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

Abstract

Summary The labeling kinetics of early products of heterotrophic and autotrophic 14 CO 2 fixation in Sulfolobus brierleyi have been studied. Glutamic acid and glutamine were the dominating labeled compounds after 30 sec and 10 min of heterotrophic 14 CO 2 fixation, with only small portions of 14 C 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 14 CO 2 fixation. It is concluded that the heterotrophic CO 2 fixation of S. brierleyi is based primarily on an exchange between the α -carboxyl group of α -keto-glutaric acid and 14 CO 2 , whereas the autotrophic 14 CO 2 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.