SOME ASPECTS OF NITRATE REDUCTION 



dealing with a typical case of incidental dissimilatory nitrate re- 

 duction. 



For our purpose it is only of importance to consider the question 

 whether bacteria which can derive their energy from the system or- 

 ganic hydrogen donator and nitrate can do so also from the system 

 molecular hydrogen and nitrate. The first indication that such organ- 

 isms exist can be derived from an observation of Lebedeff [1909] who 

 established that in cultures of bacteria oxidizing molecular hydrogen 

 with free oxygen the nitrate present in the medium was also reduced 

 and partly converted into gaseous nitrogen. The subject was more 

 systematically studied by Niklewski [19 14] who in several publications 

 gave a convincing demonstration that a pure culture of his Hydrogen- 

 omonas agilis is able to derive its energy from the system: hydrogen/ 

 nitrate. Niklewski emphasizes that his bacterium is also characterized 

 by a heterotrophic mode of life as far as free oxygen is available. Al- 

 though Niklewski is inclined to conclude that H. agilis can reduce 

 nitrate with molecular hydrogen only, the documentation for this 

 statement is very meagre. 



A critical examination of this point did not seem to be superfluous, 

 and as a first step to this it seemed worth while to investigate whether 

 typical denitrifying bacteria which are able to use very different organic 

 hydrogen donators can also use molecular hydrogen in nitrate reduction. 



This investigation was made by Mr. A. L. Koster. For a first quick 

 orientation the 'resting cell' technique seemed most suitable; one 

 might expect that denitrifying organisms which could derive their 

 energy from the molecular hydrogen/nitrate system would in any case 

 be able to bring about in the Warburg apparatus the conversion : 



(9) 2HN0 3 +5H 2 ->N 2 +6H 2 



However, from earlier manometric studies on denitrification (Van 

 Olden, 1940), later confirmed by Pollock [1946] and by Wainwright 

 and Pollock [1949] it has resulted clearly that the nitrate reducing 

 enzyme: nitratase is a typical adaptive enzyme, so that resting cells 

 only reduce nitrate without considerable lag, if they have originated 

 in a nitrate containing medium. Moreover, an analogous situation has 

 been proved to exist in the bacteria oxidizing hydrogen with free 

 oxygen for the enzyme system which activates molecular hydrogen: 

 hydrogenase. 



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