COMPARATIVE BIOCHEMISTRY 77 



van Niel's experiments showed that the photosynthetic 

 reaction of the purple bacteria could be represented by the 

 equation: — 



2H2S + CO2 -" photosynthetic product (CH20) + 2S+H20 



He therefore suggested that in these organisms the hydrogen 

 sulphide simply acted as a hydrogen donor, itself becoming 

 oxidized to free sulphur. This might be compared to the 

 photosynthetic reaction in green plants which can be repre- 

 sented as: — 



2H2O+CO2 -" photosynthetic product (CH20) + 02+H20 



where it may be suggested by analogy that water serves 

 simply as a hydrogen donor. Thus photosynthesis may be 

 considered as an oxidation-reduction reaction. This hypo- 

 thesis offers an explanation both for the requirement of 

 sulphide for carbon dioxide reduction and for the fact that 

 the product of the reaction in the bacteria is not oxygen 

 but sulphur. It is now known that a variety of hydrogen 

 donors can be used by the Thiorhodaceae, including thio- 

 sulphate, selenium compounds, and in some purple bacteria 

 simple organic substances. 



5H20+2C02+Na2S203 ^ 2(CH20)+2H20+2NaHS04 

 3C02+8H20+2Se - 3(CH20) + 3H20+2H2Se04 



The green sulphur bacteria (Chlorobacteriaceae) are 

 similar to the purple sulphur bacteria, differing in the lack 

 of the pigments imparting purple coloration. In this group 

 the sulphur granules are generally deposited outside the cells. 



THE PURPLE NON-SULPHUR BACTERIA (aTHIORHODACEAE) 



The Athiorhodaceae (with the exception of some strains of 

 Rhodopseudomonas palustris) are unable to utilize reduced 

 sulphur compounds but grow in the presence of simple 

 organic substances such as organic acids. All require the 

 addition of growth factors; some require biotin or thiamin or 

 ^-aminobenzoic acid, or combinations of thiamin and biotin, 

 or thiamin, biotin, and nicotinic acid. This requirement 

 distinguishes the Athiorhodaceae from the Thiorhodaceae. 



