66 RESPIRATION OF BACTERIA 



Colourless and coloured species are almost always found in close proximity 

 to one another the colourless forms on substrata of all kinds, the coloured 

 ones in spots exposed to a strong light. The white furry coating generally 

 consists of filamentous forms, particularly of the unbranched non-motile 

 filaments of the delicately-sheathed Thiothrix, anchored by their bases to 

 the bodies on which they grow. Here and there among them one finds the 

 free-living, slowly-oscillating filaments of Beggiatoa (Fig. 1 7 a-c) ; unicellular 

 colourless species also occur. The coloured forms (Erythrobacteria*} 

 present a greater variety. The genus Chromatium (particularly C. Okenii) 

 consists of short, plump, rod-shaped or ovoidal cells which are often present 

 in such numbers as to give the water a reddish tinge. Thiospirillum is 

 a corkscrew-shaped form. The dirty pink scum that coats stones and 

 vegetation in sulphurous waters consists generally of a mixture of non-motile 

 species, small tablets of spherical cells (Thiopedia\ cylindrical species, and 

 zoogloeae (Lamprocystis, Fig. 17 e). Winogradsky gives descriptions of 

 nine genera. 



The very fact that the red sulphur bacteria collect at those spots where 

 the illumination is strongest would seem to indicate that there is some 

 relation between the light and their physiological processes. To understand 

 this connexion, however, we must first try to gain some conception of the 

 physiology of the colourless forms. In these it is only the part played by 

 the sulphuretted hydrogen that is clearly understood, but it is highly 

 probable that they are prototrophic in habit. The waters in which they 

 naturally occur contain only traces of organic matter f ; they will not grow 

 at all upon highly nutritious media like peptonised gelatine. As a source 

 of carbon, minimal quantities of formic or propionic acid suffice, and for 

 nitrogen, traces of ammonia, all substances which are common products 

 of putrefaction. They are strictly aerobic, and do not need light. They 

 grow best in water containing 100 milligrams SH 2 per litre, but are killed 

 by a saturated solution of the gas J. If filaments of a sulphur bacterium 

 already loaded with sulphur be placed in pure water they lose it all in from 

 twenty-four to forty-eight hours (Fig. 17 a-c), and finally die from ' sulphur- 

 hunger.' But if, before it be too late, they be replaced in water containing 

 SH 2 they recover and continue to grow, the SH 2 being oxidised and the free 

 sulphur stored up in the protoplasm of the cells. The disappearance of the 

 sulphur when the bacteria are placed in pure water is due to a further 

 oxidatory process by which sulphuric acid is produced. This unites with 

 the lime dissolved in the water to form gypsum. Other bacteria common 

 in marshy places, such as Cladothrix and Spirillum, as well as mould-fungi, 



* This term is suggested as perhaps the best equivalent of the German word ' Purpurbakterien.' 

 f- In Weilbach water only 0-0048 gram per litre, yet sulphur bacteria grow vigorously in it. 

 t 4'5^ grams SH 2 per litre. The Stachelberg spring contains 0-073 gram. 



