SELECTED PAPERS 



hibited by this gas. That, despite the external variety, the same prin- 

 ciple again lies at the root of the metabolism of these so-called an- 

 aerobic organisms is immediately shown by those anaerobes which, 

 in addition to an organic substrate, also require for their energy- 

 yielding process some specific inorganic substance. This situation is 

 found with bacteria known as nitrate-, sulphate-, and carbonate- 

 reducing organisms. If now one examines the fate of these substances 

 in the metabolic process it appears that, at least under suitable con- 

 ditions, the nitrogen, sulphur, and carbon, respectively, of these salts 

 can be recovered in their most highly reduced states, viz., as ammo- 

 nia, hydrogen sulphide, and methane. Thus the perfect analogy of 

 these conversions and of the respiratory process with oxygen is im- 

 mediately emphasized, because in the latter this element is equally 

 converted into its most reduced state, water. But then the notion pres- 

 ents itself that in the energy-providing processes in question the spec- 

 ific inorganic substances have evidently adopted the role of oxygen as 

 hydrogen acceptor. Earlier we have seen that as far as energy provi- 

 sion is concerned, life has adapted itself to a great variety of hydrogen 

 donators; now we learn that oxygen by no means needs to be the ex- 

 clusive hydrogen acceptor, but that certain living organisms have 

 become adapted to the use of entirely different hydrogen acceptors. 



Finally we shall consider those anaerobic energy-yielding processes 

 in which only one single organic compound is transformed, processes 

 that are known by the terms fermentation and putrefaction. In these 

 cases the fermentable substrate appears to undergo a fission from 

 which originate on the one hand molecules that can function as hy- 

 drogen donators, and on the other hand molecules that can serve as 

 hydrogen acceptors. Consequently the principle of catalytic hydrogen 

 transfer as the basis of energy provision is encountered in these cases 

 as well. 



So far I have discussed only one aspect of metabolism, however; no 

 less important are those metabolic processes in which particular food 

 components are converted into cell materials, necessary for the repair 

 of wear and tear, and for multiplication of the organism. Here, too, 

 one is immediately struck by the great variety in requirements that 

 different groups of microbes display with respect to their nutrition. 

 Some amongst them are no less exacting than the human organism; 

 in sharp contrast to these are types, such as the hydrogen bacteria, 



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