How anaerobic bacteria and archaea conserve energy

In clostridia the exergonic reduction of crotonyl-CoA to butyryl-CoA by NADH is coupled to the endergonic reduction of ferredoxin by NADH – a process called flavin-based electron bifurcation, catalyzed by a two-FAD-containing electron transferring flavoptrotein (Etf) and butyryl-CoA dehydrogenase (Bcd). This, and similar systems are wide-spread in anaerobic bacteria and archaea, which reduce ferredoxin for H₂ formation in fermentations, for generation of ΔµNa⁺ via a ferredoxin-NAD reductase (Rnf) and in aceto-and methanogenesis for CO₂ reduction by H₂.