When Thiobacillus ferrooxidans AP19-3 that had been treated to remove iron from the cell surface was incubated aerobically with both elemental sulfur and o-phenanthroline, Fe³⁺ still present was reduced by the ferric-ion reducing (FIR) system of the cells. Ferrous ion chelated with o-phenanthroline was not be oxidized by iron oxidase, and leaked out into the reaction mixture. In the absence of o-phenanthroline, however, Fe²⁺ was not detected, suggesting that it was rapidly reoxidized by iron oxidase. Sulfur oxidation by this strain under aerobic conditions was strongly inhibited by o-phenanthroline, so Fe²⁺ was probably produced as an intermediate during the oxidation of elemental sulfur. Sulfur-grown cells that were cultured successively in a sulfur-salt medium always had high levels of iron-oxidizing activity. The strain could use Fe²⁺ produced earlier by the FIR system for growth. We concluded that when T. ferrooxidans AP19-3 grows on a sulfur-salt medium under aerobic conditions, the strain used a sulfur oxidation route not reported before that comprises both the FIR and iron-oxidizing systems.

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