STUDIES ON AZOTOBACTER CHROOCOCCUM BEIJ. 



AUGUSTO BONAZZI 



Contribution from the Ohio Agricultural Experiment Station, Wooster, Ohio, 

 Laboratory of Soil Biology 



Received for publication October 15, 1920 

 I. GENERAL 



Introduction 



The study of the metabolism of Azotobacter has been gen- 

 erally approached from the standpoint of the role of this organ- 

 ism in the nitrogen cycle in Nature, but the fact that the abiHty 

 to fix free nitrogen is regulated by the presence or absence of 

 combined nitrogen in the medium has not been given serious 

 attention although it is made plain in the works to be cited. 



Bejierink and Van Delden (1902) have shown that Azotobacter 

 chroococcum possesses the power to transform nitrates directly 

 into ammonia and Lipman (1903), Stoklasa (1908), Stranak 

 (1909) and Heinze (1906) found that small quantities of nitrates 

 ^'stimulated" nitrogen fixation by this organism. If the property 

 of nitrogen fixation were a function of the normal hfe cycle, it 

 seems strange that, although of vital importance, it should be 

 overcome with such facihty. In this connection the data pre- 

 sented by Hills (1918) are most instructive. The accompanying 

 table is a recalculation of the data given by him on pages 200- 

 203 of his contribution, and is chosen as it is the only one given 

 which was obtained by the use of a synthetic medium. 



From this compilation we see that in presence of abundant 

 stores of nitric nitrogen Azotobacter does not fix atmospheric 

 nitrogen but assimilates the nitrogen of the nitrates. A close 

 study of the original data shows that in the presence of ammo- 

 niimi nitrate the organism has a preference for the nitrate radicle 

 leaving the ammonium radicle untouched. 



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