Acetogenic bacteria are a diverse group of strictly anaerobic bacteria that utilize the Wood-Ljungdahl pathway for CO 2 fixation and energy conservation. These microorganisms play an important part in the global carbon cycle and are a key component of the anaerobic food web. Their most prominent metabolic feature is autotrophic growth with molecular hydrogen and carbon dioxide as the substrates. However, most members also show an outstanding metabolic flexibility for utilizing a vast variety of different substrates. In contrast to autotrophic growth, which is hardly competitive, metabolic flexibility is seen as a key ability of acetogens to compete in ecosystems and might explain the almost-ubiquitous distribution of acetogenic bacteria in anoxic environments. This review covers the latest findings with respect to the heterotrophic metabolism of acetogenic bacteria, including utilization of carbohydrates, lactate, and different alcohols, especially in the model acetogen Acetobacterium woodii. Modularity of metabolism, a key concept of pathway design in synthetic biology, together with electron bifurcation, to overcome energetic barriers, appears to be the basis for the amazing substrate spectrum. At the same time, acetogens depend on only a relatively small number of enzymes to expand the substrate spectrum. We will discuss the energetic advantages of coupling CO 2 reduction to fermentations that exploit otherwise-inaccessible substrates and the ecological advantages, as well as the biotechnological applications of the heterotrophic metabolism of acetogens.
Synthesis of acetate from molecular hydrogen (H 2 ) and carbon dioxide (CO 2 ) by microorganisms was discovered in 1932 by F. Fischer (1). Only 4 years later, the first acetogenic bacterium was isolated in pure culture (2). However, understanding the metabolism of acetogenesis turned out to be a challenging task and continues to be so today. Nevertheless, the road toward this goal is paved with landmark discoveries in the field of microbiology. The most characteristic feature of acetogens is their ability to produce acetate from H 2 plus CO 2 . This reaction provides only very little energy (⌬G 0= ϭ Ϫ95 kJ/mol [3]); still, acetogens evolved special adaptations to "make a living" from this conversion (4, 5). We refer to this as "autotrophic acetogenesis." Ironically, major parts to our understanding of this trait came from studies on an organism that was not known to perform autotrophic acetogenesis: Moorella thermoacetica (formerly Clostridium thermoaceticum). This organism, isolated by F. E. Fontaine in 1942 (6), attracted interest for its ability to convert glucose stoichiometrically to 3 moles of acetate, a conversion named "homoacetogenesis." Elucidation of this pathway led to the discovery of an entirely new pathway of CO 2 fixation, the reductive acetyl coenzyme A (CoA) pathway, or the Wood-Ljungdahl pathway (WLP; named after Harland G. Wood and Lars G. Ljungdahl, who made major contributions to the elucidation of this pathway) (7-9). This pathway has t...