Box 123

Radioactive or stable isotopes of C have been used extensively to study lig-nocellulose decomposition. Isotopically labeled lignocellulose preparations are prepared either chemically or by feeding plants with 14C- or 13C-labeled phenylalanine, cinnamic acid, or glucose. In a classic study, Martin et al. (1980) followed the biodegradation and stabilization of specific C of model and cornstalk lignins, lignin monomer alcohols, and wheat straw over a 2-year incubation period using 14C-labeled substrates. The beta radiation emitted by 14C makes it possible to follow small changes in C metabolism. The researchers labeled coniferyl alcohol with 14C in specific C positions (A) and then fed it to corn plant cuttings to label the lignin fraction with 14C. (B) The corn and free coniferyl alcohol was then added to soil and incubated. In addition, 14C-labeled wheat straw labeled uniformly in a 14CO2 environment was used for comparison. During the 2-year incubation under ideal moisture and temperature conditions, about 40% of the ring and two-side-chain C of coniferyl alcohol units were evolved as CO2. Loss of OCH3 carbons varied from 52 to 69%. The C mineralization rate was most rapid during 3 to 6 months of incubation. In comparison, total C losses from intact wheat straw and cornstalks after 2 years were about 72%. Most of the labeled lignin C was recovered from the humic acid fraction compared to the intact wheat straw. The 14C of aromatics enters the microbial biomass to a very limited extent. However, an extrapolation of the decomposition data using an exponential function indicated that the same amount of lignin and intact plant residue C would remain in soil after 10 years of decomposition. Though it appears lignin is difficult to degrade, it does decompose at a rate fast enough not to accumulate in soil over short periods of time measured in decades.

box 12.3 (Continued)

hydroxyl radicals (OH). This gives the brown rots the ability to degrade intact wood without completely disrupting the lignin structure. Representative organisms include Poria and Gloeophyllum.

White rot fungi are the most active lignin degraders. Several thousand species of white rots are known mainly from the basidiomycetes and ascomycetes. The basidiomycetes most studied are Phanerochataete chrysosprium and Coriolus versicolor. Pleurotus ostreatus, the oyster mushroom, and Lintinula edodes, the shiitake mushroom, are wood decay fungi that are grown commercially for food. Ascomycetes include Xylaria, Libertella, and Hypoxylon. They produce lignolytic enzymes that oxidatively cleave phenylpropane units, demethylate, convert aldehyde groups (R-CHO) to carboxyl groups (R-COOH), and cleave aromatic rings, resulting in the complete destruction of lignin to CO2 and water. Lignin degradation is repressed by more easily degraded substrates and very little lignin C is used for growth. White rots use three classes of extracellular lignin-degrading enzymes: the phenol oxidase laccase, lignin peroxidase, and manganese oxidase. Laccase and manganese peroxidase cannot directly oxidize nonphenolic structures, which constitute up to 70 to 90% of lignin. Lignin peroxidase can oxidize both phenolic and nonphenolic lignin structures. Working together, these enzymes can significantly degrade lignin but are not capable of penetrating the intact lignin structure because of their large molecular size. It is theorized that hydroxyl radicals, mentioned previously, enter the lignin structure and enlarge pores in the lignin structure to accommodate the movement of the larger enzymes (Watanabe, 2003). The OH radical may be produced from the reaction of Fe(II) with hydrogen peroxide (H2O2) via the Fenton reaction:

Other transition metals like Cu may also be used in this process. Some white rots produce these low-molecular-weight oxidants through lipid peroxidation. These potent free radicals are capable of significant lignin degradation in the absence of the larger lignin-degrading enzymes.

The degree of phenol decomposition in lignins can be described by the relative distribution of acidic and aldehydic phenolic units within the vanillyl and syringyl phenol families. As lignin is degraded, carboxylic acid units are formed from the lignin polymer during cleavage of phenylpropanoid Ca-Cß bonds. This leads to an increase in carboxylic acid-containing phenolic units with respect to phenolic units with an aldehyde side chain. The change in the acid-to-aldehyde ratio for vanillyl and syringyl units reflects the degree of lignin degradation. Kögel (1986), using the above ratio, showed that the degree of lignin decomposition increased with increasing soil depth. This approach provides for a quantitative measure of the degree of lignin phenol degradation in soil, but not of the absolute turnover of the original plant material.

Emerging molecular techniques are providing a better understanding of lignin decomposition. Extensive information on genomes containing lignin peroxidase now exists. Fungal mutants in whom N does not repress lignase activity are also available to study the mode of action and the ecology of these organisms. The white rots, such as P. chrysosporium, do not compete well with soil organisms and may be restricted to high-lignin substrates such as woody debris, indicating a complex ecology surrounding lignin degradation.

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