606 



PRINCIPLES OF SOIL MICROBIOLOGY 



Elementary sulfur was formed as an intermediary product and was actually 

 demonstrated in the cells of the bacteria (Beggiatoa and Thiothrix). Some organ- 

 isms like Thiobacillus thiooxidans oxidize the H 2 S directly to sulfuric acid without 

 forming elementary sulfur, while others, like Thiobacillus thioparus, liberate free 

 sulfur outside of their cells. 



Thiosulfate is oxidized by microorganisms, according to the following group 

 of reactions, depending on the organism taking part in the process: 



3Na 2 S 2 3 + 2iQ 2 = 2Na 2 S0 4 + Na 2 S 4 6 



(1) 



Free sulfur was found to be liberated in the process; Nathanson 16 considered this 

 to be due to the interaction of the Na 2 S 4 6 and Na 2 S 2 03. However, according to 

 Beijerinck, 17 the reaction actually takes place as follows: 



Na 2 S 2 3 + O = Na 2 S0 4 + S 



(2) 



TABLE 63 

 Oxidation of elementary sulfur to sulfuric acid by Thiobacillus thiooxidans 



* Milligrams of soluble sulfates as sulfur in flask ; averages of 3 flasks are given ; 

 the concentration of sulfates in the small flasks was greater, due to the fact that, 

 in these, a medium containing 2 gm. (jNH 4 ) 2 SQ 4 and 0.5 gm. MgS0 4 per liter was 

 used. 



It was believed that the sulfur was separated in the process of direct oxidation. 

 However, the precipitation of the sulfur is probably caused by some secondary 

 reaction. Trautwein 18 suggested that the oxidation of the thiosulfate by the 

 organism that he isolated takes place according to the following reaction: 



7Na 2 S 2 3 + 8|0 2 = 4Na 2 S 2 6 + Na 2 S 4 6 + 2Na 2 S0 4 



(3) 



No sulfur was precipitated, the reaction did not become acid. Th. thiooxi- 

 dans oxidizes thiosulfate directly to sulfate as follows: 19 



Na 2 S 2 3 + 20 2 + H 2 = Na 2 S0 4 + H 2 S0 4 



(4) 



16 Nathanson, 1902 (p. 84). These reactions have been discussed in detail 

 elsewhere, in reference to energy utilization. 



17 Beijerinck, 1904 (p. 84). 



18 Trautwein, 1921 (p. 87). 



19 Waksman, S. A., and Starkey, R. L. On the growth and respiration of 

 sulfur-oxidizing bacteria. Jour. Gen. Physiol., 5: 285-310. 1923. 



