THE SULPHUR CYCLE 243 



tion of hydrogen sulphide by bacterial agency is that brought 

 about by the higher bacteria, classified under the general term 

 of Beggiatoa (Fig. 3 (I A )). These are the organisms which 

 form the subject of Winogradski's researches above referred 

 to. They are found very often in sulphur springs and 

 wherever putrefying sewage or suchlike organic matter comes 

 in contact with air, as, e.g., on the stones of a stream in the 

 neighbourhood of a badly polluting discharge. The organism, 

 as a matter of fact, grows between wind and water, but 

 makes use of the sulphur either by decomposition of the H 2 S 

 present, or by actual absorption of the free sulphur formed 

 by its spontaneous oxidation. If a strand of Beggiatoa is 

 examined under a high-power microscope, very characteristic 

 granules of sulphur are seen to be present throughout the 

 organism, as is shown in Fig. 3 (I A ) (Chapter II). This 

 sulphur is the amorphous form soluble in carbon bisulphide. 

 Beggiatoa is capable of absorbing large quantities of sulphur 

 which it oxidises to sulphates ; for this purpose it is necessary 

 that carbonates should be present in the surrounding liquid. 

 Under its natural conditions of growth this will inevitably be 

 the case, ammonium carbonate, e.g., being always present in 

 decomposing sewage. Beggiatoa appears to use the sulphur 

 as a source of energy rather than to increase its cell substance. 

 Winogradski found that it could use up from two to four 

 times its weight of sulphur without increasing in growth. 

 Under these circumstances, comparatively small amounts 

 of organic matter will suffice to sustain it, and thus it can 

 flourish in sulphur springs, whose chief constituents, apart 

 from hydrogen sulphide, are mineral salts. 



To summarise the contents of the foregoing chapter, we 

 may conclude that sulphur enters the cycle of living nature as 

 mineral sulphates in the food of plants. By the decom- 

 position of vegetable albumin, or at a further stage from the 

 excretory products of animals, it may reappear as hydrogen 



B2 



