80 PRINCIPLES OF SOIL MICROBIOLOGY 



acting primarily upon elementary sulfur, equivalent to group 5. The 

 first four groups have their optimum on the alkaline side of neutrality, 

 while the last group has its optimum on the acid side of neutrality. 

 Only the last two groups (4 and 5) occur in the soil and play an active 

 part in the oxidation of sulfur and its compounds in the soil. The 

 first three groups do not occur to any extent in normal cultivated soils, 

 but are mentioned here for the sake of completeness. 



Group I. Colorless, thread-forming sulfur hacteria, accumulating sulfur 

 within their cells (Nos. 13-16, PI. IV). The representatives of this 

 group are able to act only upon H 2 S. In view of the fact that these 

 are primarily water and mud forms, only the general principles in- 

 volved are discussed. This group of sulfur oxidizing bacteria consists 

 of three genera : Beggiatoa, including motile organisms forming no sheaths ; 

 Thiothrix, fastened forms forming no sheaths; and Thioploca, thread- 

 forming bacteria, surrounded with a jelly-like sheath. The Beggiatoa 

 were the first organisms to attract attention as having to do with the 

 oxidation of sulfur or its derivatives. Cramer 41 pointed out that the 

 granules found within the cells of Beggiatoa consisted of sulfur. Cohn 42 

 then proposed the theory that the Beggiatoa and the purple bacteria 

 produce hydrogen sulfide by the reduction of sulfates. But it was 

 Winogradsky 43 who demonstrated that the hydrogen sulfide is pro- 

 duced by other bacteria and is oxidized by the Beggiatoa to sulfur and 

 sulfuric acid. 



This oxidation is so important for the very existence of these organ- 

 isms that, when the hydrogen sulfide is taken out of the medium, they 

 oxidize the sulfur present within their cells and, when this is used up, 

 they die out. The energy liberated in this process is utilized by the 

 organisms for the assimilation of carbon dioxide. For every gram of 

 carbon, 8 to 19 grams of sulfur are consumed. If there is enough H 2 S, 

 the presence of traces of organic substances and nitrates in the water is 

 sufficient for the development of these organisms, while the presence of 

 sugars, peptone and like nutrients will stimulate the growth of other 

 microbes but will injure these sulfur bacteria. 



41 Cramer, In Midler, C, Chemisch-physikalische Beschreibung der Thermen 

 von Boden in der Schweiz. 1870. 



42 Cohn, F. Untersuchungen liber Bakterien II. Beitr. Biol. Pflanz. 1: H. 3, 

 p. 141. 1875. 



43 Winogradsky, S. Beitrage zur Morphologie und Physiologie der Bakterien. 

 I. Schwefelbakterien. Leipzig. 1883; Ann. Inst. Past. 3: 1883, No. 2; Uber 

 Schwefelbakterien. Bot. Ztg. 45: 489, 513, 529, 545, 569, 585, 606. 1887. 



