A type of cellular respiration in which oxygen is not the terminal electron acceptor in the electron transport chain. Many bacteria and archaea can live in environments in which oxygen is limited or absent by using a range of small molecules or ions that function as the terminal acceptor; they can be chemically reduced and hence enable the generation of ATP by the chemiosmotic mechanism. Examples include the sulphate ion (SO₄^2–), as used by certain sulphur bacteria, and CO₂, which is reduced by methanogens. Obligate anaerobes are organisms that cannot use free oxygen for respiration; facultative anaerobes are normally aerobic but can respire anaerobically or use fermentation during periods of oxygen shortage. Note that anaerobic respiration is distinguished from fermentation, which generates limited amounts of ATP in some yeasts and bacteria and in muscle tissue when oxygen is absent (see oxygen debt).