Significant Changes in Microbial Communities Associated With Reef Corals in the Southern South China Sea During the 2015/2016 Global-Scale Coral Bleaching Event

Microbial communities play important roles as coral symbionts, but their changes among coral species in response to thermal bleaching events are not well understood. Therefore, we focused on the microbial communities associated with coral species in the southern South China Sea (SCS) during the 2015/2016 global-scale coral bleaching event (GCBE). Samples of eight typical coral species were collected before and during the GCBE, and the microbial communities, potential gene functions, and zooxanthellae densities (ZDs) were analyzed. We found that ZDs significantly decreased among all coral species during the GCBE. Alpha diversities of the microbial communities also significantly decreased at different taxonomic levels among these coral species. Principal coordinates analysis revealed significant differences in beta diversity among coral specimens, which were divided into two groups before and during the GCBE. Microbial gene functional prediction showed that microbial community physiology significantly changed during the GCBE, with decreased coverage in metabolism, membrane transport, replication and repair, and increased coverage in cell motility and signal transduction. Moreover, the abundance of potential pathogens such as the genusVibriogreatly increased (from similar to 0.28% to similar to 52.92%) during the GCBE, whereas the abundance of several beneficial microbes such asEndozoicomonassignificantly decreased (from similar to 26.10% to similar to 0.91%), resulting in an obvious decline in the coral-holobiont physiological functions. Thus, the GCBE greatly affected the health of coral species in the southern SCS, by reducing the biodiversity of associated microbial communities and increasing the abundance of potential pathogens.