INORGANIC SOURCES OF NITROGEN 



243 



one could ask for more quantitative data (15, 306, 419, 534, 569, 570), 

 but some strains definitely cannot grow with nitrate as the sole source 

 of nitrogen (138, 144, 393, 541). 



It is unlikely in the extreme that inability to use nitrate can serve 

 as a phylogenetic criterion. Within a genus, individual species differ 

 in nitrate utilization (150, 327, 397, 610). The capacity to use nitrate 

 can be lost by mutation (250, 255, 420, 428, 455), and survival in nature 

 of strains carrying this mutation is presumably governed by their 

 ecological situation rather than their taxonomic position. 



Failure to utilize nitrate is usually presumed to be absolute, but 

 there has been little exploration of the possibility that nitrate can be 

 metabolized once growth is initiated on another more readily available 

 nitrogen source. When spores of Streptomyces griseus are inoculated 

 into a synthetic nitrate medium, no growth occurs, but a pregrown 

 mycelium uses nitrate readily (105). 



As mentioned below, nitrite may be toxic in an acid medium. It 

 follows that the kind and the concentration of the carbon source, 

 insofar as these affect the pH, will decisively influence the response 

 of some organisms, e.g., Streptomyces coelicolor (107), to nitrate. 



Ammonium nitrate is often used in media, and it is of interest to ask 

 whether there is preferential utilization of either ion. The frequent 

 experience that utilization of ammonium nitrate is accompanied by a 

 pH drop (227, 256, 409, 415, 529) is presumptive evidence for prefer- 

 ential utilization of the ammonium ion. More direct evidence is pro- 

 vided, however, by analytical studies. In Scopulariopsis brevicaulis 

 and other fungi, nitrate utilization does not begin until virtually all 

 the ammonium has disappeared from the medium (Figure 2); this is 



Figure 2. The utilization of 

 ammonium and nitrate by 

 Scopulariopsis brevicaulis in a 

 medium containing glucose, suc- 

 cinate, and ammonium nitrate. 

 Curve 1, nitrate concentration 

 in the medium; curve 2, ammo- 

 nium concentration in the me- 

 dium. From tabular data of 

 Morton and MacMillan (375). 



2 4 6 8 



Time, days 



