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Oxidation and reduction: Redox reactions << Alternative inorganic terminal electron acceptors >> Anaerobic fermentation
Not all bacteria require oxygen as the terminal electron acceptor in respiration. Although oxygen is the most electronegative acceptor, other inorganic substances with high reduction potential, such as nitrate (NO3−), sulfate (SO42−), carbon dioxide (CO2) and ferric ions (Fe3+) are perfectly adequate substitutes for some organisms (Table 1).
Table 1. Examples of some inorganic electron acceptors utilised by gut bacteria and archaea for respiration.
| Electron acceptors | Reduced products | Examples of gut bacteria |
|---|---|---|
| NO3− | NO2− | Bilophila wadsworthia, Escherichia coli, Klebsiella pneumoniae, Schaalia odontolytica, Anaerostipes hadrus |
| NO3− | NO2−, N2O, N2 | Megasphaera elsdenii, Haemophilus parainfluenzae, Neisseria flavescens, Bacillus subtilis |
| NO3− | NH4+ | Wolinella succinogenes, Haemophilus parahaemolyticus, Haemophilus parainfluenzae, Serratia marcescens, Bacillus subtilis |
| SO42− | H2S | Desulfovibrio desulfuricans, D. fairfieldensis, D. piger |
| CO2 | CH4 | Methanobrevibacter smithii, Methanosphaera stadtmanae |
| CO2 | Acetate | Acetobacterium woodii, Clostridium carboxidivorans, Eubacterium limosum, Acetitomaculum ruminis |
| S | H2S | Desulforamulus reducens, Oscillibacter ruminantium, Wolinella succinogenes |
| Fe3+ | Fe2+ | Shewanella algae, S. xiamenensis, S. decolorationis, Geosporobacter ferrireducens, Thermolithobacter ferrireducens |
For the electrons transferred to all the terminal acceptors listed above, respiratory electron transport is accompanied by the creation of a proton gradient at a membrane and the concomitant generation of ATP. As can be seen in Table 1, a variety of diverse microbes from the gut are able to utilise inorganic substrates in the absence of oxygen; indeed, some don't even use oxygen, even when it is present. Denitrification conserves more energy than other inorganic acceptors. Methanogenesis is inhibited by sulfidogenesis (sulfate terminal acceptor), which in turn is inhibited by nitrate reduction (Kim2008). The reduction of Fe3+ is very common in anaerobic ecosystems - more so than nitrate - and is believed to account for over half of all the degradation of organic compounds (Kim2008).
What is not immediately obvious from the table is that the organisms listed are exceptions to the rule for how the gut microbiome members derive their energy. These inorganic respirers:
• are generally less dominant and/or widespread in the human gut (with a number of obvious exceptions: Bilophila wadsworthia, Escherichia coli, Klebsiella pneumoniae, for example).
• don't generally use their ability to utilise external inorganic terminal electron acceptors (e.g., nitrate) in the gut environment. Nitrate, sulfate and ferric ion concentrations are too low to support robust growth in the gut. Likewise, CO2 is less abundant given the overall reducing environment of the intestinal lumen.
• are greatly outnumbered by anaerobes that use fermentation of carbohydrates and amino acids to produce their energy.
• will only become consequential to the gut ecology when dysbiosis occurs (e.g., overgrowth of Bilophila wadsworthia, Escherichia coli, or Klebsiella pneumoniae). See next section: Anaerobic fermentation
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