Thiobacillus ferro-oxidans is capable of using the oxidation of Fe2+ by O2 at pH 2.0 as the sole source of energy for growth and CO2 fixation. The bacterium maintains an intracellular pH of 6.5 over a range of external pH from 1.0 to 8.0, as measured by [14C]acetate and [3H]methylamine distribution. The membrane potential was estimated by the distribution of the lipid-soluble cation dibenzyldimethylammonium and the anion SCN-. At pH 2.0 (the pH of growth) during Fe2+ oxidation the transmembrane pH gradient is 4.5 units with an opposing membrane potential of -10mV, giving a proton electrochemical gradient of +256mV. This gradient is actively maintained.

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