Annotation and Bioactivity Profiling of Endophytic Fungi, Penicillium singorense Species, From the Lahar‐altered Sasmuan Mangrove Forest, Pampanga, Philippines by Molecular Networking

ABSTRACT Endophytic fungi are well recognized as prolific producers of structurally diverse natural products. In this study, three endophytic fungal isolates − Penicillium singorense MA‐S1, MA‐S3, and PDA‐S2 – were obtained in Sasmuan mangrove forest, which had previously been affected and altered by the lahar from the 1991 eruption of Mt. Pinatubo. Morphological characterization and 18S rDNA/ITS sequencing confirmed their taxonomic placement in the Penicillium genus. The three isolates were subjected to fermentation using rice medium, and metabolites were extracted using ethyl acetate. Crude extracts were then fractionated using the modified Kupchan method, followed by solid‐phase extraction (SPE). Disc diffusion assays indicated that the dichloromethane and methanol fractions, as well as the 25% and 100% methanol SPE fractions, exhibited the highest zone of inhibition (ZOI) against the clinical isolates, surpassing the ZOI of voriconazole. Liquid chromatography‐tandem mass spectrometry analysis of the fractions, combined with Global Natural Products Social (GNPS) molecular networking, putatively identified the presence of known bioactive metabolites, including diketopiperazines, peptides, and terpenes. Furthermore, other clusters in the network lacked spectral matches in the GNPS library, suggesting the presence of putatively novel metabolites in the extracts. Overall, the study highlights the potential of endophytic fungi from a post‐volcanic mangrove ecosystem as valuable sources of bioactive natural products.