An efficient strategy to degrade lignin by a novel endophytic fungus Chaetomium globosum combined with ultrasound

Abstract

Green biological pretreatment of lignocellulose is the critical step in the degradation of biomass in the sustainable economy for biofuels and bio-based chemicals. In the present study, a novel laccase-producing endophytic fungus Chaetomium globosum was screened out from the flower of Hibiscus manihot L. in combination with ultrasound to improve the degradation of lignin in larch sawdust. Under the optimized conditions with the material to liquid ratio 1:20, temperature 30 °C, time 4 days, pH 7, the ultrasonic power 200 W, time 20 min, the degradation rate of lignin reached 34.29%, which increased 1.5-fold and 2-fold higher than that of fungus and ultrasonic treatment correspondingly. In addition, SEM and FTIR analysis showed that the combined treatment changed the surface morphology and chemical structure, and found that mycelium colonized in the interior of sawdust. Analysis of three types of lignin contents showed that the contents of p-Hydroxyphenyl and Guaiacyl increased from 0.46 to 0.70 mg/g and from 9.31 to 11.87 mg/g, while the contents of Syringyl propane did not change significantly, the results indicated that laccase from Chaetomium globosum might have special selectivity. After docking the three lignin monomers and laccase molecules, it was found that the hydroxyl group of Coniferyl alcohol was formed by the active groups of GLU-460 and SER-113 amino acids. The hydrogen bond distances were 1.8 Å and 2.2 Å, which were far smaller than the traditional hydrogen bond of 3.5 Å, this indicated that the compound binds strongly to the protein. The benzene ring of Sinapyl alcohol contained two methoxy groups increasing the molecular volume and hydrophobicity, which reduced laccase's ability to oxidize phenols. In general, as indicated by the results of this study, the novel endophytic fungus Chaetomium globosum from Hibiscus manihot L. has great potential to selectively degrade lignin in larch sawdust.