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pletely hydrolyses a 10 per cent urea solution and thus can tolerate 

 with impunity the alkaline reaction of a 16 per cent ammonium car- 

 bonate solution. 



I may not omit an organism such as Bacillus oligocarbophilus, for which 

 Beijerinck and Van Delden showed that it can derive its nutrients 

 from the traces of organic matter that never seem to be lacking in 

 laboratory air.* Also the group of bacteria that feed on hydrocarbon 

 vapours, studied by Sbhngen [1903], may not be neglected. 



But even this diversity is still limited in one respect. The carbon, 

 essential for the growth of the organisms so far mentioned, must be 

 supplied in the form of an organic substance, though the nature of 

 the latter may vary enormously. This implies that in the end these 

 organisms derive their food from other organisms, hence their des- 

 ignation as heterotrophs. 



Nevertheless, it is almost forty years ago since Winogradsky ex- 

 pressed the idea that even in this regard the diversity of microbial 

 metabolism is not limited. To be sure, it had long ago been estab- 

 lished that chlorophyll-containing microbes could build up their cel- 

 lular constituents from carbon dioxide and other minerals, by virtue of 

 the absorbed solar energy. But that colourless microbes, without the 

 aid of radiant energy, could do so too is a concept so daring that even 

 to-day it is surprising that a human mind has ventured to propose it. 



A genius like Winogradsky [1887] did not hesitate, however, to 

 conclude, as early as 1887, from his simple and ingenious experiments 

 that the colourless Beggiatoa alba, frequently encountered in sulphur 

 spring waters, can grow in darkness in a strictly mineral medium. The 

 organic cellular constituents would be synthesized from carbon diox- 

 ide, a process made possible because the necessary energy would be 

 obtained from the oxidation of hydrogen sulphide, first to sulphur, and 

 next to sulphate. In addition to Beggiatoa this would also apply to a 

 large group of bacteria, partly colourless, partly purple, that exhibit 

 the common characteristic of depositing free sulphur in their cells. 



Subsequent experiments have amply confirmed the correctness of 

 Winogradsky's concept. Consciously executed elective cultures have 

 shown that, in addition to the relatively large organisms that deposit 



* Later investigators are inclined primarily to implicate traces of carbon monox- 

 ide, a gas that is extremely toxic for higher animals, but can certainly be used as 

 the sole carbon source by B. oligocarbophilus. See: Centr. Bakt. II, 57, 309, 1922. 



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