224 SULPHUR BACTERIA 



the addition of 3 per cent. NagSgOg induces vigorous growth, 

 10 per cent, of this substance in a culture medium is inhibitory. 

 Still another member has been isolated which is closely associ- 

 ated with alkali deposits, and which is particularly abundant 

 in the sandy loam soils of the "black" type. This has been 

 provisionally named Thiohacillus B. It oxidizes the thiosul- 

 phate with liberation of sulphur, and the latter is then oxidized 

 to the sulphate. Its growth is best in a medium with a pW 

 value of 6 — 10. 



The activity of the thionic acid bacteria in furthering 

 changes of economic and biological importance in the soil is 

 considerable. It was estimated by Ames that 50 per cent, of 

 the sulphur combined in soil to the extent of 0-5 gram sulphur 

 per 500 grams of soil was changed into the sulphate when the 

 soil was inoculated with these organisms, and that even as 

 much as 70 per cent, was possible if the amount of sulphur 

 was present to the extent of 2-0 grams per 500 grams of soil. 

 As the higher green plants assimilate sulphur in the form of 

 sulphates, the agricultural importance of these organisms is 

 obvious. They are also important in another aspect, for the 

 greater acidity in the soil which follows their multiplication 

 enables the higher green plants to absorb a larger amount of 

 the sulphate, and a greater variety of soil constituents, than 

 is possible in a less acid medium. Many salts which would 

 otherwise be insoluble are thus made available to crop plants. 

 There is, however, a limit to the acidity beyond which harm 

 rather than good would be achieved. 



The two organisms Bacterium crystalliferwn (Gicklhorn) 

 and Bacterium retifor^nans (Gicklhorn) must also be assigned 

 to this group. They were isolated from garden soil that had 

 been covered with water containing potassium sulphide in 

 solution. After three weeks the surface of the water was 

 covered with snow-white points, each being a colony of one 

 or other of these organisms. Between the individual colonies 

 were numerous minute particles of sulphur, evidently derived 

 from the sulphide by oxidation. 



Bacterium crystalliferum is i — 2/i long and 0-3 — 0-5/^ broad, 

 and is non-motile. The individuals are closely pressed to- 



