39Q SCIENCE PROGRESS 



The Physiology of the Iron-Bacteria 



The outstanding feature of these organisms is their power 

 of attraction for iron-compounds. In the waters in which they 

 abound, unlike their co-inhabitants, large quantities of ferric 

 hydroxide are found deposited on the sheaths or membranes 

 of these bacteria. Complete agreement as to the explanation 

 of this phenomenon does not at present obtain. Not only the 

 Iron-bacteria, but also a few algae and protozoa possess the 

 same peculiar characteristic. It has been shown above that 

 except in infinitesimal quantities the presence of iron-compounds 

 is not necessary for the successful development of Iron-bacteria. 

 One may therefore dismiss at once Winogradsky 's theory that 

 the oxidation of ferrous to ferric compounds was the source 

 from which this energy was obtained. He argued that without 

 the oxidation of ferrous bicarbonate to ferric hydroxide no 

 growth was possible. This oxidation takes place according to 

 the equation : 



2FeC0 3 l + 3H 2 0+ O = Fe 2 (OH) 6 + 2 C0 2 

 H 2 C0 3 / C0 2 +H 2 



Obviously the whole theory falls to the ground when artificial 

 cultivations can be made from which iron has been rigorously 

 excluded ; and further, in Nature there are many iron-waters 

 that harbour Iron-bacteria which do not contain any iron in the 

 form of the bicarbonate. Winogradsky 's theory still holds 

 sway in many of the text-books, although he produced no 

 supporting facts. 



Another theory is supplied by Molisch. He argues that the 

 accumulation is due to a chemio-tactic irritability for iron-com- 

 pounds. These substances are attracted by the bacteria, 

 detained by the mucilaginous sheath, and there oxidised into 

 ferric hydroxide. This does away with the necessity for iron- 

 compounds, and is supported by a strong body of facts, as will 

 be seen presently. A third theory is advanced by Campbell 

 Brown, who contends that the activities of the Iron-bacteria 

 are in all cases directed towards the assimilation of organic 

 compounds. When iron is combined in solution with these 

 compounds the removal of the latter brings the iron out of 

 solution, with the result that the precipitated iron collects on 

 and in the membrane or sheath and even invades the cell. 



