66 SULPHUR BACTERIA 



demand for it by the sulphur bacteria. After the sixth week, 

 although there was a marked increase in the number of 

 saprophytes, the sulphur bacteria had increased at a far 

 greater rate, until the demand for hydrogen sulphide was 

 greater than the supply. The gradient downwards in the 

 sulphide graph is very steep between the sixth and the tenth 

 weeks. 



On the other hand, the downward decline of the oxygen 

 curve is steady during the whole of the period of investi- 

 gation, an indication that both the saprophytes and the 

 sulphur bacteria were using up this element in respiration. 



Whilst the graphs for hydrogen sulphide and for oxygen can 

 be interpreted in terms of the vital activity of the sulphur 

 bacteria, the same cannot be said of the SO4" graph. The in- 

 determinate character, of this graph indicates that, like the Ca 

 and the Mg, which have similar graphs, its rise or fall does not 

 coincide with the rise or fall in the numbers of the sulphur 

 bacteria. This fact is consistent with the supposition that the 

 change from S to the sulphate is not directly dependent on the 

 vital activities of the sulphur bacteria. 



The smiall fluctuations in the pW values reflect the small 

 changes that follow the absorption of hydrogen sulphide by 

 the sulphur bacteria, the production of other acids by the 

 saprophytic bacteria, the partial neutralization of these acids 

 by the calcium and other compounds, and the changes caused 

 by evaporation or by rainfall. The results in general confirm 

 those obtained by other investigators for other sulphur bac- 

 teria, but cannot be regarded as conclusive, because the con- 

 tribution of other organisms which were necessarily present in 

 the pool under observation could not be exactly estimated. 



The growth of Thioporphyra reached its zenith in a medium 

 containing enough organic matter to support 5,000,000 bacteria 

 per cubic centimetre. Winogradsky's conclusion that the sul- 

 phur bacteria tolerate only minute quantities of organic matter 

 is not justified. Without pure cultures it was not possible to 

 ascertain if Thioporphyra assimilates organic matter directly. 



