ThiobaciUus thiooxidans cells oxidized elemental sulfur to sulfite, with 1 mol of 02 consumption per mol of sulfur oxidized to sulfite, when the oxidation of sulfite was inhibited with 2-n-heptyl-4-hydroxyquinoline N-oxide.

Eight strains of ThiobaciUus ferrooxidans (laboratory strains Tf-1 [= ATCC 13661] and Tf-2 [ ATCC 19859] and mine isolates SM-1, SM-2, SM-3, SM-4, SM-5, and SM-8) and three strains of Thiobacillus thiooxidans (laboratory strain Tt 1= ATCC 8085] and mine isolates SM-6 and SM-7) were grown on ferrous iron (Fe2+), elemental sulfur (SO), or sulfide ore (Fe, Cu, and Zn). The cells were studied for their aerobic Fe21and S°-oxidizing activities (02 consumption) and anaerobic S°-oxidizing activity with ferric iron (Fe3") (Fe2+ formation). Fe2+-grown T. ferrooxidans cells oxidized S°aerobically at a rate of 2 to 4% of the Fe2+ oxidation rate. The rate of anaerobic So oxidation with Fe3+ was equal to the aerobic oxidation rate in SM-1, SM-3, SM-4, and SM-5, but was only one-half or less that in Tf-1, Tf-2, SM-2, and SM-8. Transition from growth on Fe21 to that on S produced cells with relatively undiminished Fe2+ oxidation activities and increased So oxidation (both aerobic and anaerobic) activities in Tf-2, SM-4, and SM-5, whereas it produced cells with dramatically reduced Fe21 oxidation and anaerobic S oxidation activities in Tf-1, SM-1, SM-2, SM-3, and

The oxidation of sulfite by Thiobacillus thiooxidans was studied at various pH values with changing concentrations of potassium sulfite. The optimal pH for sulfite oxidation by cells was a function of sulfite concentrations, rising with increasing substrate concentrations, while that by the cell extracts was unaffected. The sulfite oxidation by cells was inhibited at high sulfite concentrations, particularly at low pH values. The results from kinetic studies show that the fully protonated form of sulfite, sulfurous acid or sulfur dioxide, is the form which penetrates the cells for the oxidation.Thiobacilli oxidize inorganic sulfur compounds to sulfuric acid to obtain energy for growth. Sulfite is considered the key intermediate in the oxidation (8,20) since it is the product of sulfur-oxidizing enzyme systems (17,23) and the substrate for the sulfite-oxidizing enzyme systems (1,4,14,16). Sulfite in fact accumulates during the oxidation of thiosulfate by Thiobacillus novellus cells under certain conditions (5) and of sulfur by Thiobacillus ferrooxidans when the pH is raised (15,18). Sulfur is nearly quantitatively oxidized to sulfite by Thiobacillus thiooxidans cells when the oxidation of sulfite is inhibited by 2-n-heptyl-4-hydroxyquinoline N-oxide (21).Sulfite oxidation by thiobacilli is easily demonstrated in neutrophilic thiosulfate-grown T. novellus cells (3), but is difficult to show in acidophilic thiobacilli, e.g., T. thiooxidans (9,11,21) and Thiobacillus acidophilus (15), without raising the pH above the acid growth pH. An exception is T ferrooxidans, which has a unique sulfite:Fe3+ oxidoreductase system coupled to the Fe2+ oxidation system (16) and whose cells oxidize sulfite under acidic growth conditions (15-18). Furthermore, sulfite inhibits the oxidation of sulfur by T. thiooxidans cells only under acidic conditions (21).Sulfite ion (SO32-) can be protonated to bisulfite ion (032-plus H+ *-> H503, Ka2 = 1.02 x 10-7 or pKl2 = 7.0) and further to sulfurous acid (HSO3-plus H+ H2SO3, K~a = 1.54 x 10-2 or pKa, = 1.81). We have studied the oxidation of varied concentrations of potassium sulfite by T. thiooxidans cells at different pH values to determine which form of sulfite is used by the cells for oxidation. Our results show that the Km values for total sulfite (SO32-HS03-, H2SO3) concentrations increase with increasing pH, but the Km values calculated for sulfurous acid (H2SO3) remain relatively unchanged while the maximal rate of oxidation (Vm[) is not affected by the assay pH. These results suggest a possibility that the fully protonated sulfurous acid may be the actual substrate used by the cells. Since the cell-free sulfite-oxidizing system has an optimal pH around 7 in agreement with the reported value (9, 10) and Km values for total sulfite are largely unaffected by pH, it is concluded that either sulfurous acid or sulfur dioxide (H2SO3 <-> SO2 plus H20) may be the only form which penetrates the cells but once inside loses the preference. * Corresponding author.Sulfite oxidation by T. thiooxidan...

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Thiobacillus thiooxidans cells became more hydrophobic but less adhesive to elemental sulfur in the presence of increasing potassium phosphate concentrations. At a fixed concentration of potassium phosphate, however, there was a peak of both cell hydrophobicity and adhesion to sulfur at around pH 5. Oxidation of sulfur by the cells was affected in a complex manner by the phosphate concentration and pH, although it was inhibited by a high concentration of potassium phosphate.