Lignin unlocking process in wood-feeding termites for effective biomass sugar release.

摘要

This study explored a unique lignin unlocking process in wood-feeding termite (WFT) Coptotermes formosanus (Shiraki) for subsequent effective polysaccharide utilization during the digestion of woody biomass. Utilization of plant cell wall (PCW) for production of biofuels and biochemical is an option for alleviation of the pressure created by the increase in energy consumption and the finite supply of fossil fuels. Pretreating PCW to unlock the protection of cellulose from lignin as an essential step for its enzymatic hydrolysis to fermentable sugars is currently the major barrier for economical production of lignocellulosic biomass-based fuels. The capability of cellulose utilization by WFT worker (up to 95% cellulose within 24 hrs) makes it a promising model system for discovering novel pretreatment mechanisms and mimicking the unique processes. The study employed a series of advanced analytical techniques including advanced microelectrodes, pyrolysis-gas chromatography/mass spectrometry, gas chromatography/mass spectrometry, thermogravimetry, attenuated total reflectance fourier transform infrared spectroscopy, solide-state nuclear magnetic resonance, ultraviolet spectrophotometry, high-performance liquid chromatography and ion chromatograph. Obtained results provide evidence that termites selectively modify lignin and associated aromatic biopolymers, with minor mineralization. More specially, the effective wood assimilation in termite is initiated with lignin phenolic demethoxylation, dehydroxylation, carboxylation and lignolytic ether linkage (beta--beta', beta--5', 5--5') dissociation. These reactions enhanced lignin hydrophilicity and physically/chemically reduced its inhibition effect on cellulases. The lignolytic catalyzing system of termite appared leaving lignin backbone alone, thus consuming less energy and producing negligible lignin-derived inhibitors to subsequent cellulose hydrolysis and fermentation of the sugars produced; which is different from that of the clearwing borer. The superior performance of termite on cellulose utilization is a result of stepwise introduction of specific reactions on lignin from the chewing process to the midgut region rather than large extent lignin degradation. The termite also possesses proper micro-environments (properly distributed O2, H2O2, redox potential, pH and iron) and symbionts in the gut for lignin modification. Overall, this study provides new information on lignin unlocking mechanisms during PCW deconstruction process by termite. The information further provides insights towards development of new pretreatment processes for biochemical conversion of lignocellulose fuels and value-added chemicals.