Screening of coral probiotics and their probiotic characteristics

【Objective】This study investigated potential probiotics in coral and elucidated their probiotic characteristics，thereby contributing novel materials to the coral probiotic resource library and offering theoretical foundation for microbial control research in coral disease management.【Method】Extracellular enzyme producing strains were screened using protease and β-glucosidase activities，antagonistic active strains was screened with Vibrio coralliilyticus as indicator strain，quorum quenching active strains was screened with Chromobacterium violaceum as indicator strain，and strain identification on species and genus was performed via 16S rRNA sequencing. Then，the anti-disease function of probiotics was verified by in-tank addition test，and the probiotic properties of probiotics were predicted by whole genome analysis. 【Result】A total of 271 bacterial strains were isolated and purified from 7 coral samples using an in situ simulated cultivation method. Activity screening indicated that 136 strains exhibited protease activity，87 strains exhibited β-glucosidase activity，20 strains exhibited quorum quenching activity，and 28 strains exhibited antagonistic activity against V. coralliilyticus. The bacterial genera capable of producing extracellular enzymes and displaying antagonistic or quorum quenching activities included Bacillus sp.，Fictibacillus sp.，Exiguobacterium sp.，Ruegeria sp.，Sphingobium sp.，Pseudoalteromonas sp.，Vibrio sp.，Micrococcus sp.，Nocardia sp.，and Sphingomonas sp. The in-tank validation test of probiotic disease resistance demonstrated that the Bacillus sp. GXU-Z9 strain could effectively sustain the photosynthetic efficiency and zooxanthellae density of Pocillopora damicornis，thereby protecting the coral from bleaching induced by V. coralliilyticus. Whole genome data analysis revealed that Bacillus sp. GXU-Z9 strain possessed highly active amino acid metabolism and transport capabilities，indicating its potential to supply both essential and non-essential amino acids to coral hosts. Additionally，it could produce compounds beneficial to corals，such as antibiotics，which helped in resisting Vibrio infections，and could supplement the coral’s energy requirements by degrading fibrous or protein particulate matter. Concurrently，its genome encompassed multiple genes associated with amino acid biosynthesis and B Vitamin synthesis，which had the potential to supply essential amino acids and vitamins to the coral hosts，thereby facilitating their energy acquisition.【Conclusion】Bacillus sp. GXU-Z9 strain can effectively sustain the photosynthetic efficiency and zooxanthellae density of P. damicornis，exhibiting remarkable disease resistance. The whole genome of Bacillus sp. GXU-Z9 strain includes genes encoding chitinase and β-glucosidase，enabling the synthesis of essential amino acids and vitamins in substantial amounts. These capabilities assist the coral in digesting carbohydrate enzymes and the provision of amino acids and vitamins.