General Microbiology - Schlegel, H. 1987

Degradation of Natural Substances
Lignin

Lignin is quantitatively one of the major components of plant Tissues, second only to Cellulose and comparable in Abundance to hemicelluloses. Its content in woody tissues ranges from 18% to 30% of the dry weight. Plant tissue is incrusted with lignin, which is located in the secondary layers of The Cell wall. This plant product, formed in quite large quantities, undergoes biological degradation most slowly. Therefore, it serves as the primary source of slowly decomposing soil organic matter, particularly humic acids.

Chemically, lignin is heterogeneous. It is a highly complex compound, but this complexity does not stem from A large number of different monomeric units; all monomeric units in the lignin molecule are phenylpropane derivatives, predominantly coniferyl alcohol. The structural complexity of lignin is due to the variety of bonds linking these monomeric units together. This irregular Structure aligns with the concept that during lignin Biosynthesis, Enzymes are involved only in the generation of coniferyl alcohol radicals; these radicals then spontaneously form various linkages, with The Nature of the resulting bonds determined by the mesomeric state of the radicals.

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Fig. 14.4. Precursors for lignin biosynthesis (left) and coniferyl alcohol dimers formed as intermediates during the formation and degradation of lignin.

A number of coniferyl alcohol dimers and oligomers have been successfully isolated as intermediates of lignin synthesis (Fig. 14.4). While softwood lignin consists primarily of coniferyl alcohol, hardwood lignin contains both coniferyl and sinapyl alcohols, and cereal lignin additionally contains p-coumaric acid. These differences are reflected primarily in the methoxyl group content: it ranges from 20.5% to 21.5% in hardwood lignin, from 15% to 16% in softwood lignin, and from 14% to 15% in cereal lignin.

Phenylpropanoid units in the lignin molecule are interconnected in various ways via ether and carbon-carbon bonds (Fig. 14.4). These bonds are extremely resistant to enzymatic action. In plants, lignin exists as an inert end product that is no longer involved in METABOLISM and serves merely mechanical Functions. Only microorganisms are capable of degrading it. However, wood-decaying Fungi, as well as soil fungi and Bacteria, decompose lignin much more slowly than cellulose and hemicelluloses.

Lignin degradation. Certain fungi can degrade lignin even in living plants. Wood-decaying Basidiomycetes can be divided into two groups. Brown-rot fungi convert wood into a reddish-brown mass; they primarily break down the cellulosic and hemicellulosic components of wood and do not affect phenylpropane polymers. White-rot fungi degrade wood to form a nearly white mass; they act primarily on lignin while leaving cellulose almost untouched. Fungi that predominantly target lignin include Polystictus versicolor and several others (e.g., Stereum hirsutum). There are also fungi that simultaneously attack both lignin and cellulose, such as Pleurotus ostreatus, Ganoderma applanatum, Polyporus adustus, and Armillaria mellea. Wood degradation by pure fungal cultures proceeds so slowly that experiments stretch over months and even years. Using various Methods, The ability to degrade lignin has also been identified in representatives of several other genera (Pholiota, Clitocybe, Lenzites, Panus, Poria, Trametes, etc.).

It can be hypothesized that the initial attack during lignin degradation is carried out by exoenzymes. Since lignin-degrading fungi typically secrete phenol oxidases, it is believed that these specific enzymes are responsible for the Cleavage of Aromatic Compounds. To date, however, this function of phenol oxidases in lignin breakdown has not been conclusively proven, particularly regarding the cleavage of carbon-carbon or ether bonds.

To obtain enrichment cultures and isolate lignin-degrading microorganisms, it is best to use purified lignin from plant tissues. Isolation Methods based on the action of strong acids, alkalis, or high temperatures should be avoided. Near-native lignin can be obtained using Björkman's method (1954). In this Procedure, spruce wood is first ground with toluene to form a suspension, then separated from the toluene, dried, and lignin is extracted from the resulting wood meal using dioxane; about half of the total lignin contained in the wood passes into solution. Lignin isolated in this manner contains (by weight) 63.5% C, 6.4% H, 30.4% O, and 14.8% methoxyl groups. The isolated lignin or its derivatives (phenol-lignin or lignosulfonates) can serve as the sole carbon source not only for the fungi listed above, but also for many ascomycetes and fungi imperfecti. Enrichment cultures of certain bacterial strains (Flavobacterium, Agrobacterium, and Pseudomonas) can also be grown on partially digested lignin preparations. In mixed cultures, lignin degradation occurs faster and more completely than in pure cultures.

There is no doubt that lignin can be degraded by bacteria as well as fungi. However, its decomposition proceeds so slowly that it appears entirely negligible compared to other bacterial metabolic processes. Further searches are underway for microorganisms capable of degrading lignin or modifying it sufficiently so that other organisms can subsequently oxidize it.