Isolation and identification of actinomycetes from rhizosphere soil samples of mangrove forests in Quanzhou Bay and secondary metabolites of Streptomyces sp. W444

Abstract:

Objective To isolate and identify actinomycetes from two mangrove soil samples in Quanzhou Bay as well as secondary metabolites from the target strain Streptomyces sp. W444 with antifungal activity.Methods The rhizosphere soil samples of two different mangrove plants were collected from Luoyang River in Quanzhou Bay. Actinomycetes were isolated from the soil samples by the dilution plating method. The isolates were classified by the phylogenetic tree constructed based on 16S rRNA gene sequences. The antifungal activities of the isolates were examined by the agar diffusion method. The target strain Streptomyces sp. W444 was subjected to fermentation scale-up for the isolation of secondary metabolites. Furthermore, the biosynthetic gene clusters were analyzed to deduce the biosynthetic pathway of staurosporine.Results A total of 56 strains of actinomycetes were isolated from mangrove soil samples and categorized into 8 genera belonging to 6 families of 6 orders. Among them, Streptomyces and Micromonospora were dominant, with the relative abundance of 41.0% and 33.9%, respectively. Strain Streptomyces sp. W444 exhibited excellent antifungal activity, and three indoles (staurosporine, K252c, and streptochlorin) were isolated and identified from this strain. The biosynthetic gene cluster of staurosporine was localized in the genome of Streptomyces sp. W444 by bioinformatics analysis. The biosynthetic pathway of staurosporine was then proposed.Conclusion Actinomycetes in the rhizosphere soil of mangrove plants in Quanzhou Bay had high diversity and contained potential natural product resources. Staurosporine, K252c, and streptochlorin were isolated from strain W444. These findings lay a solid foundation for studying the diversity and secondary metabolites of cultivable actinomycetes in mangrove forests in Quanzhou Bay.