Aerobic and anaerobic thiosulfate oxidation by a cold-adapted, subglacial chemoautotroph

Geochemical data indicate that protons released during pyrite (FeS₂) oxidation are important drivers of mineral weathering in oxic and anoxic zones of many aquatic environments, including those beneath glaciers. Oxidation of FeS₂ under oxic, circumneutral conditions proceeds through the metastable intermediate thiosulfate (S₂O₃ ^2-), which represents an electron donor capable of supporting microbial metabolism. Subglacial meltwaters sampled from Robertson Glacier (RG), Canada, over a seasonal melt cycle revealed concentrations of S₂O₃ ^2- that were typically below the limit of detection, despite the presence of available pyrite and concentrations of the FeS₂ oxidation product sulfate (SO₄ ^2-) several orders of magnitude higher than those of S₂O₃ ^2-. Here we report on the physiological and genomic characterization of the chemolithoautotrophic facultative anaerobe Thiobacillus sp. strain RG5 isolated from the subglacial environment at RG. The RG5 genome encodes genes involved with pathways for the complete oxidation of S₂O₃ ^2-, CO₂ fixation, and aerobic and anaerobic respiration with nitrite or nitrate. Growth experiments indicated that the energy required to synthesize a cell under oxygen- or nitrate-reducing conditions with S₂O₃ ^2- as the electron donor was lower at 5.1°C than 14.4°C, indicating that this organism is cold adapted. RG sediment-associated transcripts of soxB, which encodes a component of the S₂O₃ ^2--oxidizing complex, were closely affiliated with soxB from RG5. Collectively, these results suggest an active sulfur cycle in the subglacial environment at RG mediated in part by populations closely affiliated with RG5. The consumption of S₂O₃ ^2- by RG5-like populations may accelerate abiotic FeS₂ oxidation, thereby enhancing mineral weathering in the subglacial environment.