P. Polak et al., The membrane potential of Methanobacterium thermoautotrophicum under different external conditions, FOL MICROB, 45(2), 2000, pp. 107-113
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.