ABSTRACT        

Coral thermal tolerance is intimately linked to their symbiotic relationships with photosynthetic microorganisms. However, the potential compensatory role of symbiotic photosynthetic bacteria in supporting Symbiodiniaceae photosynthesis under extreme summer temperatures remains largely unexplored. Here, we examined the seasonal variations in Symbiodiniaceae and photosynthetic bacterial community structures in Pavona decussata corals from Weizhou Island, Beibu Gulf, China, with particular emphasis on the role of photosynthetic bacteria under elevated temperature conditions. Our results revealed that Symbiodiniaceae density and Chlorophyll a concentration were lowest during the summer and highest in the winter. Notably, the summer bacterial community was predominately composed of the proteorhodopsin bacterium BD 1–7 _clade, alongside a significant increase in Cyanobacteria, particularly Synechococcus_CC9902 and Cyanobium_PCC-6307, which represented 61.85% and 31.48% of the total Cyanobacterial community, respectively. In vitro experiments demonstrated that Cyanobacteria significantly enhanced Symbiodiniaceae photosynthetic efficiency under high-temperature conditions. These findings suggest that the increased abundance of photosynthetic bacteria during summer may mitigate the adverse physiological effects of reduced Symbiodiniaceae density, thereby contributing to coral stability. Our study highlights a potential synergistic interaction between Symbiodiniaceae and photosynthetic bacteria, emphasizing the importance of understanding these dynamic interactions in sustaining coral resilience against environmental stress, although further research is necessary to establish their role in preventing coral bleaching.