Coral bleaching has been rapidly increasing in recent years due to abnormally elevated temperature, leading to massive damage to coral reefs worldwide. Understanding the processes and micro-ecological mechanisms of coral symbionts in response to bleaching is crucial as evidence accumulates that micro-organisms (particularly the bacteria) contribute to the health and recovery of coral, especially during heat stress. However, the exact functional mechanism of bacteria has not yet been fully elucidated. In this study, we performed metagenomic and metaproteomic analyses of healthy and bleached Acropora muricata corals to identify taxonomic and functional shifts in coral symbionts during a natural thermal bleaching event on Hainan Island. The results showed that symbiont species tended to be more abundant in bleached corals than healthy corals, and the bacterial community appeared to be crucial to coral bleaching. The relative abundance of opportunistic pathogens dramatically increased in bleached corals, accompanied by the reduction of beneficial bacteria. In addition, a direct comparison of metagenomic data sets indicated major changes in functional genes, with bleached corals exhibiting significant metabolic enrichment, while healthy corals maintained lower metabolism and energy consumption. Carbohydrate-active enzyme genes were remarkably activated, and virulence factors were highly represented in bleached corals, which was directly related to the increased abundance of pathogenic bacteria. Metaproteomic analysis also demonstrated that bleaching greatly affected photosynthesis and energy metabolism of coral symbionts. Among them, the biological processes of photosynthesis and chlorophyll biosynthesis were common in healthy corals, while pathways involved in gluconeogenesis and apoptosis were significantly enriched in bleached corals. Bacteria-mediated processes in healthy corals contribute to maintaining the basic functions of symbionts and resisting stress, whereas the proliferation of pathogenic bacteria in bleached corals leads to metabolic abnormalities of symbionts, showing enhanced energy metabolism and catalytic activity. In summary, the two omics analyzes revealed that bleaching caused enormous physiological damage to corals, and bacterial imbalance and dysfunction were the potential micro-ecological mechanisms underlying this event. IMPORTANCE: Coral reefs worldwide are facing rapid decline due to coral bleaching. However, knowledge of the physiological characteristics and molecular mechanisms of coral symbionts respond to stress is scarce. Here, metagenomic and metaproteomic approaches were utilized to shed light on the changes in the composition and functions of coral symbiotic bacteria during coral bleaching. The results demonstrated that coral bleaching significantly affected the composition of symbionts, with bacterial communities dominating in bleached corals. Through differential analyses of gene and protein expression, it becomes evident that symbionts experience functional disturbances in response to heat stress. These disturbances result in abnormal energy metabolism, which could potentially compromise the health and resilience of the symbionts. Furthermore, our findings highlighted the highly diverse microbial communities of coral symbionts, with beneficial bacteria providing critical services to corals in stress responses and pathogenic bacteria driving coral bleaching. This study provides comprehensive insights into the complex response mechanisms of coral symbionts under heat stress from the micro-ecological perspective and offers fundamental data for future monitoring of coral health.

Two Gram-staining-negative, aerobic, rod-shaped bacteria NNCM1T and NNCM2T were isolated from the scleractinian coral Acropora digitifera. NNCM1T grew with 0.5–12 % (w/v) NaCl (optimum, 3–6 %), at 18–37 °C (optimum, 28 °C) and at pH 6.0–10.0 (optimum, 7.0–8.0). NNCM2T grew with 0.5–10 % (w/v) NaCl (optimum, 2 %), at 18–37 °C (optimum, 28 °C) and at pH 6.5–9.0 (optimum, 7.0). The results of phylogenetic analysis based on 16S rRNA gene sequences indicated that NNCM1T formed a lineage within the genus Algiphilus of the family Algiphilaceae, and it was distinct from the most closely related species Algiphilus aromaticivorans DG1253T, with a 16S rRNA gene sequences similarity of 97.05 %. NNCM2T formed a lineage within the family Rhodobacteraceae, and it was distinct from the closely related genera Limibaculum halophilum CAU 1123T, Paroceanicella profunda D4M1T and Pseudoruegeria aestuarii MME-001T with 93.41, 92.78 and 91.09% identities, respectively. The major respiratory quinone was Q-8 and Q-10 for NNCM1T and NNCM2T, respectively. The predominant fatty acids (more than 10 %) were summed feature 8 (39.4 %) and C16 : 0 (19.4 %) for NNCM1T and summed feature 8 (62.8 %) and C16 : 0 (12.4 %) for NNCM2T. The DNA G+C contents of NNCM1T and NNCM2T were 63.3 and 63.4 mol% respectively. The polar lipids of NNCM1T comprised one diphosphatidylglycerol, one phosphatidylethanolamine, one phosphatidylglycerol and one unknown polar lipid, while those of NNCM2T comprised one phosphatidylethanolamine, one phosphatidylglycerol, one aminolipid and four unknown polar lipids. Phenotypic characteristics (physiological, biochemical and chemotaxonomic) also supported the taxonomic novelty of the two isolates. Thus, NNCM1T is considered to represent a novel species within genus Algiphilus , for which the name Algiphilus acroporae sp. nov. is proposed. The type strain is NNCM1T (=KCTC 82966T=MCCC 1K06445T). NNCM2T represents a novel genus and species within the family Rhodobacteraceae, for which the name Coraliihabitans acroporae gen. nov. sp. nov. is proposed. The type strain is NNCM2T (=KCTC 82967T=MCCC 1K06408T).

A Gram-stain-negative, non-motile, strictly aerobic, rod-shaped bacterium, with one polar flagellum and named D11R37^T, was isolated from coral culture seawater of Acropora digitifera. Strain D11R37^T grew with 0–6 % (w/v) NaCl (optimum, 0.5%), at 10–41 °C (optimum, 28 °C) and at pH 6.0–7.0 (optimum, 7.0). Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain D11R37^T formed a lineage within the genus Flavobacterium , and it was distinct from the most closely related species Flavobacterium suzhouense XIN-1^T and Flavobacterium suaedae G16-7^T with 16S rRNA gene sequences similarities of 95.97% and 95.48 %. The major respiratory quinone was menaquinone-6. The polar lipids comprised one phosphatidylethanolamine, two aminolipids and one unknown polar lipid. The predominant fatty acids (more than 10 % of total fatty acids) were iso-C_15 : 0 (18.0%), iso-C_17 : 0 3-OH (11.9 %) and summed feature 3 (10.9 %). The DNA G+C content was 41.3 mol%. Based on polyphasic taxonomic data, strain D11R37^T is considered to represent a novel species within the genus Flavobacterium , for which the name Flavobacterium coralii sp. nov. is proposed. The type strain is D11R37^T (=KCTC 82968^T=MCCC 1K06440^T).