Further studies on carbon dioxide fixation in Chlorobium.

Abstract

The pathway of carbon in photosynthesis of Chlorobium thiosulfatophilum strain 8346 was examined. After exposure of the cells to 14CO2, chromatography and radioautography indicated that malic, α-oxoglutaric, pyruvic, oxaloacetic, glutamic, and aspartic acids were the first stable products of photosynthesis. After 5 sec of photosynthesis, no radioactivity was detected in the area corresponding to phosphate esters on the chromatograms. 2. After 30 sec of exposure of the cells to 14CO2, all except one of the intermediates of the reductive carboxylic acid cycle were detectable on x-ray films of radioautograms. 3. An experiment was performed which showed that the specific activities of the individual intermediates of the reductive carboxylic acid cycle were within the same range, thus suggesting that the intermediates of the cycle were in isotopic equilibrium with each other. 4. Succinic acid showed a high specific activity after exposure of the cells to 14CO2, indicating that this compound might be formed by a direct carboxylation reaction as well as by the reactions of the reductive tricarboxylic acid cycle. 5. Cells of Chlorobium were shown to contain isocitrate and citrate in amounts comparable to those found in aerobic bacteria. 6. The two enzymes specific for the reductive pentose phosphate cycle, RuDP-carboxylase and phosphoribulokinase, were not detected in cell-free extracts of the organism. 7. The presence of aconitate hydratase and isocitric dehydrogenase in amounts consistent with the operation of the reductive carboxylic acid cycle was confirmed. The level of these enzymes was not repressed when acetate was present in the medium, nor was their activity inhibited by acetate or acetyl-CoA when these substances were added to cell-free extracts. 8. The data presented here are discussed in relation to the reductive pentose phosphate cycle and the reductive tricarboxylic acid cycle, or other possible mechanisms of CO2-fixation in Chlorobium.