THE METABOLISM OF THE SULPHUR BACTERIA 45 



Baas-Becking makes the erroneous statement that the 

 sulphur bacteria occur only in hard waters, and that a high 

 alkalinity is maintained in such waters by the large amount 

 of calcium and magnesium present. Organisms like Beggiatoa 

 alba are found in soft and hard, as well as in marine and fresh 

 waters. Some of these bacteria thrive in waters in which the 

 alkalinity is not maintained by the presence of carbonates 

 and bicarbonates. The factors determining the pH of sulphur 

 waters generally appear to have been determiined by this writer 

 solely from local conditions. In soft waters, under normal con- 

 ditions, the pH of the water is kept within a certain range by 

 the opposing nature of two classes of bacteria. On the one 

 hand, the saprophytic and other bacteria are constantly pro- 

 ducing acid, whilst on the other the sulphur bacteria are using 

 up sulphuretted hydrogen and carbonic acid. Between the 

 two opposing forces a balance is maintained which, if it is 

 upset, results in the disappearance of the sulphur bacteria. 

 The same conditions hold in essentials both in hard and 

 in soft waters, the differences in respect to the growth of 

 bacteria being one of degree, not one of kind. In artificial 

 cultures the maintenance of an alkaline medium is often 

 guaranteed by the addition of chalk. 



Experiment with Thioporphyra volutans. 



It was found by chemical tests that the calcium and mag- 

 nesium content of pools containing a mixture of saprophytic 

 and sulphur bacteria did not appreciably change during a 

 period of several weeks in which active growth of both classes 

 of bacteria was in progress. Hence it was concluded that 

 any changes that took place in the pH value of the water were 

 due to changes in the concentration of sulphuretted hydrogen 

 and carbonic acid. The following results were obtained : — 



