Production of Hydrogen Sulfide from Tetrathionate by the Iron-Oxidizing Bacterium Thiobacillus ferrooxidans NASF-1.

Abstract

When incubated under anaerobic conditions, five strains of Thiobacillus ferrooxidans tested produced hydrogen sulfide (H 2 S) 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 H 2 S production at pH 1.5. The mechanism of H 2 S 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 H 2 S and 0.6 μmol of trithionate were produced. Thiosulfate and sulfite were not detected in the reaction mixture. From these results, we propose that H 2 S is produced at pH 1.5 from tetrathionate by T. ferrooxidans NASF-1, via the following two-step reaction, in which AH 2 represents an unknown electron donor in NASF-1 cells. Namely, tetrathionate is decomposed by tetrathionate-decomposing enzyme to give trithionate and elemental sulfur (S 4 O 6 2− →S 3 O 6 2− +S o , Eq. 1), and the elemental sulfur thus produced is reduced by sulfur reductase using electrons from AH 2 to give H 2 S (S o +AH 2 →H 2 S+A, Eq. 2). The optimum pH and temperature for H 2 S production from tetrathionate under argon gas were 1.5 and 30°C, respectively. Under argon gas, the H 2 S production from tetrathionate stopped after 1 d of incubation, producing a total of 2.5 μmol of H 2 S/5 mg protein. In contrast, under H 2 conditions, H 2 S production continued for 6 d, producing a total of 10.0 μmol of H 2 S/5 mg protein. These results suggest that electrons from H 2 were used to reduce elemental sulfur produced as an intermediate to give H 2 S. Potassium cyanide at 0.5 mM slightly inhibited H 2 S 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 H 2 S production from tetrathionate, but not from elemental sulfur.