164 PHYSIOLOGY OF THE FUNGI 



changes in pH will take place during growth is illustrated by the work of 

 Dimond and Peltier (1945), who studied the pH changes produced by 

 Penicillium notatum as a function of the carbon and nitrogen nutrition. 

 When the initial pH was 6.0 and sodium nitrate was the nitrogen source, 

 the lowest pH values attained on different sugars were glucose, 5.1; 

 sucrose, 4.0; lactose, 3.2; maltose, 4.8; and galactose, 4.8. These were 

 the lowest pH values attained under these conditions. In another experi- 

 ment a mixture of tryptophane, asparagine, and cystine w^as used as the 

 nitrogen source. The pH again varied with the sugar used in the medium 

 The lowest pH attained with fructose was 5.3; glucose 3.5; sucrose, 4.0; 

 and an equimolecular amount of fructose and glucose, 3.5. When lactose 

 w^as used in combination with these amino acids, the pH of the culture 

 medium remained essentially constant at 7.0 



Any changes in environmental factors which affect the rate of growth 

 of a fungus may also affect the changes in pH of the culture medium. 

 Robbins and Schmitt (1945) found that the time required for Phycomyces 

 blakesleeanus to lower the pH of a glucose-asparagine medium to a given 

 level was a function of temperature of incubation. Growth and the 

 production of acid were more rapid at 26°C than at 20°C. The rate at 

 which the pH of a culture medium is changed by a fungus is also depend- 

 ent upon the volume of medium used in flasks of the same size. Some of 

 our data which illustrate this for Sordaria fimicola are shown in Fig. 27. 

 The time required for these cultures to attain maximum weight and to 

 produce perithecia correlated with the changes in pH. 



Effect of acidity on media. The composition of a medium may be 

 changed as a result of changing the pH. The various cations and anions 

 may combine to form insoluble compounds at certain pH values. Mag- 

 nesium and phosphate ions are compatible in acidic solutions, but as the 

 concentration of hydrogen ion is decreased, these ions combine to form 

 an insoluble compound, the solubility of which becomes less as the pH is 

 increased. Calcium phosphate is likewise less soluble in alakline solu- 

 tions. Ferric iron may be largely removed from media as either the 

 hydroxide or the phosphate, by making the media alkaline. If an alkaline 

 medium is filtered, certain constituents will be removed to a greater or 

 lesser extent. Lilly and Leonian (1945) found that by making a medium 

 alkaline to pH 8 and filtering, the iron concentration w^as lowered to such 

 levels that Rhizohium trifolii made about one-fifth as much growth as 

 when 250 jug of iron per liter was added to the medium. If a precipitate 

 is not removed by filtration, the situation is different. Any insoluble 

 precipitate is in equilibrium with the dissolved compound, as indicated 

 below. 



FeP04 ^ FeP04 ^ Fe+ + + + PO4" 

 solid in solution ionized 



