Physiological Requirements For Lignin Degradation By White Rot Fungi

Lignin is unable to serve as the sole carbon and energy source for white rot fungi. In order for lignin degradation to proceed, white rot fungi require an additional more readily utilizable source of carbon (Ander and Eriksson 1975; Kirk et al. 1976). It has been hypothesized that the normal mode for white rot fungi to degrade wood is by simultaneous attack of polysaccharides and lignin (Boominathan and Reddy 1992). Degradation of cellulose and hemicellulose provides glucose for the fungus and when the flow of sugar ceases, the fungus starves and thereby goes from primary to secondary metabolism.

White rot fungi degrade lignin at the onset of the secondary growth phase, when utilizable nutrients are depleted and primary fungal growth ceases (Bonnarme et al. 1991). However, it should be considered that the concepts of primary and secondary metabolism might be an oversimplification regarding lignin degradation during natural conditions where fungal growth and lignin degradation can occur at the same time. Carbon, nitrogen, and manganese are all critical nutritional variables in triggering secondary metabolism and the production of ligninolytic enzymes including LIP and MnP by P. chrysosporium and other white rot fungi (Bonnarme et al. 1991). Considering that the nitrogen content of wood is very low (C/N 350-500/1) (Cowling and Merrill 1966), it is no surprise that nitrogen plays an important role in growth and metabolism (Buswell and Odier 1987; Kirk and Farrell 1987). In P. chrysosporium, limitation of nitrogen, carbon, or sulfur (Jeffries et al. 1981) can trigger secondary metabolism and lignin degradation. Fungal degradation of lignin in wheat straw was affected by the amount of nitrogen (NH4NO3), which appears to repress lignin degradation, by most fungi on this substrate (Zadrazil and Brunnert 1980). However, nutrient nitrogen only had a moderate influence on lignin mineralization by T. versicolor and almost no influence on P. ostreatus and Lentinus edodes (Leatham and Kirk 1983). In general, nitrogen repression of lignin degradation in white rot fungi is common but it is not always a rule.

The ligninolytic activity in P. chrysosporium is also triggered in cultures where carbon becomes limiting. The activity appears when the carbon source is depleted, and this activity is associated with a decrease of mycelial dry weight. The amount of lignin degraded depends on the amount of carbohydrate provided, which in turn determines the amount of mycelium produced during primary growth (Jeffries et al. 1981).

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