SELECTED PAPERS 



species one brings about a complete, and the second one an incomplete 

 dehydrogenation of an organic substrate (acetic acid bacteria, sul- 

 phate reducing bacteria, etc.). For this reason we have extended these 

 experiments to both other bacterial species, and other substrates. 



In this respect Verhoeven has given special attention to various 

 alcohols. It should be remembered that Baars in sulphate reduction, 

 and Barker in carbonate reduction established an almost quantitative 

 conversion of ethanol into acetic acid. Using Pseudomonas aeruginosa 

 Verhoeven met with partial success, in so far as he succeeded in detect- 

 ing acetic acid, propionic acid and acetone as incomplete dehydrogen- 

 ation products in media containing ethanol, propanol and isoprop- 

 anol respectively. It should be added, however, that the amount of the 

 said products corresponded only with a small fraction (2.5% maximal) 

 of the substrates consumed. Nevertheless, these results bring a first in- 

 dication that also in dissimilatory nitrate reduction the dehydrogen- 

 ation occurs step-wise. Of course, there still remains the unsolved 

 problem why in sulphate and carbonate reduction the incomplete 

 dehydrogenations, and in nitrate reduction the incomplete hydrogen- 

 ations prevail. 



MOLECULAR HYDROGEN AS HYDROGEN DONATOR 



There is one hydrogen donator which asks for a special consideration, 

 viz., molecular hydrogen. That this gas can be utilized in incidental 

 dissimilatory nitrate reduction has already been shown several years 

 ago by Stephenson and Stickland [1931] and by Woods [1938]. The 

 latter author proved definitely that resting cells of Clostridium welchii 

 were able to reduce nitrate to the ultimate reduction stage : ammonia. 

 This result does, however, not at all imply that hydrogen can also be 

 a suitable donator in true dissimilatory nitrate reduction, for it may 

 be deemed certain that it is impossible to devise a medium which 

 does not sustain the anaerobic growth of the said bacterial species, but 

 which is able to do so on addition of nitrate. 



The same holds for the hydrogen oxidizing bacterium recently des- 

 cribed by Schatz and Bovell [1952] under the name Hydrogenomonas 

 facilis. Although resting cells of this bacterium in a hydrogen atmos- 

 phere bring about a vigorous reduction of nitrate to the nitrite stage, 

 it is described as a strictly aerobic organism. Here again thus we are 



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