Antonie van Leeuwenhoek 66:209-221, 1994. 209 @ 1994 Kluwer Academic Publishers. Printed in the Netherlands. Metabolism of homoacetogens Gabriele Diekert & Gert Wohlfarth lnstitut fiir Mikrobiologie, Universitgit Stuttgart, Allmandring 31, D-70569 Stuttgart, Germany Key words: acetate formation from CO2, anaerobic acetate degradation, autotrophic COs fixation, carbon monoxide dehydrogenase, homoacetogenic bacteria Abstract Homoacetogenic bacteria are strictly anaerobic microorganisms that catalyze the formation of acetate from C 1 units in their energy metabolism. Most of these organisms are able to grow at the expense of hydrogen plus CO2 as the sole energy source. Hydrogen then serves as the electron donor for COs reduction to acetate. The methyl group of acetate is formed from CO2 via formate and reduced Cl intermediates bound to tetrahydrofolate. The carboxyl group is derived from carbon monoxide, which is synthesized from CO2 by carbon monoxide dehydrogenase. The latter enzyme also catalyzes the formation of acetyl-CoA from the methyl group plus CO. Acetyl-CoA is then converted either to acetate in the catabolism or to cell carbon in the anabolism of the bacteria. The homoacetogens are very versatile anaerobes, which convert a variety of different substrates to acetate as the major end product. Introduction Homoacetogenic bacteria, which are often also called 'acetogenic bacteria', are strictly anaerobic microor- ganisms, most of which catalyze the formation of acetate from H2 plus COs in their energy metabolism. The formation of acetate from H2 plus COs was first reported by Fischer et al. (1932) with enrichment cul- tures from sewage sludge. In addition, the same cul- ture catalyzed the conversion of carbon monoxide to acetate. Wieringa (1936) succeeded in the purification of the acetogenic anaerobe Clostridium aceticum with H2 plus CO2 as substrates. This bacterium, howev- er, soon was lost and was later reisolated by Adamse (1980) and by Braun et al. (1981). All attempts to iso- late a chemolithotrophic acetogen with H2 plus COs failed until the purification of Acetobacterium woodii (Balch et al. 1977). In the meantime a further aceto- gen called Clostridium thermoaceticum was enriched with glucose as energy source at 60 ° C (Fontaine et al. 1942). The organism converted 1 glucose almost com- pletely to near 3 acetate as end product. This new type of fermentation was therefore referred to as 'homoace- togenic' in analogy to the homofermentative lactate fermentation. The authors suggested either the cleav- age of glucose to 3 C2 units, which are converted to acetate, or the synthesis of one of the acetate molecules from two C1 units. Fermentation of glucose in the presence of ~4CO2 yielded acetate equally labelled in both carbon atoms (Barker & Kamen 1945), supporting the idea that both C-atoms of acetate might originate from CO2. Wood (1952) then demonstrated in a sim- ilar experiment, using 13CO2 and mass analysis that part of the acetate contained two adjacent 13C-atoms. This clearly indicated that both the carboxyl and the methyl group of a distinct acetate molecule could be derived from CO2 (Fig. 1). Therefore, homoacetogenic bacteria can be defined as anaerobes catalyzing the for- mation of acetate (or higher fatty acids) from C1 units (e.g. COa) in their energy metabolism. Utilization of H2 plus CO2: the pathway of CO2 reduction to acetate in homoacetogenic bacteria Most homoacetogenic bacteria can grow chemolithoau- totrophically on H2 plus CO2 as energy and cell carbon source, indicating that CO2 reduction to acetate must be coupled with a net synthesis of ATR The pathway