OXIDATION OF SULFUR IN THE SOIL 611 



tributcd in the soil. Whether these organism form a group of 

 closely related strains or whether they form a group of organisms 

 differing in morphological and physiological characteristics still 

 remains to be seen. The characteristic growth of this group of 

 organisms is as follows: when a piece of soil is added to medium 

 8 (better growth will take place, when the medium is not steril- 

 ized), and flasks incubated at 25°C., growth will appear in 

 forty-eight to seventy-two hours. This growth consists of a 

 pale colored pellicle formed on the surface of the medium and on 

 the glass of the flask; the medium does not become turbid, 

 unless Th. thiooocddans is also present in the soil. An abundant 

 separation of the sulfur from the thiosulfate takes place, so that 

 the surface pellicle becomes sulfur-j^ellow. The thiosulfate 

 rapidly disappears with the formation of sulfates, persulfates and 

 elementary sulfur. If more thiosulfate is added to the flask, 

 it will be further transformed, so that 5 grams of thiosulfate may 

 be transformed in a 100-cc. culture in two to three weeks; this 

 will take place thus rapidly, however, only when a proportionate 

 amount of NaHCOa is added with the thiosuifate (1:5). The 

 role of the bicarbonate consists in supplying the base to the 

 foniiation of the sulfate and keeping the medium from becoming 

 acid. \\^iether the bicarbonate may also serve, at least partlj^, 

 as a source of carbon, is still unknown; however, it does not do 

 so for Th. thiooxidans. The reaction of the medium becomes 

 only slightly acid. "^Tiere intense acid fomiation accompanied 

 by the breaking up of the sulfur pelUcle has been found, Th. 

 thiooxidans has invariably been demonstrated to be present. 

 \Mien a streak is made from the culture in medium 8 upon a 

 plate containing cooled medium 9, minute colonies will rapidly 

 develop, characterized by an abundant separation of sulfur, 

 which covers the whole colony. No clear zone, or a mere trace 

 of it, is formed aroimd the colony, unless Th. thiooxidans is also 

 present. This organism thus developing on medium 8 is closely 

 related in its physiological characteristics to the Th. thioparus 

 of Beijerinck, which was demonstrated by Gehring (1915) to be 

 universally distributed in the soil. This organism will be called 

 ThiobacUlus B. This organism can be cultivated very readily, 



