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.