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

When incubated under anaerobic conditions, five strains of Thiobacillus fer rooxidans tested produced hydrogen sulfide (H2S) from elemental sulfur at p H 1.5. However, among the strains, T. ferrooxidans NASF-1 and AP19-3 were a ble to use both elemental sulfur and tetrathionate as electron accepters fo r H2S production at pH 1.5. The mechanism of H2S production from tetrathion ate was studied with intact cells of strain NASF-1. Strain NASF-1 was unabl e to use dithionate, trithionate, or pentathionate as an electron acceptor. After 12 h of incubation under anaerobic conditions at 30 degrees C, 1.3 m u mol of tetrathionate in the reaction mixture was decomposed, and 0.78 mu mol of H2S and 0.6 mu mol of trithionate were produced. Thiosulfate and sul fite were not detected in the reaction mixture. From these results, we prop ose that H2S is produced at pH 1.5 from tetrathionate by T. ferrooxidans NA SF-1, via the following two-step reaction, in which AH(2) represents an unk nown electron donor in NASF-1 cells. Namely, tetrathionate is decomposed by tetrathionate-decomposing enzyme to give trithionate and elemental sulfur (S4O62--->S3O62-+S-0, Po. 1), and the elemental sulfur thus produced is red uced by sulfur reductase using electrons from AH(2) to give H2S (S-0 + AH(2 )-->H2S + A, Eq. 2). The optimum pH and temperature for H2S production from tetrathionate under argon gas were 1.5 and 30 degrees C, respectively. Und er argon gas, the H2S production from tetrathionate stopped after Id of inc ubation, producing a total of 2.5 mu mol of H2S/5 mg protein. In contrast, under H-2 conditions, H2S production continued for 6 d, producing a total o f 10.0 mu mol of H2S/5 mg protein. These results suggest that electrons fro m H-2 were used to reduce elemental sulfur produced as an intermediate to g ive H LS. Potassium cyanide at 0.5 mM slightly inhibited H2S production fro m tetrathionate, but increased that from elemental sulfur 3-fold. 2,4-Dinit rophenol 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 completel y inhibited H2S production from tetrathionate, but not from elemental sulfu r.