246 



NITROGEN NUTRITION AND METABOLISM 



The Utilization of Nitrite Nitrogen. Nitrite serves more or less 

 well as the sole source of nitrogen for a number of fungi, e.g., Fusarium 

 niveum (622), Coprinus spp. (165), Phymatotrichum omnivorum (520), 

 Scopulariopsis brevicaulis (375), and Rhizophlyctis rosea (424). Dif- 

 ferent species of Aspergillus use it well, poorly, or not at all (305, 457, 

 494). Failure to utilize nitrite, which is the more common situation 

 in fungi, is explained on the basis of its known toxicity (233, 402, 623). 

 The fact that growth in the presence of nitrite is best in alkaline 

 media (107, 402, 454) indicates that it is the free unionized acid, rather 

 than the nitrite ion, which is the toxic species. Nitrite toxicity to 

 Fusarium lint is evidenced by accumulation of pyruvic acid (396); 

 induction by nitrite of morphological variants in Aspergillus spp. also 

 suggests toxicity (495, 497, 498). 



The Utilization of Ammonium Nitrogen. Studies on Scopulariop- 

 sis brevicaulis (336, 338, 375) provide the best general view of the 

 physiology of ammonium utilization. Both oxygen and an exogenous 

 carbon source are required, i.e., assimilation depends on respiratory 

 energy or glucose breakdown products or both. Ammonium utiliza- 

 tion increases with pH, and, unlike nitrate absorption, has no definite 

 pH optimum (Figure 3). Ammonia enters and leaves the cell by 

 passive diffusion of the undissociated NH 3 molecule; the requirement 

 for respiration is not therefore an indication of accumulation against 

 a gradient. 



In organisms generally (123, 558), in Torulopsis utilis in par- 

 ticular (620), it is believed that the major and primary reaction of 

 ammonium assimilation is the formation of glutamic acid. Other 



Figure 3. The rate of uptake of 

 ammonia and nitrate by Scopu- 

 lariopsis brevicaulis as a function 

 of pH. Curve 1, ammonia (two 

 buffers); curve 2, nitrate. From 

 Morton and MacMillan (375), by 

 permission of the Oxford University 

 Press. 



