SOME ASPECTS OF NITRATE REDUCTION 



time good growth of a uniform type of colonies occurred, and by 

 replating a pure culture was obtained which proved to be quite active 

 in the original liquid medium, provided that here too some yeast 

 autolysate was added. For this pure culture it was found in Warburg 

 experiments in which a limited amount of nitrate was added that the 

 reduction in gas volume was in excellent agreement with what should 

 be expected on the assumption of a reduction of nitrate to gaseous 

 nitrogen according to equation (9). Herewith for the first time experi- 

 mental proof has been brought that there exist bacteria which can use 

 molecular hydrogen as a hydrogen donator in nitrate reduction lead- 

 ing to nitrogen. 



A further study of the pure culture in question showed at once that 

 the organism was not at all a specific autotrophic form of life. A vig- 

 orous denitrification could also be obtained with a large number of 

 the more commonly used organic hydrogen donators. Moreover, the 

 bacterium also showed excellent development under aerobic condi- 

 tions on peptone media with and without glucose, and in the absence 

 ofnitrate. 



Microscopic examination showed immotile coccus- to spindle- 

 shaped, Gram-, non-sporeforming cells. This together with its phys- 

 iological properties made it tempting to identify the organism with 

 the bacterium described by Beijerinck as Micrococcus denitrificans. It is 

 true that there remained one difference, viz., that the type strain of 

 the species had been found unable to reduce nitrate with molecular 

 hydrogen. 



However, the studies on 'Knallgasbakterien' [Ruhland, 1924; 

 Grohmann, 1924; Kluyver and Manten, 1942] have shown that the 

 property of activating gaseous hydrogen is in these bacteria easily - 

 and often apparently irreversibly - lost. Under these conditions there 

 seems to be no reason to reject the proposed identification. 



Although Schatz and Bovell [1952] report that in their Hydrogenomo- 

 nasfacilis hydrogenase is present independent of cultural conditions, it 

 seemed worth while to study for the newly isolated strain in how far 

 the ability to reduce nitrate with molecular hydrogen was dependent 

 on the preceding cultural conditions. 



In a series of experiments with resting cells performed by Mr. M. C. 

 A. van Nievelt it became evident that for a realization of nitrate re- 

 duction with molecular hydrogen at least three conditions have to be 



497 



