Fungal pretreatments of Napier grass and sugarcane leaves for high recovery of lignocellulosic enzymes and methane production

Abstract

Effective biological pretreatment is a sought-after approach to solving the dilemma between environmental protection versus sustainable and economical production of bio-based products. In many countries, grasses are among the most attractive and widely cultivated crops for bioenergy. Here, Napier grass, including Alafal, Pakchong 1, and Merkeron varieties, and sugarcane were investigated for their potential applications in lignocellulosic enzyme and biomethane production. This was accomplished through biological pretreatments using a wood-rotting fungal Xylaria species under solid-state fermentation (SoSF) at ambient temperatures. Merkeron produced an enzymatic cocktail of xylanase and cellulase at activities of 103.1 ± 2.5 or 42.4 ± 1.61 U/g, respectively, on day 14. In contrast, using yeast Pichia pastoris expressing engineered Bacillus firmus xylanases, the highest xylanase activities were achieved between 58 and 61 U/g using Pakchong 1 or sugarcane leaves under submerged fermentation (SmF) after 2–4 days. Preferential fermentation mode between SoSF and SmF, utilized by fungi or yeast was observed during lignocellulosic saccharification. The Highest cellulase and xylanase productivity was achieved at 0.24–0.51 U/g/h, using the yeast-pretreatment via SmF. Alternatively, Xylaria-pretreated Napier grass mixed with cow manure effectively increased the daily biogas yield up to 212.8 ± 2.3 cm3/gVS with methane content over 80% and methane productivity of 2.31%/day, respectively. Thus, this study highlights potential uses of fungi in eco-friendly pretreatment of abundant feedstocks for optimal production of lignocellulosic enzymes and green energy biomethane.