Humic Acid Transformation by the Fungus Cerrena unicolor Growing on Cellulose and Glucose

Microbial degradation of lignocellulose and soil organic matter is an important process controlling CO2 flow into the atmosphere, which is mostly carried out by fungi. The alkali-soluble fraction, which is represented mostly by the so-called humic acids (HA), dark-colored compounds of phenolic nature, constitute ~30‒70% of soil Corg. This is the first report on efficient degradation of soil HA under cellulolytic conditions; cellobiose dehydrogenase (CDH), an oxidative enzyme of the cellulolytic complex, was shown to play the key role in this process. Growth of a wood-decomposing fungus Cerrena unicolor on cellulose (CDH and laccase production) and glucose (laccase production) resulted in HA decolorization by 60 and 20%, respectively. HA depolymerization in the presence of CDH and its polymerization in the presence of laccase were shown by gel filtration experiments with the culture liquid and pure enzyme preparations. HA degradation in the presence of CDH was inhibited by radical scavengers, indicating the radical nature of this process (probably the Fenton’s reaction). HA depolymerization by CDH decreased in the presence of laccase, while HA polymerization by laccase decreased in the presence of CDH. Thus, the interaction of these enzymes affects HA transformation under cellulolytic conditions. These results improve our understanding of the mechanisms of microbial degradation of the aromatic components of soil humus and of the ecological functions of wood-decomposing fungi grown under cellulolytic conditions.