400 PRINCIPLES OF SOIL MICROBIOLOGY 



According to Trautwein respiration does not depend on the oxygen 

 tension; oxidation is not affected even at a pressure of f atmosphere. 

 At iV atmosphere, however, respiration completely ceases. Th. deni- 

 trificans can grow in the complete absence of oxygen when nitrates 

 are present. The latter are reduced to atmospheric nitrogen and the 

 oxygen is used for the purpose of oxidation. 



Sulfates, even up to 5.0 per cent concentration, do not injure sulfur 

 oxidation. In this respect the thiosulfate bacteria behave differently 

 from the Nitrosomonas and Nitrobacter, since the sulfur organisms are 

 not injured by the oxidation products while the nitrite and nitrate bac- 

 teria are. The chlorine ion exerts, however, an injurious influence: 

 oxidation of sulfur is reduced by 38 per cent at 0.3 N concentration 

 of NaCl. 



The optimum reaction for the activities of Th. denitrificans lies at a 

 pH 7.9 to 9.1, with a minimum of pH 3.5. In acid media the reaction 

 tends to become more alkaline as a result of the activities of the organ- 

 ism, since no free acid is formed. 



The most extensive work on the respiration of sulfur-oxidizing 

 bacteria has been done with Th. thiooxidans. This organism is capable 

 of utilizing elementary sulfur, sulfide and thiosulfate as sources of 

 energy. The carbon is derived only from carbon dioxide. The 

 presence of glucose is not injurious to the growth and sulfur oxidizing 

 capacity of the organism but it cannot serve either as a source of energy 

 or as a source of carbon. No growth takes place when the cultures 

 are placed in a C0 2 -free atmosphere. 34 Bicarbonates, particularly 

 when the proper reaction of the medium is obtained by means of an 

 acid, may take the place of C0 2 only to a small extent, as seen from 

 table 26. The slight increase in acidity in the cultures of the ordinary 

 medium and glucose medium grown in the C0 2 -free atmosphere is due 

 to the acidity introduced with the inoculum (3 drops per 100 cc. of 

 culture medium). It may be of interest to point out, in this connec- 

 tion, that the optimum reaction for the growth of the organism is 

 pH 1.0 to 5.0. 



A modified medium may be used for this purpose, namely 0.2 gram 

 (NH 4 ) 2 S0 4 , 0.1 gram MgS0 4 , 0.01 gram FeS0 4 , 0.25 gram CaCl 2 , 3.0 

 grams K 2 HP0 4 , 10 grams sulfur in 1000 cc. water. The water is 

 distilled twice, the second time using glass containers only and pre- 



34 Waksman, S. A., and Starkey, R. L. Carbon assimilation and respiration 

 of autotrophic bacteria. Proc. Soc. Exp. Biol. Med., 20: 9-14. 1922. 



