182 THE NITEOGEN CYCLE 



heat or caustic chemicals stopped the nitrification process 

 so that ammonia passed through uncha'nged. If the passage 

 of the sewage effluent were maintained, then the culture was 

 slowly re-established and nitrification resumed after a few days. 

 It was thirteen years after this demonstration that the first 

 nitrifying organism was isolated in pure culture. In Chap. V 

 the nitrifying organisms Nitrosomonas and Nitrobacter aie 

 described as strict autotrophes whose growth is inhibited by 

 the presence of organic matter. Since it was the custom in 

 those days to attempt the isolation of organisms by growth 

 on the surface of solidified gelatine, it is understandable why 

 no successful isolation of nitrifying bacteria was achieved 

 for some time. It was not until Winogradsky invented the 

 purely inorganic solid medium consisting of inorganic salts 

 incorporated in a silicic acid gel that the first successful 

 isolation of a nitrifier was accomplished. It then became 

 obvious that two organisms are involved in the nitrifying 

 process, the first (Nitrosomonas) carrying out oxidation of 

 ammonia to nitrite, and the second (Nitrobacter) completing 

 the oxidation of nitrite to nitrate. The two processes are not 

 interchangeable, for while Nitrosomonas can grow on a 

 synthetic medium containing ammonia as source of nitrogen 

 and energy, 



2NH3 + 3O2 > 2HNO2 + 2H2O + 79 Cals. 



it cannot grow on a medium containing nitrite but no ammonia. 

 Similarly, Nitrobacter cannot grow in a nitrite-free medium, but 

 must obtain its energy from the oxidation of nitrite to nitrate : 



HNO2 + > HNO3 -f 21-6 Cals. 



The process of nitrification is peculiar to these two organisms, 

 and can consequently only take place when conditions are 

 suitable for the growth of strictly autotrophic organisms. 



(d) Reduction of nitrate to ammonia 



This change is one that can be accomplished by several 

 organisms, including Esch. coli and CI. welchii. In the presence 

 of a hydrogen donator, Esch. coli can reduce nitrate to nitrite 



