HYDROGEN-ION CONCENTRATION 163 



ion concentration of a nutrient solution may change 10,000-fold during a 

 few days as a result of the metabolic activities of a fungus. These 

 changes in pH are due to changes in the relative amounts of acids and 

 bases formed or withdrawn and to the ionization constants of these 

 compounds. Some of the metabolic processes which result in a change in 

 pH of a nutrient solution are discussed below. 



The utilization of cations, such as ammonium ion, for the synthesis of 

 protoplasm or for any other purpose whereby essentially non-ionic com- 

 pounds are formed, leaves an equivalent number of anions in the nutrient 

 solution. Since solutions are electrically neutral, an equivalent number 

 of both cations and anions must be present. Thus, when an equivalent 

 of ammonium ion is transformed into non-ionic compounds, an equivalent 

 of some other cation or cations will be formed in the nutrient solution. 

 These ''new" cations are usually hydrogen ions, which are formed from 

 water. If it is assumed that both cations and anions are adsorbed on 

 the protoplasmic membrane, the process may be thought of as replace- 

 ment. The production of acid would result from the utilization of other 

 cations as well. 



The utilization of nitrate ion or other anion such as phosphate or 

 sulfate for the formation of non-ionized compounds has the effect of 

 increasing the hydroxyl-ion concentration of the medium. We may 

 assume the same type of mechanism as before, except that the anion 

 released to the nutrient solution is the hydroxyl ion. 



Fungi produce acids from nonacidic nutrients such as carbohydrates. 

 Among these acids are carbon dioxide and various organic acids such as 

 pyruvic, citric, and succinic acids. Carbon dioxide combines with water 

 to form carbonic acid, which is unstable in the presence of stronger acids 

 and decomposes to set free carbon dioxide. Under alkaline conditions 

 carbonic acid reacts with bases to form bicarbonates. Pyruvic acid 

 accumulates in the nutrient solution in which many fungi are grown, and 

 in some instances the formation of this acid accounts for a considerable 

 part of the early depression of pH. The eventual utilization of pyruvic 

 acid causes the pH of the nutrient solution to rise. Other metabolizable 

 acids behave similarly. Ammonia is, perhaps, the most common basic 

 substance produced by fungi. Piricularia oryzae produces ammonia in 

 considerable amounts (Henry and Andersen, 1948). The production of 

 ammonia results from the deamination of amino acids and proteins. The 

 processes discussed above may occur simultaneously. Whether a culture 

 solution becomes more acid or alkaline depends upon the extent of these 

 various processes. In general, the processes which produce acid pre- 

 dominate during early growth, especially when ammonium nitrogen is 

 used. 



The importance of the composition of the medium in determining what 



