Carbon assimilation pathways in archaebacteria

Abstract

The assimilation of cell carbon from CO2 and other compounds has been studied in autotrophic representatives of the archaebactera. The Calvin cycle is not operating in these bacteria. Two different basic pathways for the assimilation of cell carbon from CO2 are found. The methanogenic archaebacteria assimilate CO2 into the central intermediate acetyl CoA via a non-cyclic pathway. The methyl group of acetate is derived from tetrahydropteridin-bound C1-compounds which are also common intermediates in the process of CO2 reduction to methane. The carboxyl group of acetate is formed by CO2 reduction to carbon monoxide. Further assimilation of the activated acetic acid involves 2 to 3 further CO2 fixation reactions and proceeds via the reductive carboxylation of acetyl CoA to pyruvate. The autotrophic sulfur-associated (—“dependent”) archaebacteria appear to use a cyclic reductive carboxylic acid pathway for CO2 assimilation; however, essential enzymes remain to be demonstrated. In the aerobic sulfur oxidizer Sulfolobus the nature of this pathway is still uncertain. In the anaerobic sulfur reducer Thermoproteus a “reductive citric acid cycle” may be operative.