anaerobic respiration

…anaerobic conditions by processes called anaerobic respiration, in which the final electron acceptor is an inorganic molecule, such as nitrate (NO₃⁻), nitrite (NO₂⁻), sulfate (SO₄²⁻), or carbon dioxide (CO₂). The energy yields available to the cell using these acceptors are lower than in respiration with oxygen—much lower

…the ocean into the atmosphere; anaerobic respiration by sulfate-reducing bacteria causes the release of hydrogen sulfide (H₂S) gas especially from marshes, tidal flats, and similar environments in which anaerobic microorganisms thrive; and volcanic activity releases additional but much smaller amounts of sulfur gas into the atmosphere.