Chemical Composition and Antioxidant Activity of Solid Fermentation Products of Soil Fungi Aspergillus fumigatus
In recent years, research on soil fungal diversity has received widespread attention. Soil contains abundant microbial resources and is one of the most complex microbial ecosystems on Earth. Due to the close connection between fungi, soil, and plant roots, they jointly participate in the absorption, preservation, and cycling of nutrients, and have unique metabolic pathways, resulting in the production of structurally novel and biologically active metabolites. The chemical composition and activity of soil fungus Aspergillus fumigatus have been reported. Mohamad et al. isolated and identified 8 compounds from the secondary metabolites of soil fungus Aspergillus fumigatus 3T-EGY. Among them, compound Juglathraquinone A-5-O-D-rhodosamine (4 ′ → 1 ′) -2-deoxy-D-glucose (4 ′ → 1 ′) – inerulose B has moderate in vitro antibacterial activity against Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans. Liu et al. isolated a new sesquiterpene derivative from the coastal saline soil fungus Aspergillus niger, named as as pergiketone. This compound exhibits cytotoxicity towards HL-60 and A549 cells, with IC50 values of 12.4 and 22.1 μ M, respectively. Liang et al. found that the soil fungus Aspergillus fumigatus is a highly active iron carrier strain, which does not inhibit plant pathogen activity, but its fermented extract can significantly promote soil biological activity. In the early stage of this study, solid fermentation cultivation of soil fungus Aspergillus niger from the Ailao Mountain National Nature Reserve valley was carried out. It was found that its bold material had a scavenging effect on DPPH (IC50=3.3mg/mL), ABTS+(IC50=0.094mg/mL), and OH radicals (IC50=1.8mg/mL). Therefore, this study conducted chemical composition research and antioxidant activity testing on the fermented extract of the ethyl acetate layer of soil fungi, laying the foundation for the development and utilization of active ingredients of this strain in the future.

This paper isolated and identified 13 compounds from the ethyl acetate layer of soil fungus Aspergillus niger through solid-state fermentation culture. Among them, compounds 1, 5, 8, 11, 12, and 13 were all polyketide compounds, compounds 6, 7, 9, and 10 were steroids, and compounds 3 and 4 were other types. Among them, except for compound 6, all other compounds were isolated for the first time from the solid fermentation product of this strain. The compounds isolated and identified from the soil fungus Aspergillus niger are mainly polyketides and steroids, and their antioxidant activity was tested. Polyketide compound 8 has a certain scavenging effect on DPPH, ABTS+, and OH radicals, while compounds 11 and 12 have a scavenging effect on DPPH and ABTS+. The free radical scavenging activity indicates that aromatic hydroxyl groups are important for free radical scavenging activity, but steroid compounds 6, 9, and 10 have no scavenging effect on DPPH and only have strong scavenging effect on OH radicals. Previous studies have shown that steroidal compounds have no scavenging effect on DPPH, but have a strong effect on OH radicals. Polyketide compounds have strong activity against DPPH, and both steroidal and polyketide compounds are the main antioxidant active ingredients in Aspergillus niger. Polyketide compounds have certain antioxidant effects because the substituents of polyketides and flavonoids are similar. Based on free radical theory, phenolic hydroxyl groups have strong reducibility, and polyketide compounds are mostly multi hydroxyl structures with strong antioxidant capacity. This study enriches the chemical composition of Aspergillus fumigatus and provides certain reference value for the solid fermentation cultivation and development of this strain in the future.