612 PRINCIPLES OF SOIL MICROBIOLOGY 



The presence of sulfur-reducing bacteria in deep layers of earth has 

 been demonstrated by Wolzogen Kt'ihr 29 who found them at depths 

 of 6 to 35 meters. One can readily imagine that processes, similar to 

 those taking place in muds, may also occur in the soil. The require- 

 ments for sulfate-reduction by Microspira desulfuricans are (1) absence 

 of free oxygen, (2) presence of organic compounds as sources of energy, 

 (3) presence of sulfate as a source of oxygen, and (4) presence of essen- 

 tial inorganic elements in available forms. The following reactions 

 will then take place: 



4C 3 H 6 3 + H 2 S0 4 = 4C 2 H 4 2 + H 2 S + 4C0 2 + 4H 2 

 C 2 H 4 2 + H 2 S0 4 = 2C0 2 + H 2 S + 2H 2 

 or 



4C 3 H 6 0s + 2H 2 S0 4 = 3C 2 H 4 2 + 2H 2 S + 6C0 2 + 4H 2 + 2H 2 



Certain actinomyces are also capable of reducing sulfates to hydro- 

 gen sulfide. 30 



Formation of // 2 S in the decomposition of organic matter. It has been 

 pointed out elsewhere that a large number of bacteria, including aerobic 

 and anaerobic forms, are capable of forming hydrogen sulfide and other 

 volatile sulfur compounds in the decomposition of organic matter 

 containing sulfur. Proteins 'may contain as much as 1.5 per cent of 

 sulfur; on hydrolysis, sulfur is liberated from them partly in the form 

 of H 2 S. This process is carried out by a large number of bacteria, 

 especially certain obligate (Bac. putrificus, Bac. sporogenes) and facul- 

 tative (Bad. coli, Bad. vulgare, Staph, pyogenes aureus) anaerobes. 



The formation of H 2 S can be tested by the use of a piece of bibulous 

 paper saturated with lead acetate to which a little glycerol has been 

 added. The paper is suspended in the upper part of the container 

 over the growing culture ; it will at first become brown, then black. To 

 determine the numbers of organisms in the soil capable of forming 

 H 2 S from a specific organic material, it is sufficient to incorporate some 

 lead acetate into an agar medium; the formation of a dark color will 

 indicate the ability of the organism to produce H 2 S. 31 



The differences in the amounts of H 2 S formed by various organisms 



29 von Wolzogen Kuhr, C. A. H. Occurrence of sulfate-reduction in the deep 

 layers of the earth. Proc. Roy. Soc. Amsterdam, 25: 188-198, 1922. (Chem. 

 Abstr., 17: 47). 



30 Sawjalow, 1913 (p. 189). 



31 Burnet, E., and Weissenbach, R. J. Valeur des renseignements founds par 

 la culture en gelose a l'acetate de plomb, pour la differenciation des bacilles 

 typhique, paratyphique A et paratyphique B. Compt. Rend. Soc. Biol., 78: 

 565-568. 1915. 



