Phylogenetic Diversity, Antimicrobial and Antioxidant Potential and Identification of Bioactive Compounds from Culturable Endophytic Fungi Associated with Mangrove Bruguiera sexangula (Lour.) Poir.

A total of 96 isolates were obtained from 375 segments, isolated from the healthy roots, stems, leaves, hypocotyls and flowers of Bruguiera sexangula (Lour.) Poir. collected at the Dong Zhai Gang Mangrove Garden on Hainan Island, and 20 independent representative isolates were identified using a combination of morphological and molecular approaches. The most frequent endophytic fungal species isolated were Diaporthe phaseolorum (relative frequency = 31.2%). The Shannon-Wiener diversity and Simpson's diversity index both showed that stems possessed the highest diversity compared to the other tissues estimated. Ethyl acetate extracts and the isolated metabolites were tested for antimicrobial activity using the serial dilution technique and for antioxidant activity using 2,2'-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonicacid) (ABTS) radical-scavenging capacity assays, respectively. The fungal isolate HL18 (Gelasinospora endodonta) cultured on Czapek's agar (CA) displayed a broad spectrum of antimicrobial activities and was significantly active against Escherichia coli (MIC = 0.0625 mg ml). Antioxidant assays showed that most of the fungal isolates (60.0%) exhibited some degree of antioxidant capacity (%RSA > 50%). The stain HL14 (Pestalotiopsis mangiferae) grown on potato dextrose agar (PDA) exhibited the highest DPPH and ABTS radical-scavenging capability with IC values of 0.717 ± 0.012 mg ml and 0.787 ± 0.027 mg ml, respectively. Furthermore, five known secondary metabolites 1-5 were isolated and identified from HL-14. Compounds 1 and 5 exhibited weak antioxidant activity.