154 



PHYSIOLOGY OF THE FUNGI 



see Umbreit et al. (1945). The useful range of a buffer extends one pH 

 unit above and below the p/v^ (or p/v^) value. The data in Table 31 

 giving the pH range of a number of buffer systems of biological interest 

 were calculated using Eq. (11) and (13). The information in Table 31 is 

 useful in selecting buffers which are active in certain pH ranges. Direc- 

 tions for preparing buffer solutions of definite pH are given by Gortner 

 (1949). In practice, media are diluted with these buffers (Lindeberg, 

 1944). 



Table 31. The Useful pH Range of a Number of Buffer Systems of Biological 

 Interest Calculated by the Use of Eqs. (11) and (13) 



A number of other considerations enter into the choice of buffers. 

 They must be nontoxic, but even a buffer composed of an essential 

 nutrient such as phosphate may be inhibitory if used in high concentra- 

 tions. The use of calcium compounds in media was found to reduce the 

 toxicity of phosphate and citrate buffers to species of Marasmius (Linde- 

 berg, 1944). The amount of a buffer required to achieve a given degree 

 of immobilization of pH during growth cannot be specified without con- 

 sidering the fungus involved and the medium used. In experiments 

 where it is desired to maintain the pH of the culture medium essentially 

 constant, the concentration of nutrients, especially the sugar, in the 

 medium should be low, and an organic source of nitrogen should be used 

 in preference to nitrates and ammonium salts. Lindeberg (1944) used 

 Af /25 phosphate buffer in a medium which contained 10 g. of glucose and 

 1 g. of asparagine, with satisfactory results. 



In general, the usefulness of highly buffered media is restricted to deter- 

 mining pH limits and for special problems. Many fungi would not 

 develop "normally" in media having a fixed pH, especially in certain pH 

 ranges. 



