13C-NMR study of autotrophic CO2 fixation in Thermoproteus neutrophilus

The pathway of autotrophic CO2 fixation has been investigated in the extremely thermophilic sulfur-respiring anaerobic archaebacterium Thermoproteus neutrophilus. [1,4-13C2]Succinate was used as a tracer since this compound was incorporated in small amounts virtually into all cell compounds without affecting the organism's ability to synthesize all cell constituents from CO2. Three representative amino acids, glutamate, aspartate and alanine were isolated from cells after growth for several generations in the presence of [1,4-13C2]succinate and their labelling patterns were determined by 13C NMR spectroscopy. The data is consistent with CO2 fixation by a reductive citric acid cycle, as proposed earlier for the green sulfur bacterium Chlorobium limicola, the sulfate-reducing Desulfobacter hydrogenophilus and the microaerophilic Knallgasbacterium Hydrogenobacter thermophilus. The presence of a reductive citric acid cycle in archaebacteria indicates that this CO2 fixation mechanism which is an alternative to the Calvin cycle is present in many anaerobic or facultative anaerobic microorganisms.