Lignocellulosic residues: Biodegradation and bioconversion by fungi

Abstract

The ability of fungi to degrade lignocellulosic materials is due to their highly efficient enzymatic system. Fungi have two types of extracellular enzymatic systems; the hydrolytic system, which produces hydrolases that are responsible for polysaccharide degradation and a unique oxidative and extracellular ligninolytic system, which degrades lignin and opens phenyl rings. Lignocellulosic residues from wood, grass, agricultural, forestry wastes and municipal solid wastes are particularly abundant in nature and have a potential for bioconversion. Accumulation of lignocellulosic materials in large quantities in places where agricultural residues present a disposal problem results not only in deterioration of the environment but also in loss of potentially valuable material that can be used in paper manufacture, biomass fuel production, composting, human and animal feed among others. Several novel markets for lignocellulosic residues have been identified recently. The use of fungi in low cost bioremediation projects might be attractive given their lignocellulose hydrolysis enzyme machinery.

Introduction

Lignocellulose is the major component of biomass, comprising around half of the plant matter produced by photosynthesis (also called photomass) and representing the most abundant renewable organic resource in soil. It consists of three types of polymers, cellulose, hemicellulose and lignin that are strongly intermeshed and chemically bonded by non-covalent forces and by covalent cross-linkages (Pérez et al., 2002). Only a small amount of the cellulose, hemicellulose and lignin produced as by-products in agriculture or forestry is used, the rest being considered waste. Many microorganisms are capable of degrading and utilizing cellulose and hemicellulose as carbon and energy sources. However, a much smaller group of filamentous fungi has evolved with the ability to break down lignin, the most recalcitrant component of plant cell walls. These are known as white-rot fungi, which possess the unique ability of efficiently degrading lignin to CO₂. Other lignocellulose degrading fungi are brown-rot fungi that rapidly depolymerize cellulosic materials while only modifying lignin. Collectively, these wood and litter-degrading fungi play an important role in the carbon cycle. In addition to lignin, white-rot fungi are able to degrade a variety of persistent environmental pollutants, such as chlorinated aromatic compounds, heterocyclic aromatic hydrocarbons, various dyes and synthetic high polymers (Bennett et al., 2002). This degradative ability of white-rot fungi is due to the strong oxidative activity and low substrate specificity of their ligninolytic enzymes. Little is known about the degradation mechanisms of lignocellulose by soft rot fungi, in contrast to white and brown rot fungi. Nevertheless, it is clear that some soft-rot fungi can degrade lignin, because they erode the secondary cell wall and decrease the content of acid-insoluble material (Klason lignin) in angiosperm wood. Soft rot fungi typically attack higher moisture, and lower lignin content materials (Shary et al., 2007). The genome sequences from different fungi such as; Phanerochaete chrysosporium strain RP8 (Martinez et al., 2004; http://genome.jgi.psf.org/whiterot1), Coprinopsis cinerea (Walti et al., 2006), Postia placenta (Stajich, 2007), Pleurotus ostreatus (Irie et al., 2000), Agaricus bisporus (Challen et al., 2007), Schizophyllum commune (Horton and Raper, 1991) and Serpula lacrymans (Bruce, 2007) have been revealed and its genomic information may greatly facilitate our understanding of the lignocellulose biodegradation process. World-wide lignocellulosic residue generation every year results in pollution of the environment and in loss of valuable materials that can be bioconverted to several added-value products (Howard et al., 2003). Lignin can be removed by chemical (Chahal, 1991, McMillan, 1994, Gong et al., 1999) or physical pre-treatment which then permits efficient bioconversion. Pre-treatment can also be carried out microbiologically. This has the advantages over non-biological procedures of producing potentially useful by-products and minimal waste (Zimbardi et al., 1999). This review will focus on the use of fungi in the biodegradation of lignocellulose, aspects of bioconversion and world-wide lignocellulosic residues.