SOME ASPECTS OF NITRATE REDUCTION 



In recent studies there is an increasing tendency to detract from the 

 importance of nitrous oxide as a regular product in dissimilatory 

 nitrate reduction. Sacks and Barker [1952] state explicitly that they 

 were unable to detect any nitrous oxide in their denitrification ex- 

 periments with their strain of Pseudomonas denitrificans. Moreover, they 

 conclude that nitrous oxide cannot be a normal intermediate in the 

 formation of nitrogen, because its utilization may be selectively blocked 

 by azide or dinitrophenol under conditions which permit the forma- 

 tion of nitrogen from nitrite, and because a lag frequently precedes 

 the utilization of nitrous oxide by resting cells, which lag is not 

 found in the conversion of nitrite into nitrogen. In their extensive 

 study of dissimilatory nitrate reduction by Pseudomonas stutzeri Allen 

 and Van Niel are initially inclined to hold the opposite view, since 

 they established a simultaneous enzymatic adaptation to nitrate, 

 nitrite and nitrous oxide. However, they finally reject the idea on 

 basis of the observation that the conversion of nitrous oxide into ni- 

 trogen is inhibited by 3 X io -4 m cyanide, whilst under the same 

 conditions nitrite is not converted into nitrous oxide - as might be 

 expected on the basis of the intermediate theory - but quite normally 

 into nitrogen. 



The arguments of both groups of American investigators seem con- 

 vincing. There may, perhaps, be one way of escape. Apparently the 

 possibility has not been considered that in the nitrite containing media 

 azide and cyanide in the very low concentrations in which they are 

 used are either completely or at least partly removed by a chemical 

 reaction with the excess of nitrite. If this would apply, nitrous oxide 

 could be restored in its position of a regular precursor of gaseous 

 nitrogen. 



For the moment we may accept that at least under certain condi- 

 tions nitrous oxide does not precede nitrogen, implying that nitrogen 

 gas can originate from a direct hydrogenation of the compound 

 H 2 X 2 2 according to equation (7). 



On the other hand I wish to emphasize that in certain cases there 

 is not the slightest doubt that nitrous oxide is a direct precursor of 

 nitrogen in dissimilatory nitrate reduction. The experimental proof 

 for this statement has been brought for bacterial species which under 

 normal cultural conditions are more disposed to produce nitrous 

 oxide than holds for both Ps. denitrificans and Ps. stutzeri. A special 



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