Organisms Degrading Lignin

Lignin is an amorphous, highly aromatic heterogenous, 3-D branched polymer (Buswell and Odier 1987; Higuchi 1985) with its substituents connected by carbon-carbon and ether linkages. There are different lignin structures consisting of various amounts of the three cinnamyl alcohol precursors present: p-coumaryl, coniferyl, and synapyl alcohols. These alcohols give rise to p-hydroxyphenyl units, guaiacyl units, and syringyl units, respectively, in lignin (Higuchi 1985; Kirk and Farrell 1987). The lignin cinnamyl alcohol precursors produce phenoxy radicals which randomly couple.

White-rot is characterized by the bleached white color of advanced wood decay due to fungi (white-rot fungi) and other organisms. White-rot can totally degrade all major wood polymers, including lignin (Crawford and Crawford 1980; Kirk 1971). Lignin is significantly de-aromatized. The degradation follows two general patterns (a) simultaneous attack on lignin and polysaccharides or (b) preferential or selective removal of lignin and/or hemicellulose without extensive depletion of cellulose (Buswell 1991). White-rot fungi degrading lignin in preference to polysaccharides are Pycnoporus cinnabarinus, Pleurotus ostreatus, Phlebia radiata, and Phlebia (Merulius) tremellosus (Ander and Eriksson 1977).

Other organisms degrading lignin include the brown-rot fungi, the soft-rot fungi, which seem capable of some demethylation and possibly degradation of side chains and aromatic rings (Ander and Eriksson 1978; Buswell 1991) and some bacteria associated with the Actinomycetes and Eubacteria (Kirk 1971). Bacteria may extensively degrade, demethoxylate, and modify aromatic rings and side chains.

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