Investigation of Anticancer and Antibacterial Metabolites from Seaweeds of the Bay of Bengal and their Associated Endophytic Fungi

Abstract

The relentless pursuit of novel anticancer and antibacterial agents has directed significant research toward marine ecosystems, with seaweeds and their endophytic fungi emerging as exceptionally promising reservoirs of bioactive metabolites. This research aimed to systematically investigate the bioactive potential of marine endophytic fungi derived from seaweeds of the Bay of Bengal, Bangladesh. Three seaweed species—Ulva sp. (Chlorophyta), Gracilaria sp. (Rhodophyta), and Sargassum sp. (Phaeophyta)—were collected, morphologically identified, and served as hosts for the isolation of endophytic fungi on potato dextrose agar (PDA) media. Twelve fungal strains were isolated and identified through a polyphasic approach combining macroscopic/microscopic morphological characterization and molecular phylogenetic analysis of ITS sequences. The fungal strains comprised Chaetomium globosum (UE-1, SE-1), Nigrospora magnoliae (UE-2), Curvularia sp. (UE- 3), Curvularia moringae (UE-4), Aspergillus terreus (UE-5, GE-2, SE-2), Collariella gracilis or C. virescens (UE-6), Aspergillus subversicolor (GE-1), Cladosporium halotolerans (GE-3), and Curvularia perotidis (SE-3). Critically, all twelve endophytic fungal strains were reported for the first time from the Bay of Bengal, Bangladesh, with Nigrospora magnoliae being reported as a fungal endophyte for only the second time worldwide. Preliminary chemical profiling of crude extracts via Thin Layer Chromatography (TLC) and Gas Chromatography-Mass Spectrometry (GC-MS) indicated a rich diversity of secondary metabolites, including steroids, terpenoids, flavonoids, and fatty acid derivatives. Bioassay-guided fractionation led to the isolation of twelve pure compounds using various chromatographic techniques, and their structures were elucidated through 1D and 2D NMR spectroscopy. From C. globosum, six compounds were isolated: the new natural compounds UC-71 [(E)-1-(2′,4′- dihydroxy-3′,5′-dimethylphenyl)-2-buten-1-one] and UC-58 [1-(3′,4′-dihydroxy-2′,5′- dimethylphenyl)-3-hydroxybutan-1-one], alongside known metabolites chaetoviridin E (UC-59), a mixture of chaetoviridin A epimers (UC-60), chaetoglobosin G (UC-62), and chaetoglobosin B (UC-63). Another six compounds were isolated and characterized for the first time from N. magnoliae: ergosta-4,6,8(14),22-tetraene-3-one (UN-172), sterigmatocystin (UN-171), dihydrosterigmatocystin (UN-176), 11-oxo-(9E)- octadecenoic acid (UN-233), ostopanic acid (UN-235), and cerevesterol (UN-411). Bioactivity screening revealed that the crude extracts possessed significant iv antimicrobial activity against Gram-positive (Staphylococcus aureus, Bacillus megaterium) and Gram-negative bacteria (Escherichia coli, Salmonella typhi, Pseudomonas aeruginosa), and antifungal activity against Aspergillus niger and A. flavus in disc diffusion assays, outperforming their host seaweeds which showed no activity. Antioxidant activity, evaluated via DPPH radical scavenging, was particularly potent in extracts of A. terreus strains (IC50 value as low as 7.88 μg/mL). Brine shrimp lethality bioassay indicated strong cytotoxicity (LC50 value less than 30 μg/mL) for numerous extracts. The isolated pure compounds also exhibited notable bioactivities; for instance, UC-60 showed remarkable antifungal activity against Candida albicans (31.3 mm zone at 100 μg/disc), and several compounds demonstrated significant cytotoxicity in subsequent MTT assay on the HeLa cervical cancer cell line. The MTT assay revealed that compound UN-235 exhibited significant dose-dependent cytotoxicity against HeLa cells (IC50 value 243.04 μg/mL), demonstrating potency comparable to doxorubicin, thereby identifying it as a promising cytotoxic candidate worthy of further investigation. These comprehensive findings underscore the immense, largely untapped potential of endophytic fungi from Bangladeshi seaweeds as prolific sources of novel chemotherapeutic agents, with the isolation of new natural compounds and first-reported metabolites highlighting their critical value in anticancer and antibacterial drug discovery pipelines.