Bioactivity and chemical screening of endophytic fungi associated with seaweeds Gracilaria sp. and Sargassum sp. of the Bay of Bengal, Bangladesh

This study explored the great potential of endophytic fungi associated with red seaweed Gracilaria sp. and brown seaweed Sargassum sp. of the Bay of Bengal, Bangladesh, for the first time. Endophytic fungi were identified taxonomically by morphological features and molecular characterisation (ITS sequence). The identification of six fungal isolates revealed five different fungal species belonging to four genera, namely, Aspergillus subversicolor, A. terreus and Cladosporium halotolerans isolated from Gracilaria sp. and Chaetomium globosum, A. terreus and Curvularia perotidis isolated from Sargassum sp. The ethyl acetate extracts of fungal endophytes were evaluated for antimicrobial activity, DPPH radical scavenging activity, and brine shrimp lethality bioassay. Amongst all the fungal extracts evaluated in this study, four showed mild to moderate inhibitory activity (10-14 mm) against the tested bacterial strains. Exceptionally, Chaetomium globosum exerted significant antibacterial activity with the highest zone of inhibition (21 ± 0.3 mm) against the Gram-negative bacterial strain Pseudomonas aeruginosa and also showed moderate antifungal activity (13 ± 0.9 mm) against A. niger. Sargassum sp.-derived A. terreus exhibited the strongest DPPH radical scavenging activity (IC₅₀ value of 7.88 ± 0.09 µg/mL). All the fungal extracts have significant lethality against brine shrimp nauplii (LC₅₀ value of ≤ 20.39 ± 4.04 µg/mL), while Curvularia perotidis and Cladosporium halotolerans were the most effective (LC₅₀ values of 9.30 ± 2.96 µg/mL and 9.94 ± 3.49 µg/mL, respectively). Gas chromatography-mass spectrometry (GC-MS) analysis of the crude extracts identified the presence of several chemical compounds. These bioactive chemical constituents might contribute to the exhibited bioactivity in this study. The current study's findings support the beneficial impacts of the fungal endophytes on exerting biological activities and consequently as valuable resources of bioactive compounds.