Life on earth evolved in the absence of oxygen with inorganic gases as potential sources of carbon and energy. Among the alternative mechanisms for carbon dioxide (CO₂) fixation in the living world, only the reduction of CO₂ by the Wood-Ljungdahl pathway, which is used by acetogenic bacteria, complies with the two requirements to sustain life: conservation of energy and production of biomass. However, how energy is conserved in acetogenic bacteria has been an enigma since their discovery. In this Review, we discuss the latest progress on the biochemistry and genetics of the energy metabolism of model acetogens, elucidating how these bacteria couple CO₂ fixation to energy conservation.

Storage and transportation of hydrogen is a major obstacle for its use as a fuel. An increasingly considered alternative for the direct handling of hydrogen is to use carbon dioxide (CO2) as an intermediate storage material. However, CO2 is thermodynamically stable, and developed chemical catalysts often require high temperatures, pressures, and/or additives for high catalytic rates. Here, we present the discovery of a bacterial hydrogen-dependent carbon dioxide reductase from Acetobacterium woodii directly catalyzing the hydrogenation of CO2. We also demonstrate a whole-cell system able to produce formate as the sole end product from dihydrogen (H2) and CO2 as well as syngas. This discovery opens biotechnological alternatives for efficient CO2 hydrogenation either by using the isolated enzyme or by employing whole-cell catalysis.

Synthesis of acetate from carbon dioxide and molecular hydrogen is considered to be the first carbon assimilation pathway on earth. It combines carbon dioxide fixation into acetyl-CoA with the production of ATP via an energized cell membrane. How the pathway is coupled with the net synthesis of ATP has been an enigma. The anaerobic, acetogenic bacterium Acetobacterium woodii uses an ancient version of this pathway without cytochromes and quinones. It generates a sodium ion potential across the cell membrane by the sodium-motive ferredoxin:NAD oxidoreductase (Rnf). The genome sequence of A. woodii solves the enigma: it uncovers Rnf as the only ion-motive enzyme coupled to the pathway and unravels a metabolism designed to produce reduced ferredoxin and overcome energetic barriers by virtue of electron-bifurcating, soluble enzymes.

Background: Hydrogen-dependent reduction of ferredoxin, a common "low-redox potential" electron carrier in anaerobes, is a highly endergonic reaction. Results: The [FeFe]-hydrogenase from an acetogenic bacterium strictly requires NAD ϩ for ferredoxin reduction and reduces

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...