Evidence for autotrophic CO2 assimilation in Sulfolobus brierleyi via a reductive carboxylic acid pathway

The labeling kinetics of early products of heterotrophic and autotrophic ¹⁴CO₂ fixation in Sulfolobus brierleyi have been studied. Glutamic acid and glutamine were the dominating labeled compounds after 30 sec and 10 min of heterotrophic ¹⁴CO₂ fixation, with only small portions of ¹⁴C detectable in aspartic acid, alanine and an unknown compound Z. Malic acid, citric acid, glutamic acid, aspartic acid and an unknown compound X were the compounds most rapidly labeled during short time autotrophic ¹⁴CO₂ fixation.

It is concluded that the heterotrophic CO₂ fixation of S. brierleyi is based primarily on an exchange between the α-carboxyl group of α-keto-glutaric acid and ¹⁴CO₂, whereas the autotrophic ¹⁴CO₂ fixation of this organism leads to a true net assimilation of carbon via a reductive carboxylic acid pathway with malic acid, citric acid, aspartic acid, glutamic acid and the unknown compound X as early products. The participation of pyruvate is doubtful, since alanine is only slowly labeled. The evolutionary implications of these findings are discussed.

It is suggested that a reductive carboxylic acid pathway, of which various modifications exist at present in Chlorobium, an early unique branch of the eubacteria and in both groups of autotrophic archaebacteria (methanogenic bacteria and Sulfolobus) may be a common heritage of all organisms, whereas the Calvin-Benson cycle was “invented” at an early evolutionary stage of the gram-negative eubacteria.