Fungi in Composting of Lignocellulosic Biomass
Composting is the controlled decomposition and subsequent stabilization of mixed organic substrates under aerobic conditions that allow the development of thermophillic temperature as a result of biologically developed heat. The organic substrates, used in composting are mostly derived from plant material and consist of cellulose, proteins, lipids and lignin. The process of decomposition is carried out by a number of microorganisms that exist within a compost pile. The capacity of microorganisms to assimilate organic matter depends on their ability to produce the enzymes needed for degradation of the substrate. The more complex the substrate, the more extensive and comprehensive is the enzyme system required.
Though, bacteria are the primary decomposing organisms, but fungi play an important role in decomposing complex polysaccharides because they share a unique nutritional strategy. Fungi can use variety of carbon sources mainly lignocellulosic polymers and can survive in extreme environmental conditions thereby exhibiting great catalytic power. Fungi are filamentous and can invade tissues more quickly, and form wind blown spores that are able to invade the substrates a greater distance away.
Fungi secrete extracellular enzymes that breakdown potential food sources. Each fungal spp. produces a limited no. of enzymes and each enzyme decomposes a limited no. of compounds. Soluble and easily degradable carbon sources, such as monosaccharides, starch and lipids are utilized by microorganisms in the early stage of composting. After the exhaustion of easily degradable carbon sources, more resistant compounds such as cellulose and hemicellulose are degraded and partly trans-formed into humus.
Cellulose is broken down into cellobiose and glucose in the presence of cellulase. The cellulases include endoglucanases and cellobiohydrolases enzymes, which act in a synergistic manner to degrade biomass. The glucose is then hydrolyzed to organic acids that are further converted to carbon dioxide and water. The major group of fungi that bring about cellulose decomposition include Fusarium oxysporium, Trichoderma reesei, Trichoderma viride and members of genus Penicillium, Aspergillus., Alternaria, Hormodendrum, Phanerochaete, Chaetomium, Pythium, Mortierella, Agaricus, brown-rot fungus Oligoporus placenta. Cellulases from brown-rot fungi are quite stable to inhibitors and denaturing conditions. Extracellular carbohydrate-degrading enzymes of brown-rot fungi are usually heavily glycosylated that accounts for their stability.
• Cellulases produce a slow, localized erosion of cellulose near the point of contact
• Trichoderma spp. and Aspergillus spp. fungi penetrate the fiber to the lumen and growth is abundant in the lumen and within the cell wall. A localized attack on cellulose fiber occurs. Trichoderma viride moves through cellulose by means of surface-bound enzymes. Fungi in contact with cellulose fibers erode the fiber surface
Hemicelluloses, the second major plant constituents and sources of energy and nutrients for soil microflora, are water-soluble polysaccharides and consist of hexoses, pentoses, and uronic acids. Xylan is the most abundant component of hemicellulose contributing over 70% of its structure. Xylanases can hydrolyze β-1,4 linkages in xylan and produce oligomers which can be further hydrolyzed into xylose by β-xylosidase. The enzymes such as β-mannanases, arabinofuranosidases or α-L-arabinanases can hydrolyse mannan-based or arabinofuranosyl-based hemicelluloses. The xylose, arabinose, galactose and mannose are further converted to organic acids, alcohols, CO2 and H2O. Uronic acids are broken down to pentoses and CO2.
Lignin accounts for about 10-30 percent of the dry matter of mature plant materials. Its content increases with the age of plant. It is one of the most resistant organic substances for the microorganisms to degrade and the enzymes required for complete degradation of lignin tend to be produced after most of the other nutrients in the compost pile have been depleted. White-rot fungi degrade lignin by means of oxidative enzymes that must be extracellular and nonspecific. Laccase, managense peroxidase and lignin peroxidase are the lignin degrading enzymes and belong to class phenoloxidases. Phanerochaete chrysosporium, Phlebia radiata, Trametes versicolor, Polyphorus varsicolor and Pleuretus sajor caju are some of the fungi that degrade lignin. Complete oxidation of lignin results in the formation of aromatic compounds such as syringaldehydes, vanillin and ferulic acid. The final cleavages of these aromatic compounds yield organic acids, carbon dioxide, methane and water.
Protein Decomposition: The process of hydrolysis of proteins to amino acids is known ammonification and is brought about by proteases or proteolytic enzymes secreted by various microorganisms. Amino acids and amines are further decomposed and converted into ammonia. During the course of ammonification, various organic acids, alcohols, aldehydes etc. are produced which are further decomposed finally to produce carbon dioxide and water. Under acidic conditions, fungi are pre-dominant, while in neutral and alkaline conditions bacteria predominate.
Inoculation with cellulolytic and lignolytic fungi hasten the process of decomposition and formation of humus during maturity of compost.
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