Optimization of the Construction of a Cellulose-Degrading Fungal Consortium and Its Effect on the Anaerobic Digestion of Rapeseed Straw

This study was conducted to improve the anaerobic digestion (AD) efficiency of rapeseed straw and to explore high-efficiency biological pretreatment strategies for lignocellulosic biomass. Five efficient cellulose-degrading fungal strains were isolated from decaying straw and were identified as Aspergillus niger, Rhizopus arrhizus, Penicillium expansum, Aspergillus microcysticus, and Trichoderma reesei. The co-culture consortium constructed with Rhizopus arrhizus, Aspergillus microcysticus and Trichoderma reesei exhibited the optimal cellulose-degrading activity and was designated as FA. Based on single-factor and response surface methodology, the optimal fermentation conditions for FA enzyme production were determined as follows: fermentation time 72.5 h, fermentation temperature 25.3 °C, and initial pH 6.0. Under these optimized conditions, the activities of carboxymethyl cellulase, filter paper activity, β-glucosidase and exo-β-1,4-glucanase produced by FA were 80.42, 31.54, 106.73 and 28.33 U/mL, respectively. For the cellulases derived from FA, the optimal reaction temperature was 50 °C and the optimal reaction pH was 6.0, and this cellulase exhibited favorable thermal stability and pH stability. FA-supernatant pretreatment effectively degraded the lignocellulosic components in rapeseed straw. On the 5th day of pretreatment, the degradation rates of hemicellulose, cellulose and lignin in rapeseed straw reached 16.51%, 9.05% and 5.82%, respectively. Pretreatment of rapeseed straw with the FA supernatant significantly enhanced methane production (p < 0.05). Taking treatment time and methane yield into comprehensive consideration, the 5-day pretreatment was identified as the optimal condition, under which the cumulative methane yield reached 167.63 mL/g volatile solids (VS), representing a 10.8% increase compared with the control, and the obtained performance was comparable to that of chemical pretreatment. This study demonstrates that FA pretreatment is an effective method to improve the AD performance of rapeseed straw.