The sodium cycle in methanogenesis. CO2 reduction to the formaldehyde level in methanogenic bacteria is driven by a primary electrochemical potential of Na+ generated by formaldehyde reduction to CH4.

CH4 formation from CO2 and H2 rather than from formaldehyde and H2 in methanogenic bacteria is inhibited by uncouplers, indicating that CO2 reduction to the formaldehyde level is energy-driven. We report here that in Methanosarcina barkeri the driving force is a primary electrochemical sodium potential (delta mu Na+) generated by formaldehyde reduction to CH4. This is concluded from the following findings. 1. CO2 reduction to CH4 was insensitive towards protonophores, when the Na+/H+ antiporter was inhibited; under these conditions delta mu Na+ was 120 mV (inside negative), whereas both delta mu H+ and the cellular ATP content were low. 2. CO2 reduction to CH4, rather than formaldehyde reduction, was sensitive towards Na+ ionophores, which dissipated delta mu Na+. 3. CO2 reduction to CH4, in the presence of protonophores and Na+/H+ antiport inhibitors, was coupled with the extrusion of 1-2 mol Na+/mol CH4, and formaldehyde reduction to CH4 was coupled with the extrusion of 3-4 mol Na+/mol CH4. Thus during CO2 reduction to the formaldehyde level 2-3 mol Na+ were consumed.