102 PHOTO- AND CHEMOSYNTHESIS OF BACTERIA CHAP. 5 



Table 5.1 

 Characteristics op the Three Groups of Photostnthesizing Bacteria 



(after van Niel) 



Green bacteria: Green-colored bacteria, occurring in hydrogen sulfide media. 

 Photosynthetic activity seems restricted to photoreduction of 

 carbon dioxide with hydrogen sulfide as hydrogen donor. 

 Oxidation proceeds only to elementary sulfur. Other sulfur 

 compounds and organic substances not used as hydrogen 

 donors. Organic growth factors not required. 

 Purple sulfur Purple to red-colored bacteria, also occurring primarily in 

 BACTERIA sulfide-containing media. Capable of oxidizing various inor- 



(Thiorhodaceae ganic sulfur compounds to sulfate with the simultaneous 

 MoUsch): photoreduction of carbon dioxide. Various organic sub- 



stances, particularly the lower fatty acids, and some hydroxy 

 and dibasic acids, can be used as hydrogen donors instead of 

 sulfide. Some species can also use molecular hydrogen. Or- 

 ganic growth factors not required. 

 Purple Purple, red or brown-colored bacteria, occurring principally 



NONSULFUR in media containing organic compounds. Capable of photo- 



bacteria chemical reduction of carbon dioxide with a large number of 



(Athiorhodaceae different organic reductanls; some species can use molecular 

 Mohsch) : hydrogen. Although some species are also capable of oxidizing 



inorganic sulfur compounds to sulfate, growth depends on the 

 presence of small amounts of complex organic materials, such 

 as yeast extract, which presumably furnish necessary organic 

 growth factors. 



bacteria normally require an organic source of hydrogen. They thrive 

 on a large variety of organic compounds, including acids, alcohols, 

 hydroxy acids, etc. 



2. The Over-All Photosynthetic Reactions of Autotrophic Bacteria 



Over-all chemical equations have not yet been established for all the 

 forms of bacterial photosynthesis by analyses comparable in precision to 

 those which lead to equation (3.6) and (3.7) for the over-all reaction of 

 normal photosynthesis. Since the oxidation products of bacterial 

 photosynthesis are solids (e. g., sulfur) or solutes {e. g.; sulfuric acid), 

 they are unsuitable for manometric assay, which is so convenient for the 

 determination of the "photosynthetic quotient" of the higher plants. 



On the other hand, bacterial photosyntheses offer the possibility of 

 determining the consumption of the redudant (e. g., hydrogen sulfide or 

 hydrogen) simultaneously with that of the oxidant (carbon dioxide), 

 whereas a determination of water consumption in normal photosynthesis 

 is practically impossible. 



