ial, red on the surface but with green and black matter underneath. In places it was 

 about Yi" thick. When pierced, the carpet released occluded gas. Microscopical ex- 

 amination at the lake side showed that it contained many sulphur granules, crystals 

 (?CaS04), many spiral bodies and a few protozoa; but the bulk of the material was an 

 amorphous mass of cellular material and sulphur granules. It was later recognised as 

 coloured photo synthetic sulphide- oxidising bacteria: Chromatium (red bacteria stor- 

 ing sulphur granules inside the cell) and Chlorobium (green bacteria which precipitate 

 sulphur outside the cell). These organisms require light for growth ; hence their mas- 

 sive development in the shallow water only. They were also detected in mud taken 

 from the deepest part of the lake (about 9 ft.) and in surface water samples. 



Subsequently, pure cultures of both Chromatium and Chlorobium were isolated 

 using the techniques described by van Niel (1931). Old cultures of these organisms 



115 



