Diversity, bioactivity, and secondary metabolites of actinomycetes associated with soft corals

Soft coral-associated actinomycetes have been increasingly recognized as promising sources of marine natural products. However, a comprehensive synthesis of their taxonomic diversity, cultivation features, and metabolic potential is still lacking. In this review, studies on coral-associated actinomycetes reported between 2007 and 2025 are comprehensively examined. The biological and ecological background of corals is outlined, and the methods used for the isolation and identification of actinomycetes are discussed. In addition, phylogenetic diversity inferred from 16S rRNA gene sequences, together with the structures and biological activities of secondary metabolites, is reviewed. To date, 172 actinomycete strains have been reported in total, of which 101 were found to contain sequence data suitable for phylogenetic analysis. In addition, 24 compounds with identified structures were confirmed, with polyketides representing the most abundant class. However, the current literature is strongly biased toward Streptomyces, a narrow range of host groups, specific geographic regions, and culture-dependent approaches. Consequently, the true taxonomic and chemical diversity of coral-associated actinomycetes is not adequately represented. Therefore, soft coral-associated actinomycetes are regarded as a promising yet still largely unexplored microbial resource. To achieve a more complete understanding of their taxonomic and chemical diversity, future research should incorporate broad sample collection and diverse cultivation strategies. In addition, advanced analytical techniques will be required for the screening and structural characterization of secondary metabolites.