15 

 natural deposits as sulfate, elemental S, and sulfide can be oxidized 

 or reduced by microorganisms in suitable environments. The types of 

 transformations are affected by the state of the S and the environ- 

 mental conditions, particularly the availability of 0. 



The predominant microorganisms concerned with the reduction of 

 sulfates are bacteria of the genera Desulfovibrio and Desulfotomaculum . 

 A number of carbohydrates, organic acids, and alcohols may serve as 

 energy sources or electron donors while sulfates are the electron 

 acceptors: 



2CH 3 CHOHCOONa+MgS0 4 ■* 2CH COONa+MgCO.+H 0+C0 o +H S . 



(Alexander, 19 77) 

 Microorganisms that reduce S are extremely important to soil fertility 

 because they reduce the availability of S in the soil, and the primary 

 product of their metabolism, H S, can be toxic to plants such as rice, 

 citrus, and other crops and trees of economic importance (Alexander, 

 1977) . 



In most well-drained, arable soils, S oxidation is of much more 

 agricultural importance than S reduction. Several reviews have been 

 made of the S-oxidation processes that occur in soils (Gleen and 

 Quastel, 1953; Vishniac and Santer, 1957; Starkey, 1966; Burns, 1967; 

 Kelley, 1968; Aleem, 1975; Alexander, 1977). Although not all soil S 

 oxidation reactions are enzymatic, and chemical oxidation of sulfides, 

 elemental S, and thiosulfates can occur, microbiological oxidation is 

 much more rapid under favorable conditions. 



Organisms capable of oxidizing reduced forms of S may be either 

 autotrophs or heterotrophs. The most significant microorganisms are 

 members of the genus, Thiobacillus. Most of these bacteria are 



