Isolation of a putative sulfur comproportionating microorganism

Sulfur comproportionation is a heretofore undiscovered microbial catabolism that was predicted based on thermodynamic calculations. Here, we report the isolation of an Acidithiobacillus thiooxidans strain from extremely low pH snottite biofilms in the karst at Frasassi, Italy. The strain grew to cell densities of >107 cells mL-1 in autotrophic sulfur comproportionation medium. Whole genome sequencing of the isolate revealed the presence of numerous genes involved in sulfur transformations that could be linked in a sulfur comproportionation pathway. We describe an experimental framework, including measurements of sulfate, sulfide, and S0 concentrations, electron microscopy, and stable and radioisotope incubations coupled with NanoSIMS, scintillation counting and isotope ratio mass spectrometry, for future searches of sulfur comproportionators. SignificanceThe prediction of and search for novel microbial catabolic reactions can be streamlined by using thermodynamics to identify energy-yielding redox reactions that may be catalyzed by microorganisms. This strategy has been used to successfully predict several previously overlooked microbial catabolic reactions, including anaerobic ammonia oxidation (anammox), anaerobic oxidation of methane (AOM), and complete ammonia oxidation (comammox). Sulfur comproportionation, or the coupled reduction of sulfate and oxidation of sulfide to form elemental sulfur, was predicted by thermodynamic calculations to exist as a microbial catabolism in low pH, low-temperature environments. In this study, we describe the isolation of the first putative sulfur comproportionating microorganism and provide a detailed experimental approach that can be applied to future investigations of this novel link in the biogeochemical sulfur cycle.