ION-TRANSPORT AND METHANE PRODUCTION IN METHANOBACTERIUM-THERMOAUTOTROPHICUM

In Methanobacterium thermoautotrophicum, the protonmotive force for th e H+-translocating ATPase consists mainly of a transmembrane electrica l gradient (Delta Psi). These cells do not establish a significant tra nsmembrane pH gradient (inside alkaline) and, in fact, if the suspendi ng medium is of pH greater than or equal to 7.0, the pH gradient may b e reversed-i.e., inside acid with respect to the extracellular pH. The se studies show by both Na-23 NMR and Na-22(+) distribution that Na+ e xtrusion with the generation of Delta Psi precedes methanogenesis in M b. thermoautotrophicum. It is calculated that the newly established Na + gradients increase Delta Psi by approximate to 50 mV (inside negativ e). There is no detectable H+ extrusion during methane synthesis; inst ead there is a high rate of H+ consumption for methane synthesis and a n increase in internal pH. This was supported by P-31 NMR experiments, which showed an internal pH shift from 6.8 to 7.6. With the cells mai ntained at an external pH of 7.2, the initial transmembrane pH gradien t of -0.4 (inside acid) at 60 degrees C is equivalent to Delta Psi of +27 mV (inside positive); after 20 min of incubation, the transmembran e pH gradient is +0.4 (inside alkaline), which at 60 degrees C is equi valent to Delta Psi of -27 mV (inside negative). Actively respiring ce lls generated a protonmotive force of -198 mV. It is proposed that ene rgy for CO2 reduction to the level of formaldehyde (the first step in methane synthesis) in Mb. thermoautotrophicum is derived from the Delt a Psi generated by electrogenic Na+ extrusion. The protonmotive force required for ATP synthesis consists primarily of Delta Psi and appears to be the result of both an electrogenic Na+ extrusion and a pH gradi ent (inside alkaline) which develops during methanogenesis.