DENITRIFICATION 39 



hof [4] to be Optimal for nitrification (NH 4" =0-005 ^-J 

 H"^ = io-^ M.; N07=3-o3 m.). 



NHt + ii02=N07+H20 + 2H+, AF=-66-5 kg. cal. 

 N02+i02=N03, AF=- 17-5 kg. cal. 



The amount of carbon assimilated as the result of the oxida- 

 tion of a known amount of NH^ or NO 2^ is determined 

 experimentally, and assuming that the reduction of i gram 

 mole of CO 2 to CHgO (the generalized formula for cell sub- 

 stance) requires 118 kg. cal. of free energy, it can be calcu- 

 lated that the energy released in the oxidations is used with 

 an efficiency of S'9% by Nitrosomonas and 7*8% by Nitro- 

 bacter [4]. These values are only approximate, and in the 

 case of Nitrosomonas it is known that the efficiency falls as 

 the culture grows older [lo]. 



Reduction of nitrate: denitrification 



In contrast to the limited number of organisms capable 

 of oxidizing ammonia and nitrite, several species accom- 

 plish the reverse process, namely, the reduction of nitrate and 

 nitrite to ammonia, nitrous oxide (N2O) and molecular 

 nitrogen. Whilst ammonia may be retained in the organism 

 or in the medium, gaseous products such as Ng and NgO 

 pass readily into the atmosphere with the result that the 

 overall nitrogen content of the organism's immediate 

 environment is decreased. In the latter instance the bio- 

 logical reduction of NO 7 and NOT is often termed de- 

 nitrification. Examples of organisms known to reduce 

 NO 7 include Ps. fluorescens, Ps. denitrificans, Ps. stutzeri^ 

 Mia-ococcus denitrificans, various spore-forming bacilli, 

 Thiobacillus denitrificans, N. crassa, Hafisenula anonialay 

 Azotobacter agilis, Esch. coli and CI. welchii. The latter five 

 species appear to be incapable of taking the reduction as 

 far as NgO or Ng . 



With some organisms, NO"^ and NO^ serve as sources 



of nitrogen for the synthesis of organic nitrogen compounds: 



alternatively, or in addition, they may function as H- 



acceptors in reactions concerned with the organism's energy 



4 



