Insights into the Galaxea fascicularis microbiome obtained from the microenvironment-based investigation

Different microenvironments shape distinct microbial communities that play a wide range of roles within coral holobionts. To better understand the capabilities of coral symbiotic microbiota, a more comprehensive characterization should be conducted. The gastrovascular cavity of coral is a distinct microenvironment that serves as a crucial link between the coral organism and the reef ecosystem. The species and functional compositions of microbiomes in the gastrovascular cavity are poorly studied. Therefore, this study was designed to investigate the microbial communities associated with mucus, tissue, skeleton, and gastrovascular cavity of Galaxea fascicularis to further explore their microscopic characteristics, species, and functional composition. The results revealed that the microbial cell abundance in the gastrovascular cavity was significantly higher than in the other three compartments, and the volume of microbial cells in the gastrovascular cavity, tissue, and mucus was significantly greater than that in the skeleton. There were significant differences in microbial community and functional composition among the four compartments of G. fascicularis. In particular, the microbial community in the gastrovascular cavity showed a relatively lower species diversity and the lowest network complexity compared to the other three compartments. Additionally, the potential functions of the microbiome in the gastrovascular cavity were enriched in substance metabolism, especially lipid metabolism. Microbial taxa and genes involved in nitrogen fixation, sulfur reduction and sulfur oxidation were less abundant in the gastrovascular cavity. Overall, our results suggested that the gastrovascular cavity was inhabited by a large number of microorganisms and it performs a strong selection process to retain microorganisms that are associated with substrate metabolism, which may be crucial to support the digestive function of the gastrovascular cavity. This also emphasizes the urgent need to explore coral microbial communities at the microscale to comprehend their role in coral health and resilience.