TRANSFORMATION OF SULFUR 611 



and microorganisms and transformed into proteins, or are gradually 

 washed out in the drainage waters, or are reduced under anaerobic 

 conditions. This phenomenon was explained 25 as due to the produc- 

 tion of nascent hydrogen by the microbes. It was suggested 26 that 

 the oxygen of the sulfate obtained in the reduction process is used for 

 the oxidation of organic matter. 



3CaS0 4 + 2C,HiOjNa = 3CaC0 3 + Na 2 C0 3 + 2H 2 + 2C0 2 + 3H,S 



This process is exothermic, resulting in the liberation of a small 

 amount of energy. The sulfate is usually first reduced to the sulfide 

 which is then transformed into hydrogen sulfide, according to the 

 reaction: 



CaS + C0 2 + H 2 = CaC0 3 + H 2 S 



The organisms concerned in this process are described elsewhere (p. 188). 

 The Microspira desulfuricans is not very sensitive to the products of 

 its metabolism, and can withstand as much as 246 mgm. of H 2 S per 

 liter of medium. The use of sulfates as a source of oxygen is limited 

 to a closely related group of organisms, of which only three have been 

 described, including a thermophilic form isolated by Elion (Vibrio ther- 

 modesulfuricans) . 27 Both sulfate and thiosulfate can be used as a 

 source of oxygen and salts of organic acids as well as other compounds 

 as sources of carbon. 



In crude cultures, the H 2 S formed from the reduction of sulfate in 

 the presence of organic matter and in contact with oxygen is again 

 oxidized by the sulfur-oxidizing bacteria. In curative muds and cer- 

 tain lakes there is an increase in the H 2 S content with depth, starting 

 from none at the surface and reaching a concentration of 30 mgm. at 

 a depth of 25 to 30 meters. The hydrogen sulfide formed in the lower 

 layers of the mud is oxidized to sulfate on reaching the surface; the 

 latter is then again reduced when reaching the lower layers. 28 



26 Petri, R., and Maassen, A. Beitriige zur Biologie der krankheitserregenden 

 Bakterien inbesondere liber die Bildung von Schwefehvasserstoff durch dieselben 

 unter vornehmlicher Beriicksichtigung des Schweinerothlaufs. Arb. K. Gesund. 

 Amt., 8: 318, 490. 1893. 



26 Beijerinck and VanDelden, 1904 (p. 188). 



27 Elion, L. A thermophilic sulfate-reducing bacterium. Centrbl. Bakt. II, 

 II, 63: 58-67. 1924. 



28 A detailed review of this subject is given by Nadson, G. On the hydrogen 

 sulfide fermentation in the Veissovo-Salt lake and the part played by the micro- 

 organisms in the formation of black mud. 1903; St. Petersburg (Russian) ; and 

 Duggeli, 1919 (p. 82). 



