Cultivable bacteria associated with Caribbean octocorals are active against coral pathogens but exhibit variable bioactivity when grown under different temperature conditions

Caribbean scleractinian corals have been declining in recent decades while octocorals appear to be thriving. Although microbial communities associated with scleractinians have been extensively studied, less is known about octocoral-associated communities. To investigate whether octocoral-associated microorganisms can provide resistance against coral pathogens, bacteria from the mucus and external surfaces of three common Caribbean octocoral species (Gorgonia ventalina, Eunicea flexuosa, and Antillogorgia americana) were isolated. Isolates were tested for bioactivity against six scleractinian coral pathogens at three temperatures to capture potential differences under varying conditions. Production of bioactive metabolites was evaluated using disk diffusion assays while growth competition assays determined whether the pathogen and isolate could establish simultaneously. Over half of the isolates, members of the phyla Actinobacteria, Firmicutes and Proteobacteria, produced compounds that inhibited the growth of one or more pathogens with some variation in bioactivity noted across temperatures. When inoculated simultaneously, most isolates were able to grow in presence of the pathogens while temperature did not have a significant impact. Collectively, these results demonstrate that octocorals support a diverse group of culturable bacteria capable of competing against coral pathogens. The putative protective roles of these bacteria provide insight into why Caribbean octocorals may be less susceptible to diseases and might explain their increasing prevalence on degraded reefs.