TRANSFORMATION OF IRON BY SOIL MICROBES 177 



reduced form serves as their source of energy for growth, and they 

 live by the oxidation of ferrous to ferric iron and, Hke other auto- 

 trophic organisms, obtain carbon for their cell development from 

 the carbon dioxide of the air. Incidentally, subsequent to oxida- 

 tion of the iron during growth of the bacteria, the ferric com- 

 pounds precipitate and accumulate about the cells in relatively 

 large amounts. Organisms of this type are but one group of 

 many that may be concerned with transformations of iron; how- 

 ever, they are probably little concerned with the changes of iron 

 in the soil (Fig. 68). 



Fig. 68. — Filament of an iron bacterium, Leptothrix crassa, showing incrusta- 

 tion of ferric hydrate (after Cholodny). 



Certain organic compounds of iron are quite soluble. After 

 the utilization of the organic portion of these compounds as food 

 by the heterotrophic microorganisms, the inorganic compounds 

 of iron which are formed in the process are precipitated, since they 

 are soluble in only very small amounts. As an example we may 

 consider ferric ammonium citrate, a double salt of the organic 

 citric acid. Upon decomposition, the citrate is changed to various 

 products, including carbon dioxide and water. This liberates the 

 ammonia as ammonium hydrate and the iron as ferric hydrate. 

 The iron hydrate precipitates by reason of its slight solubility. 

 Precipitation by iron bacteria and by the organisms decomposing 

 organic compounds is more likely to occur under aerobic conditions. 

 Under anaerobic conditions, iron may be precipitated as sulfide, 

 which is formed either from organic compounds or from the 

 reduction of inorganic compounds of sulfur. 



Solution of iron may follow the formation of any of the acids 

 produced by microorganisms in soils, among which carbonic, sul- 

 furic, nitric, and the various organic acids are especially important. 

 Since iron is so slightly soluble at reactions most commonly suited 

 to growth of higher plants, the formation of certain amounts of 



