Sulfur Bacteria 297 



organisms designated as sulfate-reducing bacteria. 

 They are anaerobic organisms widely distributed in soils, 

 lakes and streams and apparently play an important 

 part in the purification of potable waters. By means of 

 the oxygen that they withdraw from the sulfate, they 

 cause the destruction of the organic materials suspended 

 or dissolved in the water and help to make it wholesome 

 for human use. 



The sulfate of lime that accumulates on the bottom 

 of seas, lakes and streams, is reduced by one or another 

 of the bacterial groups just referred to. The sulfuretted 

 hydrogen passes upward while the lime is left behind as 

 carbonate. The sulfur bacteria at or near the surface, 

 seize the sulfuretted hydrogen and convert it ultimately 

 into sulfate of lime. This may be redeposited at the 

 bottom to go through a similar cycle of transformation. 

 It has been demonstrated that the ■ mud baths, cele- 

 brated for their curative properties in rheumatism and 

 similar diseases, are characterized by the presence of 

 sulfuretted hydrogen derived from the reduction of 

 gypsum by bacteria. 



The bacterial activities. — The curative muds of the 

 Black Sea are marked, among others, for the bacterial 

 activities just alluded to and may serve as an illustration 

 of the vast work performed by microorganisms in the 

 change of mineral as well as of organic materials. The 

 composition of the deposits of mud in land-locked waters 

 and on the ocean bed bear an intimate relation to bac- 

 terial activities. The formation and the destruction of 

 gypsum, the formation and the destructoin of lime car- 

 bonate are but a part of the far-reaching transforma- 



