Lignin Degradation and Valorization by Filamentous Fungi

The aromatic polymer lignin is one of the structural polymers present in lignocellulosic plant cell walls. After cellulose, it is the second most abundant biopolymer on Earth, and importantly the largest source of renewable aromatic compounds. Therefore, lignin has a high potential to be converted into various valuable products to replace the use of fossil feedstocks. However, efficient valorization of lignin from both natural sources and technical processes remains a challenge due to its extremely complex and recalcitrant structure. Consequently, for example, the lignin-containing side streams and waste fractions from agricultural and forestry sectors are highly underutilized. Filamentous fungi, in particular, wood decaying basidiomycete white rot fungi, have evolved an ability to efficiently degrade and modify lignin polymers in nature by secreting sets of extracellular oxidative enzymes. In addition, these fungi have ample metabolic capacities to convert the small aromatic molecules released from lignin to intermediates or final products. Many of these compounds are of industrial interest, for example, as building blocks for bioplastics and pharmaceutical compounds, and as flavor, fragrance, and antioxidizing agents. Therefore, the natural abilities of plant biomass degrading fungi makes them and their enzymes highly promising candidates for valorization of lignin through sustainable bioprocesses. In this chapter, the properties of natural and technical lignins are presented, and filamentous fungi and their enzymatic repertoire for lignin modification and degradation as well as their aromatic metabolic potential are described. Also, examples of lignin valorization using fungal enzymes and metabolically engineered fungal strains are given and the related opportunities and challenges are discussed.