Secondary Metabolites of Fungi from Diverse Ecological Sources: Chemistry and Bioactivity

Summary
Fungi represent one of the most diverse and ecologically important groups of organisms, renowned for producing a vast array of secondary metabolites that mediate ecological interactions and exhibit remarkable biological activities. These metabolites, including polyketides, nonribosomal peptides, terpenoids, and alkaloids, have historically served as valuable sources of antitumor, anti-inflammatory, immunosuppressive, and cholesterol-lowering agents, as well as antibiotics, antifungals, and anthelmintics. Despite their established importance, the potential of fungal secondary metabolism remains largely untapped,
particularly among ecologically specialized groups such as nematode-associated, entomopathogenic, and white-rot fungi.
The escalating threat of antimicrobial resistance underscores an urgent need for the discovery and development of new compounds with novel mechanisms of action, positioning fungi as invaluable yet underexplored reservoirs of chemical diversity and bioactive secondary metabolites. This thesis investigates the diversity of secondary metabolites from fungi of different ecological origins, including nine nematode-associated strains of the order Pleosporales, the entomopathogenic fungus Blackwellomyces roseostromatus, and the white-rot fungus Panus strigellus. Additionally, as part of collaborative projects, this thesis also studied the secondary metabolites of the nematode-antagonistic fungi Pochonia chlamydosporia, Polyphilus frankenii, P. sieberi, and Lachnum sp., along with fourteen entomopathogenic strains from the order Hypocreales. All studied strains were cultivated under both liquid and solid-state fermentation conditions, and their extracts were analyzed using
HPLC-DAD/MS, followed by fractionation through chromatographic techniques.

Structural elucidation of the isolated compounds was achieved through advanced spectroscopic methodologies, including 1D and 2D NMR, ECD, UV, OR, and HR-ESI-MS, along with derivatization methods such as Marfey’s and Mosher’s analyses when required. The isolated metabolites were evaluated for biological activity against a panel of Gram-positive and Gramnegative
bacteria, fungi, mammalian cell lines, and the nematode Caenorhabditis elegans.
This research led to the isolation and characterization of over 60 compounds, including unprecedented secondary metabolites with wide-ranging biological properties. Notable discoveries from nematode-associated fungi include novel cyclodepsipeptides and tetramic acids from Polydomus karssenii, which demonstrated selective antimicrobial and nematicidal activities. Compounds from Murispora sp. also exhibited strong antimicrobial and nematicidal activity, highlighting their potential for nematode biocontrol. Furthermore, four Pleosporales strains representing yet undescribed taxa produced preussomerins and macrolides with potent cytotoxic and antimicrobial activities. Remarkably, a mycotoxin was discovered from the nematode biocontrol agent Pochonia chlamydosporia, revealing an unexpected facet of its secondary metabolism. In addition, Pyrenochaeta sp. yielded novel xanthone and anthraquinone derivatives. The entomopathogenic fungus B. roseostromatus produced bioactive metabolites with selective cytotoxic and nematicidal effects, while metabolites from the white-rot fungus Panus strigellus showed antimicrobial and cytotoxic properties.
Collectively, this research underscores the remarkable chemical diversity and biological potential of fungal secondary metabolites and significantly expands the chemical and biological knowledge of nematode-associated, entomopathogenic, and white-rot fungi. The findings reveal the untapped potential of these fungi as sources of novel natural products with promising
applications in biological control and pharmaceuticals.