Metagenomic and metatranscriptomic insights into Ruegeria profundi-driven protective responses in coral holobionts against Vibrio coralliilyticus infection

In the context of climate-driven coral reef degradation, opportunistic pathogens such as Vibrio coralliilyticus are emerging as significant secondary threats, acting in synergy with thermal stress to accelerate coral bleaching and mortality. In this study, we investigated the role of Ruegeria profundi in mitigating V. coralliilyticus -induced bleaching. Specifically, the responses of coral holobiont members to pathogenic and probiotic influences were evaluated using metagenomics and metatranscriptomics. We found that the presence of V. coralliilyticus enhanced the metabolic potential of the coral-associated bacterial community, particularly regarding carbohydrate utilization and virulence. Conversely, R . profundi reduced the relative abundance of pathogenic Vibrio species by over 50% and broadly suppressed the expression of virulence genes within the coral-associated bacterial community, including a > 2-fold downregulation of genes involved in quorum sensing and flagellar assembly. Transcriptomic data indicated that immune-related genes in the host were upregulated, whereas photosynthesis-related genes in photosymbiotic microalgae were downregulated in response to V. coralliilyticus infection. R. profundi significantly promoted apoptosis resistance and antimicrobial peptide activity in the host and enhanced photosynthesis in photosymbiotic microalgae ( p < 0.05). Furthermore, R. profundi significantly suppressed virulence gene expression in the coral-associated bacterial community ( p < 0.05). Collectively, our results indicated that R. profundi orchestrates a tripartite defense mechanism involving the coral host, its associated bacterial community, and symbiotic microalgae, effectively mitigating pathogen-induced dysbiosis and bleaching. These findings have promising implications for microbiome-based strategies in coral reef restoration.