respiration

Some organisms like bacteria utilize certain inorganic exogenous electron acceptors in their electron transport chain instead of oxygen and, therefore, respire anaerobically. The major electron acceptors are nitrate (NO₃^-), sulphate (SO₄^2-), CO₂, Fe³⁺, SeO₄²⁺, etc. 
Nitrate is reduced to nitrite, nitrous oxide or molecular nitrogen in certain anerobes and sulphate is reduced to sulphide in others. In a parallel series of reactions in other organisms, CO₂ is used as electron acceptor and reduced to methane or acetic acid during oxidation of hydrogen.