70 RESPIRATION OF BACTERIA 



ing to that of the pools, the sheaths become coloured anew. It is, however, 

 only those which contain living cells that take up iron ; the empty sheaths 

 remain colourless. It is evident from this that the living bacterial cell 

 hastens the oxidation of the ferrous carbonate, just as it does the oxidation 

 of SH 2 in the Thiobacteria. In the iron bacteria, too, some energy, albeit 

 a very small amount, is gained in the process. 



Since the sheaths and cell membranes of other aquatic plants (e.g. 

 Cladothrix and Crenothrix among the bacteria, Conferva (Psichohorrmium) 

 among the filamentous algae) are also turned yellow by the deposition of 

 iron oxide, it has yet to be seen whether the iron bacteria form a separate 

 biological group. Their nutrition as regards carbon and nitrogen also needs 

 further investigation. They are doubtless prototrophic. 



It would be justifiable from some points of view to consider the acetic 

 bacteria in this section, but they can be more conveniently treated of with 

 the other fermentation bacteria. 



