BIOCHEMICAL BASIS FOR THE OBLIGATE PHOTO- 



AUTOTROPHY OF GREEN BACTERIA 



OF THE GENUS CHLOROBIUM 



ROBERT M. SMILLIE and W. R. EVANS 



Biology Department, Brookhaven National Laboratory 



Upton, N. Y. 



Acetate and a variety of other organic compounds can serve as 

 sources of carbon and reducing power for the light- dependent growth 

 under anaerobic conditions of most photosynthetic bacteria. This is 

 not true for certain species of the genus Chlorobium (C. limicola and 

 C. thiosulfatophilum). These green sulfur bacteria grow only anaero- 

 bically in light in the presence of exogenous CO2 and reduced inorganic 

 sulfur compounds (1,2), They may be classified as anaerobic, obligate 

 photoautotrophs. Such restricted requirements for growth substrates 

 are not characteristic of all green bacteria. Chlorobium chloro- 

 chromatii can grow on peptone and malate (3). Chloropseudomonas 

 ethylicum utilizes several carbon sources including ethanol, acetate, 

 and glucose (4). 



The inability of C. limicola and C. thiosulfatophilum to utilize sim- 

 ple organic compounds as the sole carbon source for growth was es- 

 tablished by van Niel (1) and Larsen (2) and subsequently confirmed 

 by others (3,5). Although these compounds cannot act as growth sub- 

 strates in the absence of CO2, several are photometabolized in the 

 presence of CO2. Larsen demonstrated a carboxylation of propionate 

 to succinate by cells of C, thiosulfatophilum . Sadler and Stanier (5) 

 showed that the growth yield of C. limicola was improved by the addi- 

 tion of certain organic supplements. Of these, acetate was the most 

 effective and doubled the cell yield. The gross intracellular distribu- 

 tion of assimilated carbon from acetate was identical with that from 

 CO2. The incorporation of acetate was strictly dependent on the pro- 

 vision of both CO2 and sulfide. It was proposed that, whereas other 

 photosynthetic bacteria oxidize acetate to provide the CO2 and reducing 

 power necessary for the assimilation of other acetate molecules, an 

 enzymic pathway for acetate oxidation is lacking in C. limicola. The 

 utilization of acetate for cell growth by this organism would then be 

 dependent upon an exogenous supply of CO2 and reducing power. 



Since the oxidation of acetate in photosynthetic bacteria such as R. 

 rubrum can proceed via an anaerobic citric acid cycle (6), extracts of 

 C. thiosulfatophilum were examined for citric acid cycle enzymes. 



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