Production of hydrogen sulfide from tetrathionate by the iron-oxidizing bacterium Thiobacillus ferrooxidans NASF-1

When incubated under anaerobic conditions, five strains of Thiobacillus ferrooxidans tested produced hydrogen sulfide (H₂S) from elemental sulfur at pH 1.5. However, among the strains, T. ferrooxidans NASF-1 and AP19-3 were able to use both elemental sulfur and tetrathionate as electron acceptors for H₂S production at pH 1.5. The mechanism of H₂S production from tetrathionate was studied with intact cells of strain NASF-1. Strain NASF-1 was unable to use dithionate, trithionate, or pentathionate as an electron acceptor. After 12 h of incubation under anaerobic conditions at 30°C, 1.3 μmol of tetrathionate in the reaction mixture was decomposed, and 0.78 μmol of H₂S and 0.6 μmol of trithionate were produced. Thiosulfate and sulfite were not detected in the reaction mixture. From these results, we propose that H₂S is produced at pH 1.5 from tetrathionate by T. ferrooxidans NASF-1, via the following two-step reaction, in which AH₂ represents an unknown electron donor in NASF-1 cells. Namely, tetrathionate is decomposed by tetrathionate-decomposing enzyme to give trithionate and elemental sulfur (S₄O₆^2-→S₃O₆^2- + S°, Eq. 1), and the elemental sulfur thus produced is reduced by sulfur reductase using electrons from AH₂ to give H₂S (S°+ AH₂→H₂S + A, Eq. 2). The optimum pH and temperature for H₂S production from tetrathionate under argon gas were 1.5 and 30°C, respectively. Under argon gas, the H₂S production from tetrathionate stopped after 1 d of incubation, producing a total of 2.5 μmol of H₂S/5 mg protein. In contrast, under H₂ conditions, H₂S production continued for 6 d, producing a total of 10.0 μmol of H₂S/5 mg protein. These results suggest that electrons from H₂ were used to reduce elemental sulfur produced as an intermediate to give H₂S. Potassium cyanide at 0.5 mM slightly inhibited H₂S production from tetrathionate, but increased that from elemental sulfur 3-fold. 2,4-Dinitrophenol at 0.05 mM, carbonylcyanide-m-chlorophenyl- hydrazone at 0.01 mM, mercury chloride at 0.05 mM, and sodium selenate at 1.0 mM almost completely inhibited H₂S production from tetrathionate, but not from elemental sulfur.