Mixed Paulownia and Sugarcane Bagasse Media Enhance Red Pigment Production by Arthrinium phaeospermum: A Potential Role for 4-Methylumbelliferone

Low-cost lignocellulosic residues offer a sustainable route to fungal pigment production. In this study, six biomass residues were evaluated as culture-medium supplements for Arthrinium phaeospermum, and their chemical constituents that may be associated with enhanced red pigment production were investigated. Fungal growth, pigment production, and pigment productivity were measured on biomass-amended potato dextrose agar. Following initial screening, Paulownia powder and sugarcane bagasse were selected, and their concentrations were optimized in mixed media. Ethanolic extracts of both materials were characterized using Fourier-transform infrared spectroscopy and ultra-performance liquid chromatography–mass spectrometry, and four major Paulownia-derived compounds were individually evaluated. The optimal medium, containing 4 mg/mL Paulownia powder and 3 mg/mL sugarcane bagasse, produced 3.09 g/L red pigment, representing a 2.02-fold increase over the control. Sugarcane bagasse primarily promoted fungal growth, whereas paulownia enhanced pigment productivity. Among the tested compounds, 0.2 mg/mL 4-methylumbelliferone (4M) showed the strongest stimulatory effect, increasing pigment production 3.01-fold over the control. These findings demonstrate that combining forestry and agro-industrial residues can simultaneously support fungal growth and pigment production. Paulownia-derived coumarins, particularly 4-methylumbelliferone, may contribute to this stimulatory effect and warrant further mechanistic investigation.