Acetogenic Bacteria

Abstract

Acetogenic bacteria are a specialised group of strictly anaerobic bacteria that are ubiquitous in nature. Together with the methane‐forming archaea they constitute the last limbs in the anaerobic food web that leads to the production of methane from polymers in the absence of oxygen. Acetogens are characterised by a unique pathway, the Wood–Ljungdahl pathway of carbon dioxide reduction with the acetyl‐CoA synthase as the key enzyme. This pathway also allows chemolithoautotrophic growth on hydrogen and carbon dioxide and it is the only pathway known that combines carbon dioxide fixation with adenosine triphosphate (ATP) synthesis. Thus, it is considered the first biochemical pathway on earth. ATP is synthesised by a chemiosmotic mechanism with Na+ or H+ as coupling ion, depending on the organism. In cytochrome‐free acetogens, energy is conserved by ferredoxin reduction followed by ferredoxin‐dependent Na+ (or H+) translocation across the membrane (Rnf complex). Acetogens may represent ancestors of the first bioenergetically active cells in evolution.

Key Concepts:

  • Acetogenic bacteria are a specialised group of anaerobic bacteria producing acetate via the Wood–Ljungdahl pathway.

  • Acetogenic bacteria are widespread in nature and are an essential link in the anaerobic mineralisation of organic matter.

  • Acetogenic bacteria are nutritionally versatile and can grow heterotrophically as well as lithoautotrophically.

  • Lithoautotrophic growth on hydrogen and carbon dioxide leads to acetate production coupled to ATP synthesis by a chemiosmotic mechanism.

  • The bioenergetic pathway and the coupled fixation of carbon into biomas links acetogens to early evolutionary processes and maybe even to the first living cell on earth.

  • The capability of acteogens to produce acetate from H2+CO2, CO, or a mixture of H2+CO2+CO (syngas) makes these organisms the prime candidates for biotechnological applications.

Keywords: metabolism; Wood–Ljungdahl pathway; carbon dioxide fixation; bioenergetics; ATP synthesis; Rnf complex; electron bifurcation; syngas fermentation; biotechnology

Figure 1.

Fermentation of hexoses to acetate by acetogenic bacteria. The fermentation yields only acetate according to eqn [III]. This fermentation is referred to as homoacetogenesis.

Figure 2.

The Wood–Ljungdahl pathway. Abbreviations: THF, tetrahydrofolate; HSCoA, coenzyme A; Pi, inorganic phosphate; e, electron; CoFe/S‐P, corrinoid‐iron sulfur protein; ATP, adenosine 5′‐triphosphate. 2[H], reducing equivalents (NADPH for the first reaction in M. thermoacetica, NADH for the second and third reaction and reduced ferredoxin for the CO‐DH reaction).

Figure 3.

Chemiosmotic mechanisms of energy conservation in acetogens during lithoautotrophic growth. (a) Proton‐dependent conservation of energy with the hypothetical involvement of different oxidoreductases that channel electrons into the electron transport chain. (b) Na+‐dependent conservation of energy by the Rnf complex that is fuelled by reduced ferredoxin generated by electron bifurcation. The NADH produced by the electron‐bifurcating hydrogenase and the Rnf complex is used to reduce CO2 to acetate.

Figure 4.

Aromatic substrates that are utilised by acetogens.

Figure 5.

Biochemistry and bioenergetics of hydrogen‐dependent caffeate respiration as carried out by A. woodii. Adapted from Bertsch et al.. © The American Society for Biochemistry and Molecular Biology.

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References

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Further Reading

Biegel E, Schmidt S, Gonzalez JM and Müller V (2011) Biochemistry, evolution and physiological function of the Rnf complex, a novel ion‐motive electron transport complex in prokaryotes. Cellular and Molecular Life Sciences 68: 613–634.

Müller V (2003) Energy conservation in acetogenic bacteria. Applied and Environmental Microbiology 69: 6345–6353.

Müller V, Imkamp F, Biegel E, Schmidt S and Dilling S (2008) Discovery of a ferredoxin:NAD+‐oxidoreductase (Rnf) in Acetobacterium woodii: a novel potential coupling site in acetogens. Annals of the New York Academy of Sciences 1125: 137–146.

Müller V, Imkamp F, Rauwolf A, Küsel K and Drake HL (2004) Molecular and cellular biology of acetogenic bacteria. In: Nakano MM and Zuber P (eds) Strict and Facultative Anaerobes; Medical and Environmental Aspects, pp 251–281. Norfolk, UK: Horizon Press.

Schiel‐Bengelsdorf B and Dürre P (2012) Pathway engineering and synthetic biology using acetogens. FEBS Letters 586: 2191–2198.

Wood HG and Ljungdahl LG (1991) Autotrophic character of the acetogenic bacteria. In: Shively JM and Barton LL (eds) Variations in Autotrophic Life, pp 201–250. San Diego: Academic Press.

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Müller, Volker, and Frerichs, Janin(Sep 2013) Acetogenic Bacteria. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020086.pub2]