CLASSIFICATION OF THE SULPHUE-BACTERIA 



149 



4. Ophidomonas sanguinea (Fig. 87a). Also a doubtful form, but 



described by Ehrenberg as one of the sulphur-bacteria. 



5. Rhabdomonas rosea (Fig. 87b). Eoughly spindle-shaped, as 



seen in the diagram. About 4-5 fi broad and 20-30 /i long. 

 This is an interesting group on account of the colouring matter, 

 bacteriopurpurin. It has been stated that it plays the same r61e 

 as the chlorophyll of green plants, by 

 the aid of which the latter are able to 

 elaborate carbohydrates from the carbon 

 dioxide and water vapour of the atmo- 

 sphere, the process being also marked by 

 the liberation of free oxygen. Though it 

 has not been conclusively demonstrated 

 that bacteriopurpurin possesses this attri- 

 bute, it is significant that these bacteria, 

 when cultivated in a glass vessel, congre 

 gate in large numbers on the side nearest 

 the light, in this respect differing from 

 all other sulphur-bacteria. Also, when 

 grown in deep vessels, it is noteworthy that the most luxuriant 

 growth is found at the bottom, where the supply of oxygen is very 

 scanty. Now, all living creatures — with a few exceptions — require 

 oxygen for purposes of respiration, so that it seems probable that 

 these bacteria are enabled to live at low depths because they use up 

 the oxygen given off in the process of assimilation mentioned above. 



Fig. 87. — (a) Opbidomouas san- 

 guinea ; (b) Bhabdomouas rosea. 

 (After Cohn.) 



§3. THE PHYSIOLOGY OF THE SULPHUR-BACTERIA. 



As has been explained above, these bacteria grow in waters rich in 

 sulphuretted hydrogen. The first change is the conversion of this 

 substance into free sulphur, this taking place according to the following 

 equation: SH^S-f 0, = 2H20-f S^. 



sulphuretted hydrogen sulphur 



The sulphur is now stored in the cells and, when required, it is 

 further oxidised into sulphuric acid, according to the equation : 

 S2-t-302-l-2H20 = 2H2SO^. 



sulphur sulphuric acid 



It will be noticed that a double process of oxidation takes place. 

 According to Winogradsky, the transformation of sulphuretted hydro- 

 gen into sulphuric acid is the process which the plant relies upon 



