78 PRINCIPLES OF SOIL MICROBIOLOGY 



Bacteria deriving their energy from the oxidation of sulfur and its com- 

 pounds. The sulfur bacteria do not form any unifonn group of micro- 

 organisms , as in the case of the nitrifying bacteria, either morphologi- 

 cally or physiologically. Morphologically they are found among the 

 Desmobacteriaceae (Thiobacteriales) and among the Bacteriaceae 

 (Eubacteriales). Physiologically they may oxidize hydrogen sulfide 

 and other sulfides, elementary sulfur, or thiosulfate and they may act 

 either in an acid or in an alkaline reaction. Some are obligate auto- 

 trophic and some are facultative. The bacteria which are found in 

 normal, fertile soils or those that become active in the soil, when intro- 

 duced, are limited chiefly to the genus Thiobacillus among the Bac- 

 teriaceae. 



All microorganisms require minute quantities of sulfur for the syn- 

 thesis of their protoplasm. Various bacteria and even some fungi seem 

 to be capable of oxidizing small amounts of sulfur. But only certain 

 bacteria work over much larger quantities of sulfur than would be neces- 

 sary for their body structure, since they utilize the sulfur or its com- 

 pounds as a source of energy. The sulfur is to the sulfur bacteria, as 



PLATE IV 

 Sulfur and Iron Bacteria 



13. Beggiatoa alba, thread forming sulfur-oxidizing bacterium: a, in liquid 

 culture rich in H 2 S; b, culture kept 24 hours in liquid freed from H 2 S; c, 48 hours 

 later in the same liquid (sulfur droplets have disappeared, cell division takes 

 place and protoplasmic contents are left), X 600 (after Omeliansky). 



14. Thread-forming, sulfur-oxidizing bacteria: (x) Beggiatoa media and (y) 

 Beggiatoa minima, X 600 (after Omeliansky). 



15. Thioploca ingrica, X 200 (after Wislouch and Omeliansky). 



16. Young threads of Thiothrix nivea, X 600 (after Omeliansky). 



17. Thiophysa macrophysa, showing drops of sulfur on periphery and oxalate 

 crystals in center, X 660 (after Nadson and Omeliansky). 



18. Thiospirillum winogradskii: a X 100 and b X 660 (from Omeliansky). 



19. Chromatium okcnii, X 660 (after Omeliansky). 



20. Achromatium oxaliferum: A, showing the calcium bodies, but not sulfur; 

 B, without the calcium bodies, but with a number of droplets of sulfur (from 

 Nadson and Wislouch). 



21. Thiobacillus thioparus, showing drops of precipitated sulfur among the rod- 

 shaped organisms, X 1000 (from Diiggeli). 



22. Thiobacillus thiooxidans, X 660 (Original). ■ 



23. Diagrammatic sketch of several typical iron bacteria: a, Spirophyllum fer- 

 rugineum; b, Gallionella ferruginea; c, Leplothrix ochracea, X about 720 (from 

 Harder, by courtesy of U. S. Geological Survey). 



24. Cladolhrix dicholoma, X 190 (after Molish). 



