The membrane potential of Methanobacterium thermoautotrophicum under different external conditions

The membrane potential (Delta psi) of whole cells of Methanobacterium therm oautotrophicum strain DeltaH was estimated under different external conditi ons using a TPP+-sensitive electrode. The results show that the Delta psi v alues of M. thermoautotrophicum at alkaline pH(out) (8.5) are comparable wi th Delta psi values under slightly acidic conditions (pH 6.8; 230 and 205 m V, respectively). On the other hand, the size of colonies on Petri dishes w as remarkably smaller at pH 8.5 than at 6.8. The Delta psi was insensitive to relevant ATPase inhibitors. At pH 6.8, the protonophore 3,3',4',5-tetrac hlorosalicylanilide (TCS) strongly inhibited Delta psi formation and ATP sy nthesis driven by methanogenic electron transport. On the other hand, at pH 8.5 the CH4 formation and ATP synthesis were insensitive to TCS and a prot onophore-resistant Delta psi of approximately 150 mV was determined. The fi nding of a protonophore-resistant Delta psi at pH 8.5 indicates that at alk aline pH(out) these cells can switch from H+-energetics to Na+-energetics, when the Delta<(<mu>)over tilde>(H+) becomes limited. The results strongly support the hypothesis that at alkaline pH(out) Na+ ions might fully substi tute for H+ in these cells as the coupling ions.