Abstract:[Background] The environmental resilience of scleractinian corals is related to the associated bacteria. However, it is unclear how these bacteria adapt to the environmental changes. Studying the biological and ecological roles of these bacteria isolated via pure culture method is a fundamental approach to decipher the environment adaptation mechanism of corals. [Objective] To study the diversity and function of the heat-tolerant bacteria associated with two species of corals and further provide new insights into the environmental resilience of scleractinian corals. [Methods] We used three media, 2216E, seawater GYP, and seawater R2A agar, to isolate the heat-tolerant bacteria associated with two coral species (Acropora pruinosa and Galaxea fascicularis) with significant differences in heat tolerance from Weizhou Island, Beibu Gulf at 32 ℃ (heat tolerance threshold of corals). The 16S rRNA gene sequencing and sequence similarity analysis were then performed for the isolated bacteria. Predominant strains were selected and tested for the heat tolerance, and their antibacterial activity was analyzed by the plate confrontation method. [Results] The diversity of the potential heat-tolerant bacteria showed significant difference between the two coral species. A total of 44 strains of potential heat-tolerant bacteria belonging to 22 genera of 4 phyla were isolated from A. pruinosa, among which Vibrio, Pseudoalteromonas, and Tenacibaculum were predominant. A total of 28 heat-tolerant bacterial strains belonging to 11 genera of 3 phyla were isolated from G. fascicularis, among which Vibrio, Pseudoalteromonas, and Rugeria were predominant. Among the isolated bacteria, 17 strains shared the 16S rRNA gene sequence similarity below 98.65%, which might be new taxa. The growth conditions of bacteria were studied at 26-37 ℃, which showed the optimal growth temperature was 34 ℃, higher than the optimal growth temperature of most marine bacteria and coral bleaching threshold, indicating that the isolated bacteria had potential heat resilience. Two strains of Rugeria had an inhibitory effect on Vibrio, a potential pathogen associated with coral diseases, and 3 strains of Tenacibaculum did not exert significantly inhibitory activity. [Conclusion] The diversity of bacteria associated with A. pruinosa and G. fascicularis is high and needs to be further researched. Although the opportunistic pathogen Vibrio was the predominant genus, the bacteria associated with G. fascicularis had an inhibitory effect on it. Based on the above results, we hypothesized that the heat resilience of scleractinian corals was related to the inhibitory effect of the coral-associated bacteria on pathogenic bacteria.
