46 SULPHUR BACTERIA 



Although some saprophytes, as, for example, the marine 

 denitrifying saprophyte examined by Cranston and Lloyd, 

 maintain an alkaline reaction, the growth of a mixed group of 

 saprophytes is practically always accompanied by a progressive 

 development of acid. We may attribute the sharp rise in 

 the p\l value between the sixth and the eighth week to the 

 consumption of the sulphuretted hydrogen (liberated by the 

 saprophytes) by Thioporphyra, which during this period was 

 observed to grow luxuriantly. The subsequent lowering after 

 the eighth week was found to be concurrent with a revival 

 in the growth in numbers of the saprophytes which had 

 suffered a slight eclipse by the previous growth of the sulphur 

 bacteria. The most vigorous growth of Thioporphyra occurred 

 on the eighth week when the pH value was 7-6, a figure which is 

 within the range set by Baas-Becking for the optimum growth 

 of the sulphur bacteria. 



The Biological Explanation of the Assimilation of 

 Hydrogen Sulphide. 



It has been shown that sulphuretted hydrogen does not 

 enter the cell in the form of food, and that the change from 

 the sulphide to sulphur does not result in a liberation of 

 energy. It has also been shown that an undoubted gain 

 to the bacteria follows from the assimilation of the sul- 

 phide. Hence the advantage cannot be a direct one, and 

 must follow because the hydrogen sulphide is a factor in a 

 wider scale of operations than its simple decomposition. It 

 is suggested that this substance enters the cells as an adjunct 

 in the respiratory process. After absorption the sulphuretted 

 hydrogen is chemically transformed in such a manner that 

 hydrosulphide ions are placed at the disposal of the bacteria. 

 As this is an endothermic process the necessary energy must be 

 supplied from the general resources of the organism, derived 

 from other metabolic changes. Then union of the sulphide 

 with the sulphur amino-acids takes place, resulting in the 

 production of free sulphur and the liberation of energy. The 

 probable changes that would then take place have been 



