398 PRINCIPLES OF SOIL MICROBIOLOGY 



s-- 



| 20 Cal. 

 HS~ 



| 3 Cal. 

 S 



| 6.5 Cal. 

 H 2 S aq. 



| 170 Cal. 



so — 



H 2 S itself is probably poisonous to Beggiatoa as it is to other bacteria. 

 The hydrogen sulfide is produced by bacteria from the decomposition 

 of organic matter, from the reduction of sulfates and other compounds 

 of sulfur and by inorganic agencies (p. 600), and is used as a source of 

 energy. 



The sulfur bacteria can be divided into three physiological groups 

 on the basis of the sulfur compounds from which they derive their 

 energy : 



1. Sulfide bacteria. These organisms obtain their energy chiefly from the 

 oxidation of hydrogen sulfide and other sulfides. They include the thread-form- 

 ing, purple, and other bacteria accumulating sulfur within their cells. According 

 to Keil, 26 only Beggiatoa and Thiothrix are autotrophic bacteria, while the 

 purple bacteria are not autotrophic. 26 



2. Thiosulfate or thionic acid bacteria. These obtain their energy primarily 

 from the oxidation of thiosulfates. Some seem to be also capable of oxidation 

 of elementary sulfur. They include Thiohacillus thioparus Beij. and the various 

 strains of Th. denitrificans Beij. The first is obligate autotrophic while the 

 second, at least some strains of it, is facultative autotrophic. According to 

 Lieske, 27 Th. denitrificans assimilated 10.9 mgm. of carbon for every gram of 

 thiosulfate oxidized. One gram of thiosulfate is equivalent to 405 mgm. of sulfur. 

 Therefore, thirty-seven parts of sulfur are oxidized to two parts of carbon assimi- 

 lated chemosynthetically. 



Nathanson first suggested the following reaction for the process of trans- 

 formation of thiosulfate, which results in the accumulation of sulfur in the 

 medium : 



3 Na 2 S 2 3 + 2K0 2 ) = 2 Na 2 S0 4 + Na 2 S 4 6 



The free sulfur, which was formed in the medium, was believed to originate by 

 the chemical interaction of the tetrathionate with some of the thiosulfate. How- 

 ever Beijerinck believed the sulfur to be produced directly from the thiosulfate: 



Na 3 S 2 3 + M0 2 ) = Na 2 S0 4 + (S) 



26 Keil, 1912 (p. 81). 

 "Molish, 1907 (p. 83). 



27 Lieske, 1912 (p. 86). 



