250 NITROGEN NUTRITION AND METABOLISM 



Cyanide as a Metabolite. Cyanide is, of course, an effective respira- 

 tory poison, acting on iron enzymes. It has also, however, been re- 

 ported to provide assimilable nitrogen to preformed mycelium of As- 

 pergillus niger (271) and to cause an apparent increase in growth of 

 Fusarium lini (395, 437). Data so far available, however, are not ade- 

 quate for a final conclusion on the utilizability of cyanide as a nitrogen 

 source; it is to be noted that Chughtai and Walker (104) found that 

 the growth-promoting effect of cyanide is not duplicated by nitrate; 

 they suggest a metabolic disturbance which diverts more carbon into 

 dry matter. An unidentified soil organism, probably an aerobic acti- 

 nomycete, makes visible growth with cyanide as the sole source of 

 both carbon and nitrogen (578). 



The liberation of hydrogen cyanide from the fruit bodies of basidio- 

 mycetes has been noted repeatedly; reports are reviewed by Bach (38), 

 Miiller (379), Robbins et al. (444), and Zellner (629). An unidentified 

 Mucor sp. was reported in 1916 to form both cyanide and benzalde- 

 hyde (219). Cyanide is produced in culture, but apparently only dur- 

 ing autolysis, not during growth (444). Fruit bodies of Pholiota aurea 

 do not contain free cyanide when freshly collected, and circumstantial 

 evidence indicates that it is released from another compound by an 

 enzymatic reaction (37). 



Production of cyanide by an unidentified basidiomycete growing 

 under snow cover is apparently responsible for crown rot in alfalfa; 

 the fungus in culture produces cyanide at low temperature (319, 320). 



2. PROTEINS AND PEPTIDES 



Nutritional Aspects. The fungi and actinomycetes as a group are 

 active in the decomposition of proteins in soil and other materials (1, 

 568, 571, 572, 573). In pure culture it appears that proteins such as 

 gelatin, casein, and egg albumin can serve as sources of nitrogen for 

 at least the common saprophytic fungi and actinomycetes (278, 574); 

 much more data are available on enzymatic destruction of common 

 proteins than on their use for growth. Utilization of keratin is 

 common only among dermatophytic fungi and, possibly, chytrids 

 (Chapter 3). Whether fungi can attack highly purified proteins in 

 the absence of any other nitrogen source is in doubt. 



Commercial peptone — a mixture of peptides of varying chain 

 length — is generally utilizable, but in recent years has been replaced 

 in culture media meant for physiological studies by materials of 

 known composition. Dipeptides and tripeptides support the growth 

 of Aspergillus spp. (2, 531) and Ustilago spp. (62, 470); the histidine 



