COMPARATIVE BIOCHEMISTRY 75 



the bacteria were photosynthetic, they should also require 

 a supply of sulphide; furthermore, in the light no oxygen 

 production could be detected and indeed the cells grew 

 only under strictly anaerobic conditions. Since they required 

 sulphide there was naturally a tendency to suppose they 

 were related to the sulphur-utilizing organisms of Wino- 

 gradsky but differed in the presence of a pigment imparting 

 a purple coloration and in the necessity of light for growth. 

 Later yet other purple bacteria were found by Molisch 

 which required both light energy and a supply of organic 

 substances for growth. Molisch suggested that in these 

 organisms light was necessary for the assimilation of the 

 organic substances. 



The difficulties which beset investigators in this field were 

 in part due to the fact that the organisms were not obtained 

 in pure culture. Later when this was achieved, van Niel 

 (1941) was able to make clear the inter-relationship between 

 the purple bacteria and the sulphur bacteria. He was able 

 to distinguish these bacteria as members of three groups, 

 namely, 



1 . The chemosynthetic colourless bacteria of Winogradsky 



which assimilate carbon dioxide as a result of the 

 oxidation of sulphide. 



2. Photosynthetic bacteria which do not require organic 



growth factors and utilize inorganic compounds in the 

 light (Thiorhodaceae). These fall into two groups: — 



a. green bacteria which assimilate carbon dioxide in 

 light in presence of sulphide and cannot utilize 

 organic compounds; 



h. purple bacteria which utilize sulphide in the light 

 but can in addition utilize some organic com- 

 pounds. (Englemann's organisms.) 



3. The photosynthetic purple bacteria of Molisch which 



require organic growth factors and in general use 

 organic hydrogen donors for assimilation of CO2 in. 

 light (Athiorhodaceae). 



