TRANSFORMATION OF SULFUR 605 



reduced by means of an enzyme, as in the case of yeasts, 13 to hydrogen 

 sulfide and the latter assimilated and utilized for protein synthesis. 

 The mechanism of autotrophic oxidation of sulfur has been studied in 

 detail and is well known. 



In considering the process of sulfur oxidation by autotrophic bac- 

 teria, we must differentiate carefully between the nature of the or- 

 ganism and the sources of sulfur. Of the different groups that have 

 been enumerated as capable of oxidizing sulfur and its compounds 

 (p. 79), only the Thiobacillus group is found in normal soils. The 

 presence of larger forms belonging to the unbranched types, accumu- 

 lating sulfur within their cells, is possible only in muds or in soils kept 

 under anaerobic conditions, where the formation of hydrogen sulfide 

 takes place. 



The Thiobacillus group is present abundantly in all soils but the 

 most active forms are found in soils receiving applications of sulfur 

 as a fertilizer, either in organic (sewage, etc.) or inorganic forms. This 

 is due either to their direct introduction into the soil or to a response 

 due to the addition of the specific nutrient. It was shown, for example, 

 that soils receiving stable manure or green manure were capable of 

 oxidizing sulfur more rapidly than the untreated soils poor in organic 

 matter. By increasing soil aeration and keeping the moisture content 

 at 50 per cent of the moisture holding capacity, favorable conditions 

 are created for the oxidation of sulfur in the soil. 



Oxidation of sulfur in the soil may be followed by (1) an increase in 

 acidity as expressed by a change in the pH; (2) an increase in the 

 amount of sulfates in the soil; (3) the disappearance of elementary 

 sulfur. The last condition can be determined by extracting the re- 

 sidual sulfur from the soil with acetone. 14 



Oxidation of sulfur by microorganisms. The biological oxidation of 

 sulfur has been studied in detail by Winogradsky. 15 As a source of 

 sulfur H 2 S was used or conditions were made favorable for its production. 



The reactions involved in the process were presented as follows: 



2H 2 S + 2 = 2H 2 + S 2 



S 2 + 30 2 + 2H 2 = 2H 2 S0 4 



H 2 S0 4 + CaC0 3 - CaS0 4 + H 2 + C0 2 



13 Morison, C. B. The production of hydrogen sulphide by yeast. Science, 

 60: 482-483. 1924. 



14 Simon, R. H., and Schollenberger, C. J. The acetone method of extracting 

 sulfur from soil. Soil Sci., 20: 393-396. 1925. 



"Winogradsky, 1887 (p. 80). 



