76 PHOTOSYNTHESIS 



It is found that H2 can act as H donor for many of the 

 organisms in groups zh and 3. Between the main groups of 

 Thiorhodaceae and Athiorhodaceae, Rhodomicrohiuni van- 

 nielii occupies an intermediate position since it requires no 

 growth factors but utilizes organic compounds in the assimi- 

 lation of carbon dioxide in the light. 



the photosynthetic purple sulphur bacteria 



(thiorhodaceae) 



The Thiorhodaceae will grow on strictly inorganic media 

 in which carbon dioxide is the sole carbon source. Growth 

 will only occur under anaerobic conditions and in an amount 

 proportional to the quantity of sulphide available (Table 

 5.1). Even using luminescent bacteria, the most sensitive 



TABLE 5.1 



THE SULPHIDE REQUIREMENT OF THIORHODACEAE 

 {from van Niel (1941)) 



way of detecting minute amounts of oxygen, no oxygen 

 production in the light has ever been observed. The sulphide 

 is oxidized to sulphur, which is deposited within the cells, 

 and may be ultimately converted to sulphate. Wieland and 

 later Kluyver considered that all the reactions concerned in 

 living organisms were oxidation-reduction processes involv- 

 ing a transfer of hydrogen or of electrons, as for example: — 



AH+B -^ A+BH 



