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It is worthwhile to add that this behaviour is not at all restricted to 

 Denitrobacillus licheniformis. On studying the denitrification process 

 caused by Pseudomonas aeruginosa with glucose as hydrogen donator it 

 was found that under normal conditions the nitrous oxide content of 

 the gas evolved always remained below 20%. However, on applying 

 the vacuum technique nitrous oxide contents of 70% and higher were 

 regularly observed. It may be remarked that this evidently means a 

 new and elegant way of trapping an intermediate product in metab- 

 olism. 



All the foregoing considerations taken together seem to justify the 

 reaction scheme (preceding page) as given by Verhoeven [1952]. 



FATE OF THE HYDROGEN DONATOR 



The unmistakable analogy between the bacteriological processes of 

 sulphate, of carbonate and of nitrate reduction seemed to suggest that 

 also in the latter process incomplete dehydrogenations might occur. 

 For sulphate reduction this phenomenon had been discovered by 

 Baars, and Barker found an analogous situation in carbonate reduc- 

 tion, the process better known as methane fermentation. On the other 

 hand nitrate reduction differs from the two other processes by the 

 fact that the occurrence of incomplete hydrogenation stages of the 

 nitrate is quite common, whilst in sulphate reduction only the ultimate 

 hydrogenation product of sulphur: SH 2 , and in carbonate reduction 

 only the analogous hydrogenation product of carbon: CH 4 are en- 

 countered. This situation suggests the possibility that in this type of 

 processes incomplete dehydrogenation and incomplete hydrogenation 

 are mutually exclusive. 



As far as known the only publication in which the fate of the organ- 

 ic hydrogen donator in true dissimilatory nitrate reduction has been 

 the subject of a thorough investigation is the article by Sacks and 

 Barker [1952]. These authors made carbon balances of the denitrifi- 

 cation caused by Pseudomonas denitrificans in a medium in which sodium 

 succinate and glutamic acid were the only organic compounds present. 

 The results were quite convincing: approximately 90% of the carbon 

 of the organic substrates consumed was found back as carbon dioxide; 

 the remaining 10% could be accounted for as the carbon present in 

 the bacteria formed. 



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