248 AGBIOULTURAL AND INDUSTRIAL BACTERIOLOGY 



eluded in the main cycle which are of great importance 

 from an economic point of view. These are denitrification 

 and nitrogen fixation. 



Denitrification. — Denitrification is a term applied to the 

 reduction of nitrates to nitrites and to the process of 

 evolution of free atmospheric nitrogen from either nitrates 

 or nitrites by the activity of microorganisms. Like the 

 term nitrification it is a misnomer, as the process is essen- 

 tially a reduction. 



Nitrates are transformed to nitrites by certain kinds of 

 bacteria when placed under anaerobic conditions and in the 

 presence of certain easily oxidizable substances. For 

 example, in the presence of organic material and the 

 absence of free oxygen, certain microorganisms remove 

 oxygen from the nitrate molecule, reducing it to nitrite, and 

 use the oxygen thus obtained for oxidizing carbon com- 

 pounds. A reaction of this type may be illustrated as fol- 

 lows : 



C.H^^Oe + I2HNO3 = 6CO2 + 6H,0 + I2HNO2 



As a result of this action energy is secured for growth. The 

 reduction of the nitrate requires less energy than is secured 

 by the oxidation of the carbon compounds, hence there is a 

 net gain in energy to the microorganism. 



Many bacteria are known which bring about this trans- 

 formation. Most of them are termed facultative, as they 

 will grow well under normal aerobic conditions, but do not 

 grow in the absence of atmospheric oxygen unless an ade- 

 quate supply of this element can be secured from sources 

 such as nitrates. The reduction of nitrates to nitrites is 

 often observed in the laboratory and is a valuable means of 

 differentiation of bacteria from each other. 



Certain bacteria, the true denitrifiers, are capable of 

 bringing about a reduction of nitrites with the evolution 

 of free nitrogen gas. The chemistry of this transformation 



