CHAPTER III 



NITRIFICATION AND DENITRIFICATION 



Nitrification 



Nitrosomonas and Nitrohacter are two genera of strictly 

 aerobic chemosynthetic autotrophs which respectively 

 obtain their energy by the oxidation of ammonia to nitrite 

 (NOo), and nitrite to nitrate (NO3 ). 



Nitrosomonas, NH3+iJ02=HN02+H20 

 Nitrohacter, N02+i02=N07 



These organisms play an important role in the formation 

 of NO"^ from NH3 and organic nitrogen compounds, a pro- 

 cess occurring in soil and the beds of sewage purification 

 works [7] and known as nitrification. Most organic nitrogen 

 compounds are only nitrified by Nitrosomonas an^ Nitro- 

 hacter after they have been degraded by heterotrophs to 



NH3. 



The first evidence in favour of Pasteur's suggestion that 

 nitrification was due to micro-organisms came from the 

 classic experiments of Schloessing and Muntz [27]. After 

 sewage effluent had percolated through a column of sand 

 and chalk for about twenty days, they noted that NH3 was 

 being converted almost quantitatively into NO"^. This con- 

 version was completely stopped if the column was subjected 

 to conditions injurious to life, e.g. heat or chloroform, but 

 commenced again after washing with non-sterile water 

 derived from soil. Of the organisms known at that time, 

 none could oxidize NH3 to NO 7, and the first attempts to 

 isolate the causative agents by plating soil on nutrient 

 gelatin media all ended in complete failure. The lack of 

 success was traced to the inhibitory effect of organic sub- 

 stances on the growth of nitrifying organisms and resulted 

 in the introduction of media containing only inorganic 



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