6 Wood Discoloration

3.5 pH Value and Acid Production by Fungi 71
are however fungi that even grow at about pH 0 like a Cephalosporium species.
Among the bacteria, the Archaea Picrophilus oshimae and P. torridus have their
pH-optimum at pH 0.7 and even grow at pH −0.06 (Anonymous 1996).
Various wood fungi can change pH values near the extremes by means of pH
regulation through their metabolic activity (Rypáček 1966; Humar et al. 2001).
Alkaline substrates are acidified by the excretion of organic acids, particularly
oxalic acid/oxalate (Jennings 1991). Oxalic acid is synthesized by oxaloacetase
(EC 3.7.1.1) from oxalic acetate of the citric acid cycle (Micales 1992; Akamatsu
et al. 1993a, 1993b) and can also derive from the glyoxylate cycle (Hayashi et al.
2000; Munir et al. 2001). Table 3.9 shows the amount of oxalic acid produced
by some house-rot fungi in vitro and the resulting pH value.
Figure 3.2a shows the change of the pH value by Schizophyllum commune
as an example of the pH-regulation curve of fungi. If there would not have
been a pH-change caused by the fungus, the diagonal in Fig. 3.2a would have
resulted. Nutrient liquids with acidic initial pH values become alkalized. For
example, the initial pH of 4.2 changed stepwise to the final pH of 7.5. After
3–4 weeks of culture, a nearly straight plateau of pH 7.5 derived from the initial
pH values 4.2, 5.1, 6.0 and 7.5. In contrast, the alkaline initial pH value of 7.5
was acidified in the first 2 weeks of culture (Schmidt and Liese 1978).
Aerobic bacteria alkalize their substrates by ammonia release from proteins
and amino acids (Schmidt 1986) and anaerobic bacteria alkalize the wetwood
in trees by methane formation (Ward and Zeikus 1980; Schink and Ward 1984).
Is less intensively examined by which metabolic pathways fungi alkalize acid
media. This may occur by the consumption of anions or by the formation of
ammonia from nitrogen compounds (Schwantes et al. 1976).
While unbuffered laboratory nutrient media approach the natural habitat
of wood fungi and show the physiologically produced pH value of a fungus,
buffered media of different initial pH values results in that pH-range, within
which a fungus can grow without adjusting the pH. The pH-optima received
Table 3.9. Content of oxalic acid (g/L) and pH-value in nutrient liquid after 2 months of
incubation (from Schmidt 1995; Schmidt and Moreth 2003)
Species Isolate (g/L) pH
Antrodia vaillantii FPRL14 1.85 2.4
R112 0.63 2.8
BAM 65 0.65 2.8
DFP 2375 1.20 2.4
Antrodia sinuosa MAD 2538 1.10 2.6
Oligoporus placenta FPRL 280 0.25 2.2
Coniophora puteana Ebw. 15 0.04 4.2
Serpula lacrymans BAM 133 1.85 2.4
Donkioporia expansa MUCL 29391 0.16 4.6
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