Effects of micro-organisms in the transformation of lignin to humic substances

Abstract

Conditions important in the formation of humic substances are mentioned briefly. Because of the relatively fast decomposition of cellulose and the slower rate of decomposition of lignin, the latter can be considered to be the essential parent material of humic substances. Transformation of the lignin fraction isolated from rotted plant material, with respect to time, are discussed. During the decomposition of plant material, the lignin fractions are oxidatively decomposed. The most remarkable alteration are the increase in nitrogen content and the decrease in methoxyl content. Infrared spectroscopic investigations of lignin fractions isolated at different rotting times illustrate that these are progressively transformed into humic substances. During the decomposition of plant materials, different lignin decomposition products, such as guaiacyl-and p-hydroxyphenyl derivatives, could be identified in the aqueous extracts together with a number of unknown phenolic compounds. Syringyl derivatives have not yet been found. During oxidation of different lignin decomposition products, the essential reactions are decomposition of the side chain, demethylation, oxidation to quinones, dimerization and polymerization, and cleavage of the ring. Examples of these oxidation reactions of the lignin decomposition products and the resultant products are discussed. The transformation of lignin decomposition products by means of lignin-rotting fungi was also studied. The lignin decomposition products were added to the cultures of fungi as well as submitted to oxidation catalyzed by purified phenoloxidases. For this purpose, phenylacrylic derivatives were labelled at different positions in the carbon atoms of the side chain. Polymerizates are formed through intermediate radical-like or quinoid steps during oxidation catalyzed by the phenoloxidases. Cleavage of methyl ethers is also an important reaction during oxidation of lignin decomposition products. Lignin decomposition products labeled at the methyl group of the methyl ethers, when added to the cultures of fungi, split off labeled carbon dioxide. Part of the activity can also be found in protein synthesized by the fungi. Most of the activity of the protein is found in the methionine and serine. The methyl group of the methyl ethers of the lignin decomposition products can, therefore, serve for methylation of the precursors of these two amino acids. In the presence of phenyloxidases, polymerizates are formed from the compounds labeled at the methyl ether group; total activity remains in the polymerizates. If these polymerizates are added to cultures of fungi, a slow cleavage of the methyl ether takes place. Cleavage of the ring also takes place during the enzymatic oxidation of lignin decomposition products. The mechanism of this cleavage was elucidated using carboxyl-labeled protocatechuic acid. According to the experimental results, the formation of humic acids from lignin, that is, the decomposition products formed by the action of micro-organisms, can be described as follows: polymerizates are formed simultaneously with degradation of the side chain. Crosslinking of the polymers through the side chains decreases with continued decomposition, and linkage of the rings takes place to an increasing degree. Increasing demethylation enables increasing condensation with nitrogen-containing compounds.