4 SULPHUR BACTERIA 



or sulphur to sulphates, the necessary oxygen being obtained 

 by the reduction of nitrates. 



3. Siilphale-redncing Bacteria, which effect the reduction 

 of sulphates, sulphites, and thiosulphates to sulphuretted 

 hydrogen. 



4. Saprophytic Bacteria, which by their activities liberate 

 sulphuretted hydrogen from the organic molecule. 



Although these four classes of bacteria are concerned with 

 the sulphur metabolism, they are not termed sulphur bacteria. 



Subdivision of the Sulphur Bacteria. — The first division is 

 based on the presence or absence of colour. 



1. Colourless sulphur bacteria. 



2. Coloured sulphur bacteria. 



The colouring matter is composite in structure, and the 

 sum total of the ingredients imparts to the organism a purple 

 colour. The purple colouring matter is also found in bacteria 

 other than the sulphur bacteria. Whilst it has not yet been 

 proved that the association of this colouring matter with the 

 sulphur metabolism is other than accidental, it is a note- 

 worthy fact that all the coloured sulphur bacteria are coloured 

 with the same group of pigments. Although there are con- 

 siderable differences in the tints of the various growths of 

 sulphur bacteria, they differ only in degree, not in kind. The 

 differences are due to variations in the proportions in which 

 the constituent colouring substances are present. 



Geographical Distribution of the Sulphur Bacteria. 



If we consider the conditions which favour the multiplica- 

 tion of the sulphur bacteria, it is not surprising that their 

 distribution is universal. They thrive equally well in salt 

 water and fresh water. They occur wherever saprophytic 

 decomposition results in the production of sulphuretted hydro- 

 gen, and wherever, in addition, the water is shallow, the supply 

 of oxygen not abundant, and the temperature suitable. As 

 these conditions are obtainable in all parts of the world, with 

 the exception of certain areas with special conditions (polar 

 regions, deserts, etc.), the sulphur bacteria are universally 



