Reduction of ferric iron by strains of moderately thermophilic, acidophilic bacteria grown in anaerobic cultures containing 20 mM ferric sulfate and 5 mM potassium tetrathionate (pH 2.0) and incubated at 45°C. Symbols: •, S. acidophilus ALV; ▪, S. acidophilus THWX; ▴, S. acidophilus YTF1; ○, S. thermosulfidooxidans TH1; ▿, A. ferrooxidans TH3; +, cell-free control.