78 PHOTOSYNTHESIS 



The organisms were at first regarded as heterotrophic, 

 directly assimilating organic compounds in the light, but 

 van Niel suggested that there was another possibility, 

 namely that the organic substances might act simply as 

 hydrogen donors in a photosynthetic process. This was 

 shown by Foster to be the case for a substrate such as iso- 

 propanol which was quantitatively converted to acetone at 

 the same time that an equivalent quantity of carbon dioxide 

 was consumed, in accordance with the equation: — 



2CH3CHOHCH3+CO2 - (CH20)+2CH3COCH3+H20 



Some results are given in Table 5.2. The work w^as sub- 

 sequently confirmed by Seigel and Kamen using labelled 

 carbon dioxide and labelled alcohol. On the other hand w^ork 

 with labelled acetate showed that acetate does not serve 

 exclusively as hydrogen donor but is in part directly 

 assimilated by the cell without passing through the stage of 

 CO2. Thus certain substrates are used entirely as hydrogen 



TABLE 5.2 



UTILIZATION OF ISOPROPANOL BY A STRAIN OF PURPLE 



NON-SULPHUR BACTERIA 



{from van Niel) 



donors whereas other substrates are in part used as hydrogen 

 donors to carbon dioxide and in part assimilated hetero- 

 trophically. Indeed some strains of Athiorhodaceae can live 

 heterotrophically in the dark under aerobic conditions by 

 oxidizing organic substance whereas in the light under an- 

 aerobic conditions they utilize a photosynthetic mechanism; 

 i.e. these strains are facultatively anaerobic. 



