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by cells - grown in hydrogen on a Grohmann medium containing 

 nitrate - in an atmosphere containing 50% H 2 , 10% O a , 40% N 2 in 

 the absence of nitrate, and curve B the gas absorption by the same 

 cells in a medium containing KNO s in an atmosphere of pure hydro- 

 gen. Taking into account that in the first case the amount of hydrogen 

 consumed is only two thirds of the gas consumption observed, and in 

 the second case five fourth of this consumption, it is clear that in both 

 conversions the hydrogen consumption is practically the same, im- 

 plying that the capacity of the hydrogenase is in both cases controlling 

 the rate of the reaction. 



Fig. 3 dealing with cells grown on a Grohmann medium without 

 nitrate in an atmosphere containing both H 2 and 2 shows that under 

 these conditions the cells maintain the ability to catalyze the 'Knall- 

 gasbakterien', and therefore contain hydrogenase (curve A), but lack 

 of nitratase practically prevents nitrate reduction with hydrogen 

 (curve B). 



We may summarize the foregoing by stating that the heterotrophic 

 denitrifying bacterium Micrococcus denitrificans produces under certain 

 conditions a hydrogenase which enables it to use molecular hydrogen 

 as a hydrogen donator in nitrate reduction. However, both nitratase 

 and hydrogenase are typical adaptive enzymes, and therefore resting 

 cells are only active, if both nitrate and hydrogen were available in 

 their cultivation. The bacterium can at the same time transfer hydro- 

 gen to free oxygen, or in other words it belongs to the physiological 

 group of 'Knallgasbakterien'. To a certain extent this heterotrophic 

 bacterium can be considered as a facultative autotroph, although 

 evidently certain organic growth substances are essential. 



THE INFLUENCE OF FREE OXYGEN ON NITRATE REDUCTION 



For all bacteria capable of a true dissimilatory nitrate reduction it is 

 characteristic that this property is a facultative one, in as far as they 

 are able to proliferate as well in the absence of nitrate, provided that 

 free oxygen is available. In other words nitrate and free oxygen are 

 interchangeable as hydrogen acceptors. This point of view - which 

 contrasts strongly with what holds for sulphate and carbonate reduc- 

 tion - lends a special interest to a study of the influence of free oxygen 

 on dissimilatory nitrate reduction. 



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