp microbes. Polyps were dominated by aerobic bacteria such as Blastocatella and Pseudoalteromonas , and archaea such as Candidatus Nitrosopumilus. Skeletons were dominated by anaerobic bacteria such as Chlorobium and Prosthecochloris , and the anaerobic archaea Bathyarchaeia. Statistical analyses suggest that the diversity and community composition of bacteria and archaea in coral polyps and skeletons varied significantly among seasons. At the genus level, the dominant coral-associated bacteria and archaea varied significantly seasonally. In winter, coral polyps had a higher abundance of putative opportunistic and potentially pathogenic bacteria, such as Vibrio , and potentially disease-resistant bacteria, such as Pseudoalteromonas . Skeletons contained a high abundance of Terasakiellaceae. The dynamic changes of microorganisms in the coral polyp and skeletons help us better understand the adaptation of symbionts in marginal reefs to seasonal changes.