The intracellular pH of Clostridium paradoxum, an anaerobic, alkaliphilic, and thermophilic bacterium

Wen the extracellular pH was increased from 7.6 to 9.8, Clostridium paradoxum, a novel alkalithermophile, increased its pH gradient across the cell membrane (Delta pH, pH(in) - pH(out)) by as much as 1.3 U. At higher pH values (> 10.0), the Delta pH and membrane potential (Delta psi) eventually declined, and the intracellular pH increased significantly. Growth ceased when the extracellular pH was greater than 10.2 and the intracellular pH increased to above 9.8. The membrane potential increased to 110 +/- 8.6 mV at pH 9.1, but the total proton motive force (Delta p) declined from about 65 mV at pH 7.6 to 25 mV at pH 9.8. Between the extracellular pH of 8.0 and 10.3, the intracellular ATP concentration was around 1 mM and decreased at lower and higher pH values concomitantly with a decrease in growth rate.