THE LIGHT FACTOR IN PHOTOSYNTHESIS 1043 



CO2 + 2H2S -> CH2O + H2O + 2S 



and in the case of the purple sulfur bacteria 



2CO2 + H2S + 2H2O -> 2CH2O + H2SO4 



The question is in what manner the oxidation of the H2S is involved 

 in the reduction of the CO2. Kluyver and Donker (52) have suggested 

 that in these organisms the H2S is a hydrogen donor and the CO2 the 

 hydrogen acceptor. The concept has been elaborated by van Nicl 

 to the effect that in hydrogen sulfide the hydrogen is loosely bound and 

 is already present in an active form while the green pigments may be 

 regarded as the agents which cause the activation of the hydrogen accep- 

 tor, the carbon dioxide. According to this view, in ordinary autotrophic 

 plants water must serve as the hydrogen donor and in this case oxygen is 

 liberated. Any substance can act as a hydrogen donor to the carbon 

 dioxide provided its hydrogen can be photochemically activated in the 

 organism, and the reaction may be written: 



CO2 + 2AH2 -^ CH2O + H2O + 2A 



Van Niel suggested that certain organic substances could serve as 

 hydrogen donors for the reduction of carbon dioxide. Muller (82) 

 cultured purple bacteria in the presence of organic substances and 

 found that the bacteria developed only in the light. The difference in 

 the amount of carbon dioxide taken up per unit of substrate consumed 

 varied with the oxidation value of the substrate. Gaffron (31) has 

 followed the photosynthesis of the nonsulfur bacteria which absorb 

 carbon dioxide only in the light when grown on sodium salts of fatty 

 acids. The ratio of the carbon dioxide absorbed to the organic acid 

 used varied with different acids. For each increase in CH2 in the carbon 

 chain of the fatty acid there was an increase of 0.5 mole of carbon dioxide 

 absorbed. 



The results of experiments on photosynthesis in intermittent light 

 have been used to elucidate the relation of the photochemical and Black- 

 man reactions. Warburg (133) made comparisons between the effects 

 of continuous and intermittent illumination. With high intensity of 

 illumination, equal amounts of radiant energy reduced more carbon 

 dioxide when this was intermittent than when it was continuous. The 

 excess of photosynthesis in intermittent light depended upon the fre- 

 quency of the flashes; when these were 8000 per minute this was almost 

 100 per cent, while with 4 per minute it was about 10 per cent. Warburg 

 suggested that these results might be explained either on the assumption 

 that the reduction of carbon dioxide proceeds during the dark periods 

 with uninterrupted rate, or that the reduction is interrupted in the dark 

 periods and proceeds at double the rate in the light periods. The latter 

 of these he considered as the more probable. 



