BACTERIA OF THE SOIL 267 



roots and built up again into protein. Under anaerobic 

 conditions sulphates may be changed into sulphides. Cer- 

 tain bacteria are also known which can produce free sul- 

 phur, and others which can oxidize free sulphur to sul- 

 phates. 



Org-anisms Oxidizing Hydrogen Sulphide and Free Sul- 

 phur. A large group containing many families and numer- 

 ous genera of the so-called purple or sulphur bacteria (thio- 

 bactcria) are known which evidently utilize hydrogen 

 sulphide, very largely in their metabolism. Many springs 

 and ground waters contain considerable amounts of sul- 

 phides which, upon aeration, give off free hydrogen 

 sulphide. Bacteria belonging to this group are active in 

 bringing about oxidation in such waters. Among the most 

 common forms are members of the genera Beggiatoa and 

 Thiothrix. These organisms, by the oxidation o5 hydrogen 

 sulphide, produce granules of free sulphur inside of the 

 cells. When examined microscopically these free sulphur 

 granules will be noted as glistening globules. If such bac- 

 teria are dried upon a slide and a small amount of a sulphur 

 solvent, such as carbon disulphide is added and allowed to 

 evaporate, sulphur crystals may be detected. These bacteria 

 also possess the power of oxidizing the free sulphur to sul- 

 phates. It is not probable that these organisms are par- 

 ticularly significant in the soil as hydrogen sulphide is 

 rather readily oxidized to sulphates by chemical means and 

 by various catalytic agents in the soil. The species of bac- 

 teria which may bear some relationship to this phenomenon 

 in the soil are not well known. The process has been some- 

 times termed sulphofication. 



Bacteria belonging to this group may be of some signifi- 

 cance in the destruction of concrete structures such as 

 cement tile through the formation of sulphuric acid and 

 consequent disintegration. 



Sulphates are quite as essential to plant growth as are 



