OXIDATION OP SULFUR IN THE SOIL 233 



within the cells of Beggiatoa consisted of sulfur. Cohn (1875) 

 then proposed the theory that the Beggiatoa and the purple 

 bacteria produce hydrogen sulfide by the reduction of sulfates. 

 But it was Winogradsky (1883, 1887, 1888) who demonstrated 

 that the hydrogen sulfide produced by other bacteria is oxidized 

 by the Beggiotoa to sulfur and sulfuric acid. 



2HS +02 = 2HoO + Sj + 122 Cal. 

 Sj + 3O2 + 2H,0 = 2H0SO4 + 282 Cal. 



This oxidation is so important for the very existence of these 

 organisms that, when the hydrogen sulfide is taken out of the 

 medium, they oxidize the sulfur present witliin their cells and, 

 when this is used up, they die out. The presence of traces of 

 organic substances and nitrates in the water is sufficient for the 

 development of these organisms, as long as there is enough hydro- 

 gen sulfide, while the presence of sugars, peptone and like 

 nutrients will stimidate the growth of other microbes but will 

 injure these sulfur bacteria. 



According to Winogradsky (1888), the sulfuric acid formed is 

 neutraUzed by the calcium carbonate present in the water. 



H2SO4 + CaCOa = CaS04 + CO2 + H.O 

 H2SO4 + CaH, (003)2 = CaS04 + 2COo + 2H2O 



The reaction of the water cultures of the acid producing bac- 

 teria was not found to become acid. Beggiatoa has been 

 obtained in pure culture by Keil (1912). _ No definite physiolog- 

 ical studies were made by Winogradsky. In general, the results 

 of this investigator are summarized under the following four 

 headings: (1) The sulfur bacteria oxidize hydrogen sulfide and 

 accumulate sulfur in the form of small spheres, consisting of soft 

 amorphous sulfur which never crystalUzes in the hving cells. (2) 

 Thej^ oxidize the sulfur to sulfuric acid, which is at once neutra- 

 Uzed, by the carbonates present, into sulfates. (3) Without 

 sulfur, the organisms soon die off. (4) They can live and mul- 

 tiply in liquids containing only traces of organic substances. 



This last point was refuted by Keil (1912), who demonstrated 

 that the organisms are autotrophic and do not need organic 



