PHYSIOLOGY OF THE SULPHUE-BACTERIA 151 



of sulphuretted hydrogen. This is taken up by the sulphur-bacteria, 

 and ultimately changed into sulphates. The sulphates are then taken 

 up by the roots of plants and built up once more into the albuminoid 

 molecule. This can be diagrammatically represented as shown above. 

 This, of course, does not represent the whole of the changes that take 

 place, only the broad cycle of events. 



THE IRON-BACTERIA. 



§4. INTRODUCTION TO IRON-BACTERIA. 



The iron-bacteria are so called because the membrane of these 

 organisms is usually covered with a reddish-brown deposit of ferric 

 hydroxide. They abound in iron waters, and hitherto have not been 

 found growing elsewhere. Such waters usually ooze out of the earth, 

 the iron in them being present at first in the form of the soluble 

 bicarbonate, FeH2(COg)2 : they are common in almost all parts of 

 Great Britain, and particularly in the neighbourhood of Glasgow. 

 The immediate neighbourhood of the spot where this kind of water 

 wells out is covered with a reddish-brown deposit. In the same 

 ■way the beds of the streams which are fed by this water become 

 reddish-brown in colour. The colour is due to the formation of 

 the insoluble ferric hydroxide into which the soluble bicarbonate 

 ■changes as it reaches the surface. This change can be represented as 

 follows : 



aFeCOgi -I- SHjO + = 'E'e^(OB.)s + 2CO3. 



y ieviic hydroxide 



HjCOgJ H.,0h-C02. 



The accumulation of the precipitated ferric hydroxide is often a 

 source of trouble, and it has either to be periodically removed, or else 

 -conducted into a stream, so that it may be carried down with the 

 water. 



If, now, the reddish-brown deposit be microscopically examined, 

 the ferric hydroxide will be found, in the vast majority of samples, to 

 be deposited on the membranes of organisms ; in fact, it may be stated 

 that the deposit consists of organisms, the membranes of which are 

 coated with ferric hydroxide. Sometimes this coating is so thick that 

 its width is four or five times that of the organism which it envelops. 

 In almost all cases the organisms belong to the bacteria. lu some 



