Thiosulfate oxidation by Thiomicrospira thermophila: metabolic flexibility in response to ambient geochemistry

Abstract

Summary Previous studies of the stoichiometry of thiosulfate oxidation by colorless sulfur bacteria have failed to demonstrate mass balance of sulfur, indicating that unidentified oxidized products must be present. Here we present reaction stoichiometry and kinetics under variable pH conditions during the growth of Thiomicrospira thermophila strain EPR85, isolated from diffuse hydrothermal fluids at the East Pacific Rise. At pH 8.0, thiosulfate is stoichiometrically converted to sulfate. At lower pH, the products of thiosulfate oxidation are extracellular elemental sulfur and sulfate. We were able to replicate previous experiments and identify the missing sulfur as tetrathionate, consistent with previous reports of the activity of thiosulfate dehydrogenase. Tetrathionate was formed under slightly acidic conditions. Genomic DNA from T. thermophila strain EPR85 contains genes homologous to those in the Sox pathway (soxAXYZBCDL), as well as rhodanese and thiosulfate dehydrogenase. No other sulfur oxidizing bacteria containing sox(CD)2 genes have been reported to produce extracellular elemental sulfur. If the apparent modified Sox pathway we observe in T. thermophila is present in marine Thiobacillus and Thiomicrospira species, production of extracellular elemental sulfur may be biogeochemically important in marine sulfur cycling.

The effect of O2 and pressure on thiosulfate oxidation by Thiomicrospira thermophila

“…At 54.75 hr, the pump was set to 0.1 ml/min (Table ). This flow rate was chosen based on previous batch growth rates of this strain during oxidation of thiosulfate to tetrathionate ( μ : 0.2 hr −1 ; see Houghton et al., ). However, dissolved oxygen remained low and cell density continued to decline over the next 24 hr (Table ).…”

“…Tetrathionate was quantified using the cyanolysis protocol described in Houghton et al. () with a detection limit of 0.1 mM based on replicate standards and an analytical error of 0.04 mM based on the abiotic control (Table ). Thiosulfate was also detected using cyanolysis, but as it is calculated by difference and relies on the tetrathionate measurement (see Houghton et al., ), we are choosing to use the independent IC‐based method (above) to quantify thiosulfate concentrations in these experiments.…”

“…Elemental sulfur was determined by absorbance at 270 nm on a Thermo Evolution 60 UV‐Vis spectrometer after extraction from fluid samples using dichloromethane as described in Houghton et al. ().…”

“…To study the effect of rapid fluctuations in substrate availability and transient redox gradients, such as those found in shallow sediments and advection‐dominated systems, a flow‐through system can be employed to control the system and allow variations in redox conditions (Hoehler & Jørgensen, ). Here, the effects of oxygen availability on thiosulfate oxidation by Thiomicrospira thermophila strain EPR85 (Houghton et al., ) were investigated at a range of different pressures (0.1–10 MPa) using a bioreactor designed for use at high pressure to simulate realistic seafloor hydrothermal conditions (Foustoukos & Pérez‐Rodríguez, ; Figure ). Previous work with this strain demonstrated rapid changes in the reaction stoichiometry in response to pH during growth at ambient pressure (Houghton et al., ).…”

“…Here, the effects of oxygen availability on thiosulfate oxidation by Thiomicrospira thermophila strain EPR85 (Houghton et al., ) were investigated at a range of different pressures (0.1–10 MPa) using a bioreactor designed for use at high pressure to simulate realistic seafloor hydrothermal conditions (Foustoukos & Pérez‐Rodríguez, ; Figure ). Previous work with this strain demonstrated rapid changes in the reaction stoichiometry in response to pH during growth at ambient pressure (Houghton et al., ). Although this strain was isolated from deep‐sea hydrothermal deposits, its physiology at in situ pressure has not yet been evaluated.…”