However, in some prokaryotes, oxygen is replaced by other acceptors, such as organic salts, sulfates, bicarbonates, nitrates, and iron. Electron donors are either organic or inorganic matter. Anaerobic respiration enables living things to survive in environments where oxygen is limited or completely absent.
Several common types of anaerobic respiration:
Nitrate anaerobic respiration or Nitrate Reduction or Denitrification
Nitrate is incredibly prevalent in biosphere and has a high redox potential. As a result, it is the favored electron acceptor for bacteria in an anoxic environment. In denitrification, electron is captured by nitrate (NO₃⁻) that is reduced to intermediate nitrogen oxides whose sequence is nitrite (NO₂⁻), nitric oxide (NO), nitrous oxide (N₂O), and nitrogen gas (N₂) is final product. Coupling of free energy decrease to ATP synthesis in prokaryotes is much less efficient than eukaryotes, since there are no mitochondria and complex cristae to maintain a stable proton gradient. About 10 ATP are produced when one glucose is completely oxidized.
NO₃⁻ + 2e⁻ + 2H⁺ → NO₂⁻ + H₂O
NO₂⁻ + e⁻ + 2H⁺ → NO + H₂O
2NO + 2e⁻ + 2H⁺ → N₂O + H₂O
N₂O + 2e⁻ + 2H⁺ → N₂ + H₂O
Anaerobic ammonia oxidation or Anammox
Scientists have already noticed the unexplained loss of ammonium in denitrifying bioreactors and anoxic zones of marine environment. According to thermodynamic theory, the Austrian physicist Broda (1977) predicted some bacteria could anaerobically transform ammonium into nitrogen gas. In 1995, anammox bacteria were discovered in a denitrifying bioreactor by Gijs Kuenen and his colleagues.
Nitrate is transformed into nitrite by other bacteria. Nitrite is metabolized by anammox bacteria into nitric oxide that is couple with ammonium salt for synthesis of energy-rich hydrazine. In the final dehydrogenation process, hydrazine is used to produce nitrogen gas and a proton gradient for ATP generation. This occurs in anammoxosome of Gram-negative anaerobe, a membrane-bound organelle that prevents leakage of toxic hydrazine into cytoplasm.
NO₂⁻ + e⁻ + 2H⁺ → NO + H₂O
NH₄⁺ + NO + 2H⁺ + 3e⁻ → N₂H₄ + H₂O
N₂H₄ → N₂ + 4H⁺ + 4e⁻
Sulfide and Sulphur anaerobic respiration
Sulfate-reducing bacteria are another common type of anaerobic bacteria. They use sulfate to accept electrons from organic matter such as short-chain fatty acids or alcohols. Sulfur, with a valence of +6, is reduced to sulfur with a valence of -2 via a series of chemical reactions (intermediates are sulfites and other sulfur oxides). Meanwhile, short-chain fatty acids or alcohols are incompletely oxidized to acetate, or fully oxidized to water and carbon dioxide. Some bacteria and archaea use sulfur as the electron acceptor. Sulphur anaerobic respiration occurs in sulfur-rich environments, such as sulfur springs or submarine volcanic vents. The final products of both metabolisms are hydrogen sulfide (H₂S). Since the free energy decrease in these reactions is small, and 2 ATP are consumed to activate sulfate into APS, the ATP produced is very limited and comparable to fermentation.
CH₃COOH + SO₄²⁻ ⟶ H₂S + 2HCO₃⁻
2C₃H₆O₃ + SO₄²⁻ ⟶ 2CH₃COOH + H₂S + 2HCO₃⁻
Bicarbonate anaerobic respiration
Carbon dioxide (in water as bicarbonate, HCO₃⁻) can also serve as an electron acceptor. For example, methane and ATP are produced by methanogens from hydrogen and carbon dioxide. Methanogens are strictly anaerobic and very ancient organisms. They have inherited ancient enzymes and lack modern ones, so they can only use a limited range of substances to synthesize organic matter. Besides hydrogen and carbon dioxide as common raw materials, some species of methanogens use formic acid, methanol, and acetic acid, but they cannot utilize methylamine. Some strains found in deep-sea sediments can use methylamine. Methane production and organic synthesis are coupled in these bacteria: intermediates in methane anabolism and carbon dioxide form acetyl-CoA that participates in the synthesis of carbohydrates, lipids, and amino acids.
H⁺ + HCO₃⁻ + 4H₂ → CH₄ + 3H₂O
Iron anaerobic respiration
Ferric compounds (Fe³⁺) are reduced to ferrous compounds (Fe²⁺) by iron-reducing bacteria to obtain energy. The greatest challenge is that ferric compounds (iron oxide or hydroxide) is often insoluble and unavailable precipitate. Bacteria employ various strategies to make iron available. Flagella and pili on the cell wall contact ferric compounds closely. Citric acid is secreted to dissolve the precipitate and form soluble ferric chelate. Electrons are transferred from cytoplasm to ferric compounds through conductive proteins containing metals. Chelating agents are also released into surrounding environment to make neutral soluble ferrous chelate, preventing ferrous iron from adsorbing onto the cell wall or becoming a precipitate that covers surface of oxide.