Redox-balance
In anaerobic chemoheterotrophic organisms where ATP production occurs in fermentation pathways by phosphoryl group transfer, production of the necessary high phosphoryl group transfer metabolites involves intermediary oxidation reactions where NAD or NADP are used as the oxidizing agents. Since oxygen is not available as the ultimate electron acceptor for regeneration of the oxidized forms of these intermediary electron carriers a metabolite of the original organic growth substrate has to be used for this purpose.  Examples are the reduction of pyruvate to produce lactate or the reduction of acetaldehyde to produce ethanol.  Rumen microorganisms utilize a diverse array of these and other such redox couples to achieve redox balance.