Insights into the culturable community and symbiosis-related traits of bacteria from long-term aquarium and Red Sea tropical octocorals

Corals are under unparalleled pressure due to climate change, with octocorals (Octocorallia, Cnidaria) being affected by rising temperatures, pathogens, and anthropogenic disturbances too. Host-microbe interactions play key roles in octocoral health, but our ability to benefit from such interactions to preserve octocorals remains underexplored. This project examines the role of aquarium facilities in octocoral microbiome conservation and aims to develop a "probiotic cocktail" to mitigate climate change-induced stress in octocorals. Three tropical octocoral species, Litophyton sp., Lobophytum sp. and Sclerophytum sp., from a long-term aquarium mesocosm (Oceanário de Lisboa), and two octocoral species, Litophyton sp. and Sclerophytum sp., from the Red Sea, were sampled. A total of 152 bacterial strains were isolated in our culture-dependent approach with strains grouping into six bacterial classes, 14 orders and 27 genera. Additionally, six unclassified isolates were obtained, likely representing new bacterial families in the Alteromonadales and Cellvibrionales orders. The collection comprised multiple, "hard-to-cultivate" genera such as Endozoicomonas, Fictibacillus, and Flammeovirga. The Endozoicomonas genus has been frequently suggested as a coral health indicator. Based on genus-level diversity, phenotypic screenings for host-beneficial properties of 25 bacterial isolates were performed. Twenty-two isolates presented antioxidant properties, eighteen cellulose- and nine chitin-degradation capabilities, with Actinobacteria frequently degrading both polysaccharides. Antimicrobial activity was tested against coral bacterial and fungal pathogens under control (26ºC) and stress (34ºC) temperatures, and 19 isolates displayed antagonism towards pathogens under heat stress conditions. The preservation of core symbionts of corals in captivity highlights the possibility of using sustainable, man-made ecosystems as repositories of stable and healthy coral microbiomes. The observed antimicrobial and enzymatic activities indicate potential beneficial traits among our isolates. Mesocosm experiments are now underway to determine their probiotic effects in octocorals.