118 Perspecfives in Microbiology 



quires for success that an external source of nitrogen come 

 into some sort of equilibrium with its opposite number 

 inside the cell or even in an enzyme reaction chain. Often, 

 such equilibration does not occur, so if a negative result is 

 obtained it may not mean much; but, to our surprise, it 

 succeeded (8). Azotobacter cells fixing ordinary N2 were 

 furnished with a small external supply of labeled ammonia; 

 at intervals, aliquots were taken and the level of N^^ de- 



Table I. Dilution of Exogenous Ammonia by Azofobacfer vinelandli 



Atom % Excess N^^ in 



* Reisolated by alkaline distillation from culture supernate before 

 complete utilization occurred. 



t 18-hour culture supplied 40 jxg/ml N15 as ammonium acetate at time 

 (31.9 atom % excess Ni5). 



X 19-hour culture supplied 20 pig/ml Ni4 as normal ammonium acetate. 

 Gas phase 0.1 atm N^15 (31.9 atom % N15), 0.5 atm O2. 



termined. As shown in table I, the label in the external 

 source of nitrogen decreased rapidly and steadily, indicat- 

 ing that it was equilibrating, with N^*H4+ appearing as 

 a result of fixation. Likewise, when supplied N2-^^, the 

 organism equilibrated fixed N^^H4+ with an external 

 source of N^^H4+. Although not so clear-cut evidence as 

 that obtained with C. pasteurianum, it is a welcome addi- 

 tion to the several other types of findings that implicate 

 ammonia as the key end product in nitrogen fixation by 

 Azotobacter, 



