ENERGY TRANSFORMATION 397 



The influence of organic matter on nitrite and nitrate-forming bac- 

 teria has been studied in detail by Winogradsky and Meyerhof . These 

 results indicate that soluble organic substances have a distinct in- 

 jurious effect upon the respiration and growth of both the nitrite and 

 nitrate-forming bacteria. The injurious effect of amines was found to 

 be parallel to their "lipoid solubility." The importance of this phenom- 

 enon in soil processes is discussed elsewhere. A detailed study of the 

 influence of various ions and ion antagonism upon the respiration of 

 these two groups of organisms has been made by Meyerhof. 



Energy utilization from the oxidation of sulfur and its compounds. 

 Winogradsky 23 was the first to demonstrate that certain bacteria living 

 in sulfur springs, like Beggiatoa, Thiothrix and others, use H 2 S as a 

 source of energy. This is first oxidized to sulfur and water: 



H 2 S + h (0) 2 = H 2 + S + 61.1 Cal. 



The reaction H 2 S = H 2 + S is endothermic, but the simultaneous 

 oxidation of the hydrogen makes the process exothermic. The sulfur 

 accumulates within the cells and is further oxidized, in the presence 

 of carbonates (CaC0 3 ). 



(S) rh + l\ (0 2 ) + H 2 = H 2 S0 4 + 142.2 Cal. 



H 2 S is a weak acid and undergoes hydrolytic dissociation, which 

 increases with a decrease in the hydrogen-ion concentration: 



K! [H,S] = [H + ] [HS-] 



in which 



Ki = 0.91 X 10 ~\ at 18°C. 

 K 2 [HS-] = [H+] [S— ] 

 K 2 is very small, about 10 ~ 15 



Baas-Becking 24 assumed that not the H 2 S as such, but rather the 

 hydrosulfide is used by the sulfur bacteria as a source of energy, since 

 the H 2 S has a lower energy value. This is seen by arranging the sul- 

 fur compounds according to their free energy levels. 



23 Winogradsky, 1887 (p. 80). 



24 Baas-Becking, L. G. M. The source of energy of the sulfur bacteria. Proc. 

 Soc. Exp. Biol. Med., 22: 127-130. 1924; Studies on the sulfur bacteria. Ann. 

 Bot., 39: 613-650. 1925. 



