PHOTOHETEROTROPHIC BACTERIA 



107 



requires the presence of carbon dioxide (which is " coassimilated " with 

 the acid), makes it probable that the initial step is a photochemical 

 reaction between the organic substrate and carbon dioxide; but the 

 subsequent stages of the process could include the direct "heterotrophic" 

 assimilation of the organic material. 



Under these conditions, it is important that Foster (1940) found 

 organic substrates capable of yielding only two hydrogen atoms to carbon 

 dioxide in light, and resisting any further assimilation. These were 

 secondary alcohols. The carbon chain in these compounds is not attacked 

 by purple bacteria, and the reaction in light is restricted to the transfer 

 of two hydrogen atoms from the alcohol to carbon dioxide, according to 

 the equation: 



(5.9) 



Ri Ri 



\ light \ 



: CHOH + CO2 > 2 C=0 + {CH2O) + H2O 



Ra R2 



For example, isopropanol is quantitatively converted into acetone: 



(5.10) 2 (CH3)2CHOH + CO2 > 2 (CH3)2CO + {CH2O} + H2O - 14 kcal 



Foster found for this reaction, the " photosynthetic quotients" given 

 in table 5.IV. 



Table 5.IV 

 Photosynthetic Quotients for Bacterial Photosynthesis with Isopropanol 



We shall now return to the assimilation of fatty acids and attempt 

 to analyze it in terms of photoreduction combined with direct assimila- 

 tion. A "pure" photoreduction of fatty acids, if it is to lead to carbo- 

 hydrates as the only products, must involve either "coassimilation" or 

 hberation of carbon dioxide. When the substrate is " overreduced " 

 (compared to the carbohydrates), it must be diluted with carbon dioxide; 

 when it is "underreduced," some carbon dioxide must be eliminated. 



