AUTOTROPHIC BACTERIA 91 



The medium is prepared as usual and sterilized at 15 pounds pressure for 

 fifteen minutes. Plates and slants are inoculated from a vigorous liquid culture 

 and incubated at 25° to 30°C. Growth appears in five to six days in the form of 

 minute straw yellow to cream-colored colonies. Under the microscope, each 

 colony is found to be surrounded with crystals of gypsum due to the action 

 of the sulfuric acid, formed from the oxidation of the thiosulfate, upon the 

 CaCl 2 . This phenomenon is particularly prominent in media containing tri- 

 calcium phosphate in place of the chloride; a clear zone is formed around each 

 colony, due to the disappearance of the insoluble calcium salt. 74 



When, instead of an ordinary neutral or acid soil, an alkaline soil is 

 used for composting, the sulfur is also oxidized to sulfuric acid with 

 the result that the alkalinity of the soil is decreased. When enough 

 sulfur is added, black alkali soil having a reaction of pH 9.8 can be 

 made neutral and even acid. At first it was thought that Thiobacillus 

 thiooxidans is responsible for this oxidation, particularly since the same 

 acid composts were employed. It is possible, however, that the or- 

 ganism concerned in the oxidation of elementary sulfur in alkali soil is 

 of an entirely different nature, approaching more the Th. thioparus of 

 Beijerinck, in its cultural and some physiological characters, rather than 

 the Th. thiooxidans. The latter, however, is also found in alkaline 

 composts, and it is possible that both organisms take an active part in 

 the oxidation of the sulfur under alkaline conditions. The Th. thioparus 

 group has its optimum on the alkaline side (pH 7.0 to 9.0), as shown by 

 Trautwein, while the Th. thiooxidans has its optimum on the acid side. 

 Both organisms may, therefore, act upon the sulfur under alkaline con- 

 ditions. However, the phylogenetic relationship of the various species 

 of Thiobacillus, as well as the differences in their chemical action still 

 remain to be investigated. 



Oxidation of selenium and its compounds. Brenner 75 isolated from the 

 soil an organism (Micrococcus selenicus, less than 0.5/z in size), which is 

 capable of oxidizing selenides and using the energy obtained for its 

 activities. Sodium selenite, sodium thiosulfate or sodium selenate, as 

 well as litmus, methylene blue or indigo carmin, can be used as hydro- 

 gen acceptors (or sources of oxygen) but not nitrates, sulfates, sulfites 

 or tellurites. In addition to selenide, the organism can also use various 



74 The occurrence of the Thiobacillus group in the soil has been further studied 

 by Brown, H. D. Sulfofication in pure and mixed cultures, with special refer- 

 ence to sulfate production, hydrogen-ion concentration and nitrification. Jour. 

 Amer. Soc. Agron. 15: 350-382. 1923. 



76 Brenner, W. Ziichtungsversuche einiger in Schlamm lebenden Bakterien 

 auf selenhaltigem Nahrbodem. Jahrb. wiss. Bot. 57: 95-127. 1916. 



