102 PHYSIOLOGY OF THE FUNGI 



form of a particular amino acid. Cantino (1949) found that Blastocladia 

 pringsheimii is deficient for methionine and perhaps other amino acids. 

 Presumably other amino acids are used to supply a portion of the metabolic 

 nitrogen of this species. The same situation may exist in the nitrogen 

 utilization of amino-acid-deficient mutants of Neurospora. Leonian and 

 Lilly (1938) reported Coprinus lagopus and Pleurotus corticatus to grow on 

 a mixture of five amino acids and not on ammonium nitrate as a source of 

 nitrogen. 



Inorganic sources of nitrogen. The nitrates commonly used in prepar- 

 ing media are potassium nitrate, sodium nitrate, and calcium nitrate. 

 These salts are equivalent in so far as they supply the same kind of nitro- 

 gen. They are not equivalent in that different cations are involved. 

 Calcium ion may precipitate a varying amount of phosphate, depending 

 upon the concentrations of the two ions and the pH of the medium. 



Some fungi utilize nitrite (N02~) nitrogen. Blakeslea trispora makes 

 some growth on nitrite nitrogen (Leonian and Lilly, 1938). Owing to the 

 instability of nitrites in acid solution and the destructive effect of nitrous 

 acid on proteins and amino acids, nitrite nitrogen is little used in making 

 media. Nitrite is produced by many fungi from nitrate and may accumu- 

 late in the medium under certain conditions. The toxic effect is related 

 to the pH of the medium, being greatest at low pH. Wirth and Nord 

 (1942) attributed the accumulation of pyruvic acid in the nitrate medium 

 on which Fusarium lini grew to the presence of the nitrite, which inac- 

 tivated thiamine pyrophosphate (cocarboxylase). 



Yeasts utilize nitrate nitrogen poorly as a general rule. Pirschle (1930) 

 studied the relative value of nitrate and ammonium nitrogen for a yeast 

 and concluded that poor utilization of nitrate nitrogen was due in part to 

 the accumulation of nitrite in the medium. This was shown by the yields 

 of aerated and nonaerated cultures on media containing nitrate and 

 ammonium nitrogen as well as by analyses of the culture medium for 

 nitrite. Aeration prevented the accumulation of toxic amounts of nitrite 

 or its decomposition product nitrogen trioxide. In other experiments 

 Pirschle showed that nitrite inhibited the growth of yeast on ammonium 

 nitrogen. By adding sufficient nitrite to a medium containing ammonium 

 sulfate, growth was depressed below that obtained on potassium nitrate. 

 How far these conclusions may be applied to other fungi which do not 

 utilize nitrate nitrogen is not known. 



Inorganic and organic ammonium salts are equivalent in that they 

 furnish inorganic nitrogen; i.e., ammonium ion. The nitrogen of all 

 ammonium salts is the same, but the physiological effects of the anions 

 are not. The ammonium salts of strong inorganic acids generally tend to 

 make a culture medium more strongly acidic than when an ammonium 

 salt of a weak acid is used. However, the situation is far more compli- 



