Chapter XII 



BACTERIA WHICH TRANSFORM SULFUR 

 COMPOUNDS 



Sulfur is essential for the growth of plants and is utilized partly as 

 sulfate by most of them. Animals obtain their sulfur requirements largely 

 from plants, other animals, or bacteria. Sulfur is liberated by bacteria, 

 almost exclusively as hydrogen sulfide, from the remains of animals arid 

 plants. Specialized bacteria oxidize H2S to elementary sulfur, sulfate, or 

 intermediate compounds of sulfur. Elementary sulfur, thiosulfate, 

 tetrathionate, and other compounds intermediate between sulfur and 

 sulfate may be oxidized by bacteria. Under certain conditions sulfate 

 may be reduced to elementary sulfur or H2S by bacteria. Sulfite, thio- 

 sulfate, and other sulfur compounds likewise may be reduced by bacteria 

 and allied microorganisms. 



Besides their manifold effects on the sulfur cycle, bacteria which trans- 

 form sulfur or its compounds may influence the pH, Eh, color, carbonate 

 content, oxygen tension, and other properties of water or bottom depos- 

 its. Bacteria concerned with the sulfur cycle in the sea may render water 

 or mud uninhabitable by other organisms or, on the other hand, they may 

 be growth promoting. Sedimentary sulfur desposits have been ascribed 

 to the activities of sulfate-reducing and sulfur-depositing bacteria. The 

 importance of bacterial activities in the sulfur cycle in marine sediments 

 is outlined by Galliher (1933). 



Liberation of sulfur from organic compounds : — Most organic sulfur 

 occurs in cystine, cysteine, methionine, or other sulfhydryl amino acids. 

 Lesser quantities are bound in the form of sulfatides, organic sulfates, 

 sulfonates, sulfureted glucosides, etc. The liberation of sulfur from albu- 

 minous material is a process which is more or less incidental to its decom- 

 position. While all bacteria are not able to liberate H2S from sulfur- 

 containing organic matter, those which are endowed with this ability are 

 so widespread in the sea that, except for special compounds, this process 

 can be regarded as universal and non-specific. 



In bottom deposits from the Clyde Sea, Ellis (1932) demonstrated 

 from 10,000 to 3,000,000 saprophytes per gram, nine-tenths of which 

 liberated H2S from albuminous material. ZoBell (1938a) found from 

 10,000 to 1,000,000 H2S-producing bacteria per gram of bottom sediments 

 from the Pacific Ocean off the coast of California. 



The bacterial decomposition of animal and plant remains generally 

 results in the liberation of H2S. Abundant aeration seems to induce many 

 bacteria, active in the decomposition of proteins, to transform sulfhydryl 

 sulfur directly to sulfate instead of H2S. Such bacteria probably produce 

 H2S, but under favorable conditions the latter is abiogenically oxidized as 

 fast as it is liberated from the decomposing protein. Under anaerobic 

 conditions organic sulfur is converted almost quantitatively into H2S by 

 raw cultures of saprophytic bacteria. Traces of mercaptans may be pro- 

 duced (Bunker, 1936). 



