Lagoon coral microbiomes and their potential relationship with adaptation of coral holobionts to extreme high-temperature environments

Numerous corals can survive in high-temperature environments, but little attention has been paid to the microbiome of lagoon corals and their potential adaptation to thermal environments. In this study, we explored this issue by analyzing the coral-symbiotic Symbiodiniaceae, Symbiodiniaceae densities, and bacterial communities among coral species and seawater in the lagoon regions of the southern South China Sea (SCS). Galaxea fascicularis had the highest Symbiodiniaceae density and relative abundance of heat-tolerant Symbiodiniaceae (Durusdinium trenchii) , and Acropora cytherea had the lowest density, but the highest diversity, of Symbiodiniaceae. The Symbiodiniaceae diversity of Fungia fungites was the lowest. Moreover, the Symbiodiniaceae community and diversity differed significantly between the lagoon and open-ocean seawater. In addition, the relative abundance of Vibrio was higher in coral holobionts than in the lagoon seawater samples. The relative abundance of Ruegeria was highest in A. cytherea , indicating that coral-associated bacteria in the lagoon were affected by high thermal stress during summer. However, corals in the lagoon may adopt different adaptation strategies for high-temperature environments. G. fascicularis corals may adapt to lagoon environments by symbiosis with heat-tolerant Symbiodiniaceae at high densities and abundance, whereas A. cytherea may survive in the lagoon environment based on a highly flexible symbiosis associated with the high photosynthetic efficiency of Symbiodiniaceae and bacteria.