AUTOTROPHIC BACTERIA 87 



sulfate and sodium tetrathionate, which are completely oxidized to 

 sulfate. 



The energy liberated in the process is sufficient both for the reduction 

 of the nitrate, which is an endothermic phenomenon, and the assimila- 

 tion of C0 2 from the carbonate and bicarbonate. The reason why the 

 same organism can oxidize various compounds of sulfur, while other 

 autotrophic soil bacteria, like those concerned in nitrification, can act 

 only on one definite compound, is explained by Lieske to be due to the 

 step-like oxidation of the sulfur compounds. For every 100 gm. of 

 Na 2 S 2 3 oxidized to sulfate, Lieske found that 1 gm. of carbon is 

 assimilated. 



The Thiobacillus denitrificans is of universal occurrence in various soil 

 types, the number of bacteria increasing with the increase in the carbon 

 content of the soil. 65 By adding thiosulfate and bicarbonate to the 

 soil, intensive nitrate decomposition takes place. The organism oc- 

 curring in various soils varies in activity, so that the forms from com- 

 posts, forest soils and peat belong to a group which is four times as 

 active as the form from cultivated soils. Thiobacillus denitrificans 

 offers, according to Beijerinck, 66 the natural connecting link between 

 sulfur oxidizing bacteria and denitrifying bacteria. 



Trautwein 67 isolated an organism from the soil which was classified 

 with Thiobacillus denitrificans Beij.; this organism is 1 to 2 by 0.5^ in 

 size, motile, can reduce nitrate, but can grow also under aerobic condi- 

 tions; it grows well on organic media and does not precipitate any 

 sulfur from thiosulfate. The organism is facultative autotrophic, 

 since it can obtain its carbon both from C0 2 (with thiosulfate as a 

 source of energy) and from organic substances (in the absence of thio- 

 sulfate). It was grown on the following medium: 



KNO, l.Ogram NaHC0 3 l.Ogram 



(orNH 4 Cl) 0.1 gram MgCl 2 0.1 gram 



Na 2 HP0 4 0.2gram Distilled water 1000 cc. 



Na 2 S 2 3 -5H 2 2.0 grams 



To prepare a solid medium, agar is added to the above solution. As organic 

 media, ordinary bouillon or nutrient agar can be employed. 



65 Gehring, A. Beitrage zur Kenntnis der Physiologie und Verbreitung deni- 

 trifizierender Thiosulfat Bakterien. Centrbl. Bakt. II, 42: 402^38. 1915. 



66 Beijerinck, 1920 (p. 547). 

 6T Trautwein, K. Beitrag zur Physiologie und Morphologie der Thi 



bakterien (Omelianski). Outrbl. Bakt. II, 53: 513-548. 1921. /<\Vj\0 



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