Antagonism between coral pathogen Vibrio coralliilyticus and other bacteria in the gastric cavity of scleractinian coral Galaxea fascicularis

Scleractinian corals host numerous microbial symbionts with different types of interactions. The gastric cavity of scleractinian coral, as a semiclosed subenvironment with distinct chemical characteristics (e.g., dissolved O 2 , pH, alkalinity, and nutrients), harbors a distinct microbial community and a diverse array of bacteria that can be pathogenic or beneficial. Galaxea fascicularis is one of the dominant massive scleractinian coral species on inshore fringing reefs in the northern South China Sea. Although the abundance of coral-associated bacteria has been investigated in G. fascicularis , less is known about the microorganisms in the gastric cavity. In this study, we specially isolated cultivable bacterial strains from the gastric cavity of G. fascicularis collected from Hainan Island using a noninvasive sampling approach. Among the 101 representative bacterial strains, one Vibrio coralliilyticus strain, SCSIO 43001, was found to be a temperature-dependent opportunistic pathogen of G. fascicularis. The antagonistic activity between the 100 strains and V. coralliilyticus SCSIO 43001 was tested using a modified Burkholder diffusion assay. Our results showed that V. coralliilyticus SCSIO 43001 inhibits the growth of Erythrobacterflavus and Sphingomonas yabuuchiae. Additionally, we found that three Pseudoalteromonas strains showed moderate to high anti-bacterial activity against V. coralliilyticus SCSIO 43001 and several other coral-associated Gram-negative bacterial strains. These results suggest that competition between the coral pathogen and other bacteria also occurs in the gastric cavity of coral, and Pseudoalteromonas strains in the gastric cavity of G. fascicularis may provide a protective role in the defense against co-inhabiting coral pathogens at elevated temperature.