What is Anaerobic respiration?

Anaerobic respiration is respiration — an electron-transport chain that pumps protons and makes ATP by chemiosmosis — with a terminal acceptor other than oxygen. Denitrifiers (Pseudomonas, Paracoccus) reduce nitrate to nitrite, nitric oxide, nitrous oxide and finally $\mathrm{N_2}$, which escapes to the air; they use their aerobic chain with a few added enzymes and switch to nitrate only when oxygen is gone. Sulfate reducers (Desulfovibrio) reduce sulfate to sulfide, blacken the mud with iron sulfide and give it its smell. Methanogens, which are archaea, reduce $\mathrm{CO_2}$ with $\mathrm{H_2}$ to methane ($4\mathrm{H_2} + \mathrm{CO_2} \to \mathrm{CH_4} + 2\mathrm{H_2O}$, $\Delta G^{\circ\prime} = -131\,\mathrm{kJ}/\mathrm{mol}$), or split acetate into methane and $\mathrm{CO_2}$; they are killed by oxygen and live in cow rumens, rice paddies, marshes and landfills, from which they release a greenhouse gas that is the subject of Chapter 27. Iron and manganese oxides serve other groups. The Year 1 volume treated oxygen as the acceptor; for most of Earth’s history, and in every sediment today, it is only the first of a series.