182 PLANT PHYSIOLOGY 



organic residues into nitrites and further into nitrates. The 

 classical investigations of Vinogradsky (1889) have proved that 

 this oxidation is accomplished by two groups of bacteria. The 

 first group of nitrite bacteria includes the genus Nitrosomonas. 

 These bacteria are small mobile cocci that oxidize ammonia to 

 nitrous acid according to the equation: 



2NH3 + 3O2 = 2HNO2 + 2H2O + 158 cal. 



The other group of nitrate bacteria includes the genus Nitro- 

 hader, a nonmobile short bacillus that oxidizes the nitrous acid, 

 prepared by the bacteria of the first group, into nitric acid accord- 

 ing to the equation: 



2HNO2 + 02 = 2HNO3 + 38 cal. 



It is obvious from these equations that the second process 

 gives much less energy than the first oxidation step. Nitrite 

 bacteria oxidize on the average 35 molecules of ammonia for 

 the assimilation of 1 atom of carbon, while the nitrate bacteria 

 require for the assimilation of the same amount of carbon 135 

 molecules of nitrous acid. 



By the oxidation of nitrogenous compounds as their source of 

 energy, and using carbon dioxide of the air as their source of 

 carbon, the nitrifying bacteria do not require any other organic 

 compounds as respiratory materials, and they are not capable 

 of using them in respiration or sjnithesis. Moreover, organic 

 substances indispensable to the respiration of most organisms, 

 including even the sugars, are toxic to nitrifying bacteria. In 

 spite of this singularity in metabolism, these bacteria are com- 

 posed of the same compounds as other plants; e.g., they contain 

 the same protein substances. That carbohydrates participate 

 in the structure of their protoplasm is an hypothesis that remains 

 as yet insufficiently investigated. 



Nitrifying bacteria represent a special physiological type, 

 sharply differing from the green autotrophic plants as well as from 

 the usually colorless heterotrophic saprophytes and parasites. 

 The exothermic process of oxidation of ammonia into nitrous 

 acid is used by the nitrifying bacteria both for the primary 

 synthesis of organic substances and for the maintenance of other 

 vital processes, such as growth and movement; such processes 

 are maintained in other organisms, autotrophic as well as hetero- 



