IRON BACTERIA 69 



way as those of the visible spectrum. The Erythrobacteria have therefore 

 a double source of energy, the oxidation of sulphur and the absorption of 

 sunlight, a great advantage to them in the struggle for existence, inas- 

 much as they would be able to subsist in places where, from want of 

 sulphuretted hydrogen, the colourless Thiobacteria would perish. What 

 products arise from the assimilation of the C0 2 is not known ; starch has 

 not been detected. 



The Erythrobacteria are among the most sensitive phototactic organisms 

 known, the slightest diminution of brightness repelling, the slightest increase 

 attracting, them. The actively motile chromatia can be led about under the 

 microscope at will by suitable manipulation of the light. 



The importance of the part which the sulphur-bacteria play in the 

 economy of nature lies in their behaviour to sulphuretted hydrogen. They 

 take this gas — a useless and injurious product of organic decay — and work it 

 up into sulphates, substances which are assimilated by other organisms, and 

 used to build up new life. 



Hardly less remarkable in their nutrition than the sulphur bacteria are 

 the ' iron-bacteria ' (ferrobacteria, 38), which resemble them somewhat in 

 their prototrophic respiration. Our knowledge of these organisms is still 

 very fragmentary. Stagnant pools in marshy places are often covered by 

 a greasy-looking brownish scum, that consists mainly of ferric hydroxide, 

 Fe(HO) 3 , together with organic matter and some phosphate of iron. 

 The ferruginous matter is deposited as bog iron ore. By the action of 

 reducing substances arising from putrefaction, the ferric compounds, 

 particularly ferric hydroxide, are reduced to ferrous compounds, which 

 are dissolved by and unite with the C0 2 in the water to form ferrous 

 carbonate. The atmospheric oxygen alone is sufficient to convert this 

 back slowly again into ferric hydroxide, and cause its precipitation, but 

 Winogradsky has shown that the change is not purely chemical, and that 

 here, too, bacteria step in and accelerate the process. In the iridescent 

 deposits of the pools enormous numbers of short brittle tubes are found. 

 These are the broken sheaths of an unbranched filamentous bacterium, that 

 may be called for the present Leptothrix ochracea. These yellowish-brown 

 sheaths are coloured blue by hydrochloric acid and ferrocyanide of 

 potassium ; they contain ferric hydroxide. Specimens of bog iron ore 

 from Siberia, Sweden, and North Germany contained, in thirty-three 

 samples, only three with large amounts of these sheaths. 



Besides the empty sheaths vigorously growing mats of Leptothrix are 

 always found, some with the cylindrical cells still in situ, others where they 

 have already swum away in the form of gonidia (as in Cladothrix). By 

 placing the bacteria in water containing C0 2 the iron may be dissolved out 

 of the sheaths, leaving them colourless. If these bleached filaments be then 

 put into a weak solution of ferrous carbonate of a concentration correspond- 



