Anaerobic respiration
For the electron transport chain to work, there must be a final electron acceptor at the end of the chain. This allows electrons to pass through the chain. In aerobic organisms, this final electron acceptor is oxygen.  Molecular oxygen is a highly oxidizing agent and so it is an excellent acceptor. In anaerobes, other less-oxidizing substances such as sulphate (SO42−), nitrate (NO3−), sulphur (S) are used.  These terminal electron acceptors have smaller reduction potentials than O2, so less energy is released per oxidized molecule.  Anaerobic respiration is therefore less efficient than aerobic respiration except, of course, when oxygen is scarce or not apparent.