248 NITROGEN NUTRITION AND METABOLISM 



of ammonium carbonate or phosphate (177), increase in buffer con- 

 centration (122), use of mixed nitrogen sources (254), or inclusion of 

 utilizable organic acids in the medium. This last phenomenon has 

 given rise to much confusion in the literature, the improvement of 

 growth by use of organic acids in media relying on ammonium nitro- 

 gen being taken for a specific nutritional or metabolic effect of the 

 acid. Morton and MacMillan (375) correctly and with adequate evi- 

 dence dispose of this argument; the organic acid effect is a pH effect. 

 Illustrative evidence for the effect itself is abundant — work on the 

 Mucorales (29), higher basidiomycetes (275), and miscellaneous fungi 

 (221) may be considered typical. 



This point has been treated at some length because a requirement 

 for "organic nitrogen," i.e., inability to grow with ammonium nitro- 

 gen, may reflect only failure of the organism to grow at low pH. 

 Allomyces javanicus var. macrogynus (340) and Chytridium sp. (113) 

 illustrate this phenomenon. No claim of failure to utilize ammonium 

 nitrogen can stand unless the pH effect is excluded experimentally. 



Other possibilities of error in claims for a non-specific organic nitro- 

 gen requirement may be mentioned. The amino acids which appear 

 to be essential may be acting to reduce, by complex formation, the 

 inhibitory effect of heavy metals present in the basal medium; copper 

 is especially likely to be removed by amino acids (16, 17), and histidine 

 specifically antidotes cobalt toxicity (411, 462). Apparently essential 

 sources of organic nitrogen may be effective by virtue of their content 

 of vitamins (234,591). 



The apparent glutamic acid requirement of Allomyces javanicus var. 

 macrogynus disappears when the concentrations of glucose, potassium, 

 and minor elements are changed and sulfate is eliminated from the 

 medium (339). 



It seems therefore that many reports of failure to utilize ammonium 

 nitrogen (318, 343, 359, 506) must remain sub judice. There does 

 appear, however, to be in some of the lower phycomycetes — Blastocla- 

 diella emersonii (42), Sapromyces elongatus (204) and, probably, 

 Leptomitus lacteus (463) — a genuine inability to grow with am- 

 monium as the sole nitrogen source, but no specific amino acid re- 

 quirement. Contradictory and only semi-quantitative data on Apo- 

 dachlya brachynema (200, 461) do not permit of a decision. Finally, 

 the relatively few naturally occurring fungi with specific amino acid 

 requirements (p. 259) and the many known deficient mutants are, of 

 course, exceptions to the rule that fungi as a group can, at least under 

 specified conditions, grow with ammonium ion as the sole source of 

 nitrogen. 



