SELECTED PAPERS 



usually stated that it is completely oxidised to carbon dioxide and 

 water, but the documentation for this statement can scarcely be con- 

 sidered satisfactory. Finally our knowledge of the catalytic systems 

 active in the dissimilatory process is still very incomplete. 



FATE OF THE HYDROGEN ACCEPTOR 



Almost from the beginning of the study of dissimilatory nitrate re- 

 duction it has been clear that the nitrogen of the consumed nitrate 

 can be recovered in different reduction stages depending both on the 

 bacterial species used, and on the experimental conditions. This has 

 naturally led to the idea that these various reduction stages are just 

 intermediates in the conversion of nitrate into the ultimate reduction 

 stage of the nitrogen : ammonia. A critical examination of the experi- 

 mental evidence for this point of view seems appropriate. 



Taking into account that in the dehydrogenation of organic sub- 

 strate nearly always two hydrogen atoms are removed simultaneously, 

 it is difficult to escape the conclusion that the first step in nitrate re- 

 duction will lead to the formation of nitrite : 



(i) HNO3+2H -» HN0 2 +H 2 



This view is strongly supported by the fact that in experiments on dis- 

 similatory nitrate reduction as a rule the presence of nitrite in the 

 medium can easily be established, although mostly in small quantities 

 only. It is true that bacterial species have been described which ap- 

 parently are unable to bring about a further conversion of the nitrite, 

 so that in such a case this compound accumulates in the medium. 

 However, relatively low concentrations of nitrite usually have a toxic 

 effect on the organisms, so that nitrate reduction remains incomplete, 

 thus proving the suicidal character of the process. 



Accepting nitrite formation as the first step in nitrate reduction it 

 is only logical to postulate as the second step : 



(2) HN0 2 +2H -> HNO+H 2 



It should be remarked at once that any direct experimental evidence 

 for the correctness of this assumption is lacking, in so far as the occur- 

 rence of a compound HNO in a denitrification medium has never 

 been demonstrated. This, however, is not surprising since the studies 

 of Raschig and others leave no doubt that the compound which they 



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