92 PRINCIPLES OF SOIL MICROBIOLOGY 



alcohols (ethyl-, iso-butyl) as well as asparagine and glucose as sources of 

 energy, but not proteins. The carbon is obtained, however, only from 

 organic substances (ethyl alcohol). The possible oxidation of elemen- 

 tary selenium, in the absence of organic matter, with an increase of 

 acidity of the medium has also been suggested. 76 The Th. thioparus and 

 Th. thiooxidans were found to be inactive in both cases. The relation 

 between the oxidation of selenium and of selenide and autotrophic proc- 

 esses is unknown. 



Bacteria oxidizing iron compounds. The nitrifying bacteria are found 

 to be strictly autotrophic and, in the case of the nitrite formers, are even 

 injured by the presence of organic matter. The sulfur bacteria are 

 not injured by the organic matter present in the medium (except Th. 

 thiooxidans which is injured by nitrogenous substances above 0.2 per 

 cent) and some of them can even utilize organic materials. The 

 utilization of complex organic substances is true even to a greater extent 

 in the case of iron bacteria, only one or two forms of which are known 

 to require iron compounds as a source of energy, while the majority are 

 able to derive their energy heterotrophically. The strictly iron bacteria, 

 or those organisms that are capable of oxidizing ferrous to ferric iron, 

 whereby the energy obtained is used for the chemosynthetic assimila- 

 tion of carbon, should be distinguished from those organisms that can 

 absorb or accumulate iron, when living in media containing iron. 

 Unlike the latter process the precipitation of iron by true iron bac- 

 teria is a direct result of utilization of energy from the oxidation of 

 iron. 77 



As early as 1836, Ehrenberg, 78 found that microorganisms play an 

 important part in the formation of ochraceous deposits of bog iron ore. 

 The iron precipitating organisms are present universally in nature, 

 wherever iron-bearing waters occur. They belong chiefly to the thread- 

 forming bacteria, although a number of them have also been found to 

 belong to the Eubacteria. Here again we find a similarity between 

 the iron and sulfur bacteria, a large number of forms belonging to 

 distinctly different morphological groups. The majority of these forms 

 belong to the higher bacteria, according to the following classification 88 : 



76 Lipman, J. G., and Waksman, S. A. The oxidation of selenium by a new 

 group of autotrophic microorganisms. Science, N. S. 57: 58. 1923. 



77 Winogradsky, 1922 (p. 61). 



78 Ehrenberg, C. G. Vorlaufige Mittheilungen fiber das wirkliche Vorkommen 

 fossiler Infusorien, und ihre grosse Verbreitung. Poggendoff's Annalen 38: 

 213-227. 1836. 



