A novel bacterial thiosulfate oxidation pathway provides a new clue about the formation of zero-valent sulfur in deep sea

Zero-valent sulfur (ZVS) has been shown to be a major sulfur intermediate in the deep-sea cold seep of the South China Sea based on our previous work, however, the microbial contribution to the formation of ZVS in cold seep has remained unclear. Here, we describe a novel thiosulfate oxidation pathway discovered in the deep-sea cold seep bacterium Erythrobacter flavus 21-3, which provides a new clue about the formation of ZVS. Electronic microscopy, energy-dispersive, and Raman spectra were used to confirm that E. flavus 21-3 effectively converts thiosulfate to ZVS. We next used a combined proteomic and genetic method to identify thiosulfate dehydrogenase (TsdA) and thiosulfohydrolase (SoxB) playing key roles in the conversion of thiosulfate to ZVS. Stoichiometric results of different sulfur intermediates further clarify the function of TsdA in converting thiosulfate to tetrathionate (^-O₃S-S-S-SO₃^-), SoxB in liberating sulfone from tetrathionate to form ZVS and sulfur dioxygenases (SdoA/SdoB) in oxidizing ZVS to sulfite under some conditions. Notably, homologs of TsdA, SoxB, and SdoA/SdoB widely exist across the bacteria including in Erythrobacter species derived from different environments. This strongly indicates that this novel thiosulfate oxidation pathway might be frequently used by microbes and plays an important role in the biogeochemical sulfur cycle in nature.