44 SULPHUR BACTERIA 



bacteria is that used by Licskc in his investigation of the iron- 

 bacteria. This is made up as follows : — 



Ammonium sulphate ((NH4)oS04) 



Potassium chloride (KCl) 



Magnesium sulphate (MgS04) 



Mono-hydric-potassium phosphate (K2HPO4) 



Calcium nitrate (Ca(N03)2) 



Calcium carbonate (CaCOg) 



Distilled water ..... 



The use of the comparatively large quantities of ammonium 

 sulphate and particularly of calcium carbonate in this mineral 

 solution is due to the circumstance that the nitrogen in the 

 sulphate is required for the building up of the protoplasm ; 

 whilst the carbonate is required for purposes of neutralization. 



These organisms are indifferent to common salt (NaCl). 



Issatchenko (4) gives a list of sulphur bacteria that were 

 found by him in solutions containing either NaCl or Na2S04 

 in proportions varying from I to 23 per cent. 



6. Hydrogeii-ioii Concentration. — The range in the pW 

 values within which growth takes place must be somewhat 

 circumscribed, for whilst the medium of growth must be 

 alkaline the two substances that are required in greatest 

 amounts in the metabolism of these organisms, namely, 

 sulphuretted hydrogen and carbonic acid, are of an acid 

 character. Hence growth is inhibited by the presence of 

 sulphuretted hydrogen in quantity. Baas-Becking found that 

 the optimum for the waters containing sulphur bacteria near 

 Stanford University was 7-6 — 8-6 ; his figures agree with those 

 determined by Atkins. Under natural conditions when active 

 growth is in progress the pW is progressively decreased by 

 the acid substances liberated by the saprophytic bacteria 

 (CO2, H2S, organic acids, etc.). When a certain degree of 

 acidity is reached the growth of the sulphur bacteria will 

 cease altogether, and will recommence only when the surplus 

 acids have once more been used up by saprophytic and other 

 organisms. 



