CHAPTER 10. Biological Pretreatment of Biomass in Wood-Feeding Termites

Abstract

Wood-feeding lower termites are able to digest wood with high efficiency, which makes them an ideal model system for studying lignocellulose utilization in nature. The efficient cellulose extraction ability of termites is attributed not only to the advantages of the termite cellulolytic system, but also the unique pretreatment system that attacks the protection of cellulose provided by the structures of lignin and hemicellulose association. Through evolution, wood-feeding termites have evolved ideal physiochemical environment for pretreatment of lignin within the digestive system that starts from the chewing process and continues to the foregut and midgut. The termite pretreatment system selectively modifies lignin instead of completely degrading lignin molecules. The structural modifications were demonstrated as phenolic/aliphatic dehydroxylation, phenolic esterification and carboxylation, as well as selective disruption of β–β′, β–5′, and lignin-hemicellulose association, with reservation of the major lignin units of β–O–4′. Recent evidence suggests that the lignolytic system responsible for the lignin modification consists of not only lignases, but also non-enzymatic redox factors. Such energy efficient lignin modification mechanisms results in decrease inhibitory effect of lignin on downstream enzymatic hydrolysis of cellulose. The inspiration from nature-evolved biological systems such as termites can lead to innovations in the development of new generation of biorefinery.