A standalone reverse dissimilatory sulfite reductase (rDsr) pathway enables sulfur oxidation under oxygen limitation

Abstract

Sulfur oxidation drives nutrient and energy flow in marine sediments. Bacteria typically oxidize thiosulfate, a key sulfur intermediate, either through a complete Sox system or through an incomplete Sox system that feeds zero valent sulfur into the reverse dissimilatory sulfite reductase (rDsr) pathway. Whether rDsr alone can oxidize thiosulfate has been unclear. Here we show that ten intertidal Ruegeria strains carry a complete rDsr pathway but lack all sox genes. Under low oxygen (2% O2) but not under air, thiosulfate activates the pathway, enhances growth, and is associated with accelerated nitrate reduction. Using a CRISPR base editing tool for Ruegeria, we demonstrate that disrupting dsrA, which encodes a subunit of the core rDsr enzyme, significantly delays thiosulfate consumption, suggesting that rDsr is actively involved in oxidation. Previous work only documented rDsr acting downstream of an incomplete Sox system; our results establish that a standalone rDsr pathway is sufficient for thiosulfate consumption under oxygen limitation and that this activity can feed electrons into the denitrification pathway. Metagenomic surveys show rDsr genes from this group are widespread in low oxygen marine environments, including coastal sediments where they account for up to 26% of all rDsr genes. These findings revise the view that thiosulfate oxidation depends on Sox and highlight rDsrs ecological role in deoxygenated habitats.