Ferredoxin can also be reduced by using NADH (-320 mV) or  (-414 mV), but these processes are coupled to the consumption of the membrane potential to power the "boosting" of electrons to the higher energy state. The Rnf complex is a widespread membrane protein in bacteria that reversibly transfers electrons between NADH and ferredoxin while pumping  or  ions across the membrane. The chemiosmotic potential of the membrane is consumed to power the unfavorable reduction of  by NADH. This reaction is an essential source of  in many autotrophic organisms. If the cell is growing on substrates that provide excess , the Rnf complex can transfer these electrons to  and store the resultant energy in the membrane potential. The energy converting hydrogenases (Ech) are a family of enzymes that reversibly couple the transfer of electrons between  and  while pumping  ions across the membrane to balance the energy difference.