Abstract
Chlorobium limicola has been proposed to assimilate CO2 autotrophically via a reductive tricarboxylic acid cycle rather than via the Calvin cycle. This proposal has been a matter of considerable controversy. In order to determine which pathway is operative, the bacterium was grown on a mineral salts medium with CO2 as the main carbon source supplemented with specifically labeled 14C-pyruvate, and the incorporation of 14C into alanine (≙intracellular pyruvate), aspartate (≙oxaloacetate), glutamate (≙α-ketoglutarate), and glucose (≙hexosephosphate) was measured in exponentially growing cells in long term labeling experiments. During growth in presence of pyruvate, 20% of the cell carbon were derived from pyruvate in the medium, 80% from CO2. Since pyruvate was not oxidized to CO2, only those compounds should become labeled which were synthesized from CO2 via pyruvate.
The three amino acids and glucose were found to be labeled. Alanine had one fifth the specific radioactivity of the extracellular pyruvate, indicating that 20% of the intracellular pyruvate pool were derived from pyruvate in the medium, 80% were synthesized from CO2. Glucose had twice the specific radioactivity of alanine, showing that hexosephosphate synthesis from CO2 proceeded via the pyruvate pool. The latter finding is not consistent with the operation of the Calvin cycle, in which pyruvate is not an intermediate. The specific radioactivities of aspartate (≙oxaloacetate) and of glutamate (≙α-ketoglutarate) were practically identical but considerably lower than that of alanine (≙ intracellular pyruvate). These findings are compatible with the operation of a reductive tricarboxylic acid cycle as mechanism of autotrophic CO2 fixation. Degradation studies of the cell components support this interpretation.
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Fuchs, G., Stupperich, E. & Jaenchen, R. Autotrophic CO2 fixation in Chlorobium limicola. Evidence against the operation of the Calvin cycle in growing cells. Arch. Microbiol. 128, 56–63 (1980). https://doi.org/10.1007/BF00422306
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DOI: https://doi.org/10.1007/BF00422306