HYDROGEN-ION CONCENTRATION 1C5 



As the ions are utilized, more and more of the precipitate will dissolve. 

 The effect of a change in pH of the solution as a result of the metabolic 

 activities of the fungus must be considered. An acid reaction will hasten 

 solution of the precipitate, while an increase in alkalinity will delay the 

 process. It is possible that the harmful effects sometimes noted in 

 alkaline media may be due, in part, to an induced iron deficiency. 



The influence of pH on the solubility of certain ions may be modified 

 by the presence of other compounds, especially those which form com- 

 plexes. The solubility of iron in alkaline solutions is greatly increased in 

 the presence of hydroxy organic acids such as citric, tartaric, and malic 

 acids. Ammonia and amino acids also form complexes with certain ions, 

 e.g., copper. The presence of any complex-forming compound may 

 modify the availability of the ions with which it forms complexes. The 

 chemical changes in media due to alteration of pH, whether imposed 

 from the outside or caused by the fungus, affect metabolic processes. 

 The pH of a culture medium changes during the growth of a fungus, and 

 these changes may affect the composition of the medium and thus the 

 response of the fungus. 



pH and oxygen supply. The solubility of oxygen in water is slight, 

 being less than 10 mg. per liter at 20°C. The rate of diffusion of oxygen 

 into media is dependent upon the composition and the pH. Rahn and 

 Richardson (1941) have described a simple and elegant method of measur- 

 ing the rate of diffusion of oxygen into agar media. Methylene blue, an 

 organic dye which is colorless when reduced and blue when oxidized, w^as 

 used as an indicator. When this dye (1/200,000) is autoclaved with 

 media which contain easily oxidized constituents such as glucose, the dye 

 is reduced to the leuco, or colorless, form. As oxygen diffuses into the 

 medium, the reduced dye is oxidized, and the rate at which the blue zone 

 advances into the medium is a measure of the rate of oxygen diffusion. 

 The pH of the medium also affects the ease with which certain constitu- 

 ents are oxidized. Some data of Rahn and Richardson (1941) on the rate 

 of oxygen diffusion into a peptone medium are shown in Fig. 28. The 

 amount of oxygen available to submerged mycelium is greater in acidic 

 than in alkaline media. 



Effect of pH on utilization of nutrients. Before any substance (ion 

 or molecule) is utilized, it must first pass through the cell wall and the 

 protoplasmic membrane. The cell wall is nonliving and consists of 

 polysaccharide-like compounds. For a discussion of the nature of the 

 cell wall and literature citations, see Brian (1949). The protoplasmic 

 membrane appears to be composed of proteins and lipoid-protein com- 

 plexes. Proteins are colloidal amphoteric compounds. An amphoteric 

 compound possesses both acidic and basic properties and may form salts 

 with either acids or bases. The protoplasmic membrane has acidic 



