Diversity of culturable endophytic fungi from Zhanjiang mangroves and secondary metabolites of the isolate Stemphyliums sp. SCSIO 40436

Abstract:

[Background] Mangroves, trees and shrubs adaptive to salt, grow along the coast at tropical or subtropical latitudes. Mangrove endophytic fungi are the second largest community of marine fungi, which are rich in secondary metabolites with diverse chemical structures and excellent bioactivities. Therefore, mangrove fungi have proven to be an essential source of natural products and novel drug leads. [Objective] To study the diversity of culturable endophytic fungi from Zhanjiang mangroves and to identify bioactive secondary metabolites from the isolate Stemphyliums sp. SCSIO 40436. [Methods] Endophytic fungi isolated from Zhanjiang mangrove forests were identified based on internal transcribed spacer (ITS) sequence. The production of secondary metabolites on rice and oat solid media was evaluated by HPLC-DAD and filter paper diffusion method. The crude extract was subjected to column chromatography with silica gel and Sephadex LH-20, medium-pressure liquid chromatography, or semi-preparative RP-HPLC. The metabolite structures including absolute configuration were determined by spectroscopic methods (HRESIMS, NMR), X-ray crystallographic analysis, and comparison of electronic circular dichroism. Antimicrobial activities were measured against four indicate strains via the broth microdilution method. [Results] A total of 52 strains of culturable endophytic fungi were isolated from the Zhanjiang mangrove forests and categorized into 29 genera. The strain Stemphylium sp. SCSIO 40436 was found to be prolific in metabolites. Extensive chromatographic separation of the ethyl acetate extracts afforded five polyketides: stemphol (1), macrosporin (2), altersolanol A (3), alterporriol E (4), and alterporriol D (5). The structures of 1 and 3 were confirmed by X-ray crystallographic analysis. Stemphol (1) exhibited a good activity against the Gram-negative bacterium Acinetobacter baumannii ATCC 19606 with a minium inhibitory concentration (MIC) value of 8 μg/mL. [Conclusion] A total of 52 endophytic fungal strains are isolated from the Zhanjiang mangrove forests, among which several strains shared <95% similarity of ITS rDNA sequences, which enriches the diversity of mangrove endophytic fungi. Stemphol (1) exhibits a moderate activity against the Gram-negative bacterium A. baumannii.