6 Wood Discoloration

72 3 Physiology
Fig.3.2. pH value regulation by Schizophyllum commune (a) and mycelial dry matter after
growth with different initial pH values (b) for 7–28 days (from Schmidt and Liese 1978)
in buffered and unbuffered media can differ. For example, Schizophyllum commune
grewbestonbufferedagaratpH4.7–5.1,butreachedinunbuffered
nutrient liquid the highest mycelial dry matter at pH 7.5 (Fig. 3.2b). Two pHoptima
may occur (Fig. 3.2b). Frequently, the optimum pH value of enzyme
activity of enzymes isolated from a fungus differs in vitro considerably from
the pH value for the corresponding fungal growth.
Most brown-rot fungi accumulate oxalic acid (oxalate) in rather large quantities
and acidify their microenvironment usually to a greater extent than do
the white-rot fungi (Table 3.9: Donkioporia expansa). pH-reduction by brownrot
fungi was thought to favor the activity of some non-enzymatic systems
hypothesized to be active in these fungi, as well as cellulolytic enzyme activity
(Goodell 2003). In brown-rot fungi, oxalate serves as an acid catalyst for the
hydrolytic breakdown of wood polysaccharides (Chap. 4). The acid attacked
the hemicelluloses and the amorphous cellulose, thus increasing the porosity
of the wood structure for hyphae, enzymes and low-molecular degrading substances
(Green et al. 1991a; Shimada et al. 1991). The enzyme system to produce
oxalate was also found in the white-rot fungi like Trametes versicolor (Mu et al.
1996). White-rot fungi accumulate smaller amounts of oxalate and use it in
connection with the enzymatic lignin degradation by lignin peroxidase and
manganese peroxidase. Under extracellular condition, the mediators, veratryl
alcohol cation radicals and Mn 3+ , produced by lignin and manganese peroxidase,
respectively, catalyze the decomposition of oxalate to CO2 (Shimada
et al. 1994). During intercellular metabolism, oxalate is formed by oxalate
decarboxylase (EC 4.1.1.2) to formate and CO2, and the formate produced
is converted to CO2 by formate dehydrogenase (EC 1.2.1.2), yielding NADH
(Watanabe et al. 2003). Oxalate may be also metabolized by oxalate oxidase
(EC 1.2.3.4) to CO2 and H2O2. There are, however, exceptions within both fun-
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