Aerobic and anaerobic respiration
Respiration serves to oxidize glucose into CO2 and energy (ATP). The oxidation occurs in 3 serial steps:
1) Glycolysis (requires ATP to start, but is net positive of ATP)
2) Citric acid cycle (produces NADH, FADH2 and CO2)
3) Electron transport chain in mitochondrial membrane (oxidative phosphorylation, uses NADH and FADH2 as electron donors, leading to proton influx and phosphorylation of ADP to ATP. The electron is finally donated to O2, which serve as electron acceptor in aerobic respiration.)
Under anaerobic conditions alternative electron acceptors are required.
Here is a list of alternative electron acceptors:
| Electron Acceptor | Process | Reduced Product(s) | Example Reduction Reaction | Typical Organisms/Examples |
|---|---|---|---|---|
| Nitrate (NO₃⁻) | Denitrification (or nitrate respiration) | Nitrogen gas (N₂), sometimes intermediates like NO₂⁻, NO, N₂O | 2NO₃⁻ + 12H⁺ + 10e⁻ → N₂ + 6H₂O (overall for full denitrification) | Paracoccus denitrificans, Pseudomonas spp., some Escherichia coli strains |
| Sulfate (SO₄²⁻) | Dissimilatory sulfate reduction | Hydrogen sulfide (H₂S) | SO₄²⁻ + 10H⁺ + 8e⁻ → H₂S + 4H₂O | Desulfovibrio spp., Desulfobacter spp., some Archaeoglobus |
| Fumarate | Fumarate respiration | Succinate | Fumarate²⁻ + 2H⁺ + 2e⁻ → Succinate²⁻ | Escherichia coli, some anaerobic bacteria (e.g., Wolinella) |
| Carbon dioxide (CO₂) | Methanogenesis | Methane (CH₄) | CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O | Methanogenic archaea (e.g., Methanosarcina barkeri) |
| Ferric iron (Fe³⁺) | Iron reduction (dissimilatory Fe(III) reduction) | Ferrous iron (Fe²⁺) | Fe³⁺ + e⁻ → Fe²⁺ (often involving Fe(III) oxides/hydroxides) | Geobacter spp., Shewanella spp. |
| Elemental sulfur (S⁰) | Sulfur respiration | Hydrogen sulfide (H₂S) | S⁰ + 2H⁺ + 2e⁻ → H₂S | Desulfuromonas spp., some sulfur-reducing bacteria |
Other less common acceptors include manganese (Mn⁴⁺ reduced to Mn²⁺), perchlorate (ClO₄⁻), or certain organic compounds like trimethylamine N-oxide (TMAO), but the ones listed above are among the most widespread in microbial ecology.