NITRATE FORMATION 137 



results, first giving rise to nitrite; when a large part of the ammo- 

 nia has disappeared, nitrate is formed in increasing amounts at 

 the expense of the nitrite. 



The two groups of bacteria concerned in the transformation are 

 thus quite distinct from one another, but they are alike physio- 

 logically in that they are both autotrophic, that is, they do not use 

 organic substances as food but can use the energy liberated in the 

 specific oxidation processes for their growth and development. 

 The carbon necessary for the synthesis of their cells is taken from 

 the carbon dioxide of the atmosphere and used in a manner similar 

 to its utilization by green plants. 



CO2 + H2O = HCHO + O2 



Formalde- 

 hyde 



6HCH0 = CgHi20g (sugar) or other organic compounds. 



The organisms forming nitrite from ammonia have been 

 designated as Nitrosomonas or Nitrosococcus, depending upon their 

 morphology. The organisms producing nitrate from nitrite are 

 called Nitrobacter. These forms are all very similar in morphology 

 being spherical or short oval cells, either motile or non-motile 

 (see Figs. 13 and 14). The difficulties involved in isolating 

 these organisms in pure culture have delayed the description of 

 more than a few species. The fact that very few species have been 

 obtained is of less significance than the fact that nitrifying organ- 

 isms exist in practically all soils. Since Winogradsky first obtained 

 the organisms in 1891, they have been found to be active prac- 

 tically everywhere, with the exception of certain acid forest and 

 peat soils. After drainage, cultivation, and liming, even these 

 soils can be made a favorable medium for nitrification. 



The optimum reaction for the activities of the organisms is 

 about pH 7.0 to 8.0, but some strains will still grow at as high an 

 acidity as pH 3.5 and at an alkalinity of pH 10.0. Under condi- 

 tions of increasing acidity or alkalinity there develop certain 

 acid-tolerant or alkali-tolerant strains which have different reaction 

 requirements from those strains commonly encountered in most 

 arable soils. However, at an acidity greater than pH 3.5, the 

 nitrifying bacteria practically cease functioning. Acidity is but 

 one of the factors inhibiting development of nitrifying organisms in 

 acid peat bogs. The limited supply of oxygen in the water- 



