Lignin biodegradation: reduced oxygen species
Lignin degradation, is quite common in nature and is an important link in the natural carbon cycle. A large variety of microorganisms are know to degrade lignin in nature as well as in contrived fermentation systems. White-rot and soft-rot fungi, as well as Actinomycetes, are apparently the most active lignin degraders in nature.
The large, cross-linked, polymeric structure of the lignin macromolecule makes its direct uptake, during the initial stages of its degradation, by microbial cells improbable. Moreover, the fact that the lignin macromolecule is composed of different monomeric units linked by a large variety of non-hydrolyzable intermonomeric bonds precludes hydrolytic cleavage of the biopolymer. Despite the fact that many extracellular and membrane-bound enzymes have been suspected in the initial breakdown of lignin, such activities have not yet been found. A close review of the literature indicates that the initial breakdown of the lignin macromolecule may be nonenzymatic. In addition, the degradation of the lignin polymer appears to follow an exo-degradation mechanism. That is, many lignin degrading microorganisms are apparently incapable of splitting the lignin molecule into intermediate molecular weight polyphenolic moieties which are further degraded; instead, they attack the periphery of the macromolecule.
The possible involvement of reduced oxygen species produced by white-rot fungi in the initial breakdown of the lignin macromolecule, during its biodegradation, was investigated. Using Coriolus versicolor as a representative of white-rot fungi, I demonstrated that C. versicolor exports superoxide radical and hydrogen peroxide during lignin degradation, into the lignolytic medium. Results presented in this study indicate that a correlation between the concentration of extracellular superoxide radical in the medium and the extent of lignin degradation may exist. Moreover, I have shown that superoxide radical is produced in the cell membrane, and not the organism's mitochondria. This precludes the possibility that such reduced oxygen species are produced as a result of normal respiration by the organism.
An investigation of the effects of aeration and agitation indicated that agitation has a detrimental effect on the extent of lignin degradation. On the other hand, increased oxygen tension in lignolytic cultures appeared to enhance the extent of lignin degradation. Another interesting finding was the fact that conditions leading to the formation of reproductive fruits in the lignolytic microorganism favored the degradation of the lignin fraction in lignocellulosic materials.
A comparative study of two different fennentation schemes, designed to degrade lignin in 1ignocellu1osic materials on a large scale, indicated that solid state fermentation of such materials led to greater lignin degradation. Fluidized bed fermentations, on the other hand, appeared to favor the degradation of the carbohydrates rather than the lignin fraction of lignocellulosic materials.
Studies of the biodegradation of monomeric lignin model compounds do not shed light on the initial step(s) involved in the breakdown of the lignin polymer. Such studies assume that microbial breakdown of lignin model compounds is similar to microbial breakdown of lignin an assumption that may not be correct. It is true that degradation of monomeric lignin model compounds can conceivably elucidate the mode of degradation of low molecular weight moieties resulting from initial breakdown of the lignin macromolecule. However, the chemical identities of these low molecular weight intermediates are not yet known. The efficacy of studies using aromatic, monomeric lignin model compounds in attempts to identify intracellular pathways for metabolism of lignin depends on the assumption that lignin breakdown products are indeed mononuclear phenolic materials.
Careful analysis of soluble and insoluble residual lignin resulting from lignin fermentations is a critical step in assessing the lignolytic ability of microorganisms. Furthermore, such analyses are essential in understanding the steps involved in lignin metabolism by microorganisms. To date the methods for residual lignin analyses are complex, time consuming and error prone. There is an urgent need to develop a quick and simple method for residual lignin analysis that will yield accurate and reproducible results capable of elucidating structural changes in residual, biodegraded lignin. The development of such an analysis technique will undoubtedly lead to a better understanding of the complex problem of lignin biodegration.