CHANGES IN INORGANIC CONSTITUENTS. 289 



We owe much of our knowledge concerning the sulphur bacteria to 

 Winogradski. This investigator showed that in places where hydrogen 

 sulphide is generated in considerable quantities sulphur bacteria grow 

 vigorously and accumulate granules of sulphur within their cells. When 

 the cells containing sulphur granules are removed to suitable media, in 

 which no hydrogen sulphide is present, the sulphur seems to be gradually 

 oxidized and disappears and the bacteria finally die of starvation. Thanks 

 to the sulphur bacteria, the higher plants are enabled to utilize again the 

 sulphur once locked up in plant and animal tissues, and liberated thence 

 by decay bacteria. The circulation of sulphur is thus made possible 

 and the cycle is completed when the sulphates are again used by plants to 

 build protein compounds. It may also be noted in this connection that 

 " Thiobacillus denitrificans," described by Beyerinck, may also oxidize 

 elementary sulphur. In this case, however, the oxygen is derived from 

 nitrates instead of the atmosphere. Thus: 



SULPHATE REDUCTION. The fact that sulphates may be reduced to 

 sulphides in the presence of organic matter has been known for many 

 years. In compost heaps, and at the bottom of seas, lakes and rivers, 

 the reduction of calcium sulphate is of common occurrence. Similarly, 

 ferrous sulphate may be reduced in water-logged soils and in swamps 

 and may give rise to deposits of bog iron. But while sulphate reduction 

 is of common occurrence in certain localities, it has been shown by Bey- 

 erinck and also by van Delden, that the reduction can be accomplished 

 in artificial media by specific microorganisms. Two species isolated by 

 these investigators have been named Sp. desidpliuricans and Msp. 

 (estuarii. When grown under anaerobic conditions in culture media sup- 

 plied with combined nitrogen and organic nutrients these organisms were 

 found capable of reducing sulphates. The oxygen withdrawn from the sul- 

 phates was used for the oxidation of organic matter in a manner analogous 

 to that in nitrate reduction where the oxygen is derived from the nitrates. 

 Apart from the two microspiras that cause the specific reactions just 

 noted, there are many common soil bacteria that may be responsible for 

 sulphate reduction in a less direct manner. Nadson has observed that 

 when the supply of oxygen is limited calcium sulphate may be reduced 

 to sulphide by B. my co ides and by B. (Proteus') vulgar is. The calcium 

 19 



