SELECTED PAPERS 



of cell protein which means that the nitrogen is completely, or 

 almost completely, found back in organic compounds in which the 

 nitrogen occurs in the reduction stage of ammonia. This process 

 can aptly be designated as assimilatory nitrate reduction, or simply nitrate 

 assimilation. 



Next to these there are organisms which bring about a process of 

 nitrate reduction in which the reduction products are mainly excreted 

 into the medium. Here obviously the nitrate is involved in some dis- 

 similatory reaction. 



A closer inspection leaves no doubt that in this dissimilatory nitrate 

 reduction two cases can still be distinguished. In the first case the 

 nitrate just interferes with the normal energy yielding process of the 

 cell by acting as many other added hydrogen acceptors would do. 

 This holds for instance, if nitrate is added to various sugar fermenting 

 bacteria like Bacterium coli, Clostridium welchii a.o. The main effect of 

 the addition is usually a reduction of the nitrate either to nitrite, or 

 to ammonia, which conversion is accompanied by some changes in 

 the nature of the fermentation products. It is clear that this best 

 studied type of nitrate reduction is for the cells in question only of 

 minor importance, and the designation of the process as incidental dis- 

 similatory nitrate reduction seems quite adequate. 



The full splendour of bacterial nitrate reduction, however, reveals 

 itself in the case in which the nitrate is involved in an energy yielding 

 process which, at least conditionally, is essential for the well-being and 

 proliferation of the organism. It has been definitely established that 

 certain bacterial strains, if inoculated into some well-balanced medi- 

 um, will die off unless either free oxygen or nitrate is added to the 

 medium. In this case apparently the nitrate can replace the function 

 of oxygen as a hydrogen acceptor in the respiration process, and in 

 doing so the nitrogen of the nitrate consumed is found back in reduced 

 nitrogen compounds. 



Here the addition of nitrate decides over life and death of the or- 

 ganism, and it is this true dissimilatory nitrate reduction which seems to 

 offer the best chances for a closer investigation, and which for this 

 reason will be exclusively considered in the following. 



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