OBUGATE PHOTOAUTOTROPHY OF GREEN BACTERIA 157 



The experiments described in this paper indicate that a closely 

 related bacterium C. thiosulfatophilunA is deficient in aconitate hy- 

 dratase. With the exception of a-ketoglutarate dehydrogenase which 

 was not assayed, all the remaining enzymes necessary for the oxida- 

 tion of acetate via the citric acid cycle are present. A deficiency of 

 aconitate hydratase would, firstly, preclude the possibility of acetate 

 incorporation via a glyoxylate cycle such as occurs in Chromatium 

 (23), and secondly, prevent oxidation of acetate via a citric acid cycle. 

 While metabolism via the citric acid cycle cannot result in a net up- 

 take of carbon, it can provide the CO2 and reducing power necessary 

 for acetate assimilation by a pathway such as that occurring in R. 

 rubrum (24), A deficiency of aconitate hydratase could thus explain 

 why acetate alone cannot support the growth of C. thiosulfatophilum. 

 Assuming that these cells, like C. limicola, can assimilate acetate 

 provided CO2 and reduced sulfur compounds are present, a pathway 

 for acetate assimilation exists which involves neither the glyoxylate 

 nor citric acid cycles. 



In the presence of thiosulfate, the growth of C, thiosulfatophilum 

 could theoretically be supported by any substance which is converted 

 by the organism to CO2. The inability of dicarboxylic acids like suc- 

 cinate and malate to function as growth substrates could be attributed 

 in part to a deficiency of aconitate hydratase. However, in R. rubrum 

 these acids can be diverted to form CO2 and Cs-fragments under con- 

 ditions where the citric acid cycle is blocked by monofluoroacetate 

 (32), This alternative to metabolism via the citric acid cycle does not 

 appear to operate in C. thiosulfatophilum. 



ACKNOWLEDGMENTS 



The research at Brookhaven National Laboratory was carried out under the 

 auspices of the U. S, Atomic Energy Commission. The assistance of Miss H. 

 Kelly, Mrs, O. Ritter, and Mr. N. Rigopoulos in cultivating the photosynthetic 

 bacteria was greatly appreciated. 



SUMMARY 



Experiments were carried out in an attempt to establish a bio- 

 chemical basis for the obligate photoautotrophy of Chlorobium thio- 

 sulfatophilum. It is suggested that this organism cannot utilize acetate 

 in place of CO2 as the growth substrate because it lacks aconitate 

 hydratase. This precludes assimilation of acetate by a glyoxylate 

 cycle as well as oxidation via the citric acid cycle to provide the CO2 



C. thiosulfatophilum is distinguished from C. limicola by the inability of the 

 latter to oxidize thiosulfate (7). 



