526 PRINCIPLES OF SOIL MICROBIOLOGY 



phere free from C0 2 (but containing C0 2 in the medium), although at 

 a slower rate. The cells of the organism are strongly catalytic and 

 are capable of liberating oxygen from a peroxide. Ferric hydrate has 

 a stimulating effect upon the oxidation of ammonia to nitrate. 26 Since 

 iron was found in active cultures partly in a ferrous state, Bonazzi 

 suggested that it acts as a carrier of oxygen, thereby hastening the 

 oxidation process; iron fulfills in these cultures the functions of the 

 peroxide, due to its mechanism of auto-oxidation resulting in a com- 

 bination with oxygen, while the cells liberate the oxygen thus bound. 



Fe" + O -> Fe"' • O 



Fe"' • O + peroxidase -f NH 3 -> HN0 2 + peroxidase + Fe". 



In 100 cc. of medium and at 35°, 20 mgm. of nitrogen in the form of 

 ammonium sulfate can be oxidized in 6 hours. Nitrite formation 

 comes to a standstill, when the solution contains about 1.5 to 2.0 per 

 cent N0 2 . The concentration of the substrate which is most favor- 

 able for the oxidation process lies at 0.005 M NH4, while with a tenth 

 molar solution of ammonia oxidation is nil. 



The more recent theories tend to indicate that the oxidation of am- 

 monia to nitrous acid goes through the hydroxylamine and hyponitrous 

 acid stages. 26 * 



/H 

 H /H 



N(-H + O -> N^-H + H 2 0, 

 \\H M)H 



\OH Hydroxylamine 



/H .H /H /H 



Nf-H + O -» N< , N/ + H 2 -* N^-OH 

 M)H ^O ^O X)H 



/H Jd 



\OH x OH 



Mechanism of nitrite oxidation. The oxidation of nitrite to nitrate 

 takes place according to the following reaction: 



NaN0 2 + h (0 2 ) = NaN0 3 



This was demonstrated to hold true by measuring the nitrite and oxy- 

 gen consumption. With optimum concentration of the nutrients and 



28 Ashby, 1907 (p. 394). 



26a Kluyver, A. J., andDonker, H. J. L. Die Einheit in der Biochemie. Chem. 

 d. Zelle u. Gewebe., 13: 134-190. 1926. 



