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Value and Acid Production by Fungi


             The  value influences germination of spores, mycelial growth, enzyme ac-tivity (wood degradation), and fruit body formation. The optimum for wood fungi is often in slightly acid environment of pH 5-6 and for wood bacteria at pH 7. Basidiomycetes have an optimum range of pH 4-6 and a total span of about 2.5-9 (Thornqvist et al. 1987).





 Ascomycetes, particularly soft-rot fungi, may tolerate more alkaline substrates to about pH 11. Thus, the pH values from 3.3 -6.4 in the wood capillary water of living trees and in aqueous extracts of wood and bark samples from trees of the temperate zones and from trading timbers (Sandermann and Rothkamm 1959; Rayner and Boddy 1988; Fengel and Wegener 1989; Landi and Staccioli 1992; Roffael et al. 1992a, 1992b) cor-respond with the pH demands of wood fungi. Over the tree cross section, pH differences can occur, that is for example the heartwood of oaks and Douglas fir is more acid than the sapwood.

 Furthermore, an initial pH value can be changed in the context of microbial succession, because bacteria may acidify or alkalize the substrate by their metabolites (fatty acid production in acid wetwood or methane or ammonia formation in alkaline wetwood ).

 Outside about pH 2 and 12, respectively, microbial activity is commonly pre-vented. The acid pH-extreme of Aspergillus niger is 1.5 (Ress3 1997). There  pn value and Acid Production by Fungi 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 (Rypaeek 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.

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 frnm nitrocen 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 Antrodia vaillantii 

Isolate 
Antrodia sinuosa (iligoporus placenta Coniophora puteana Serpula lacrymans 1)onkioporia expansa 

FPRL14 R112 BAM 65 DFP 2375 MAD 2538 FPRL 280 Ebw. 15 BAM 133 MUCL 29391 

(g/L) pH 1.85 2.4 0.63 2.8 0.65 2.8 1.20 2.4 1.10 2.6 0.25 2.2 0.04 4.2 1.85 2.4 0.16 4.6 



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