Key enzymes and genetic traits in sulfur autotrophic denitrification: Electron donors, functional microbes, and...

Nitrate pollution in water bodies is a pressing global environmental issue. Sulfur autotrophic denitrification (SAD) offers a viable solution for treating low carbon-to-nitrogen ratio wastewater, as it requires no external organic carbon. However, the application of SAD is constrained by sulfate accumulation and system acidification. This review synthesizes recent advances in SAD, focusing on the interplay between electron donors, microbial communities, and functional genes. It highlights that establishing an endogenous sulfur cycle can effectively reduce effluent sulfate concentrations. Different sulfur-based electron donors have distinct characteristics: elemental sulfur is cost-effective but prone to acidification; biogenic sulfur preserves nitrous oxide reductase (Nos) activity, mitigating N₂O emissions; sulfide, at optimal S/N ratios, promotes nitrite accumulation, which can be harnessed for integration with the Anammox process; thiosulfate offers the fastest reaction kinetics but generates high sulfate levels; sulfite can also serve as an electron donor, but excessive S/N ratios may inhibit denitrifying enzymes. Key microbes, including Thiobacillus and Sulfurimonas, drive denitrification via genes such as sulfur oxidizing enzyme (SOX), narG, nirS and nosZ. genes. Synergistic interactions among sulfur-oxidizing bacteria, sulfate-reducing bacteria and sulfur disproportionating bacteria can establish an endogenous sulfur cycle that mitigates sulfate accumulation. Leveraging these microbial mechanisms, integrated processes offer synergistic benefits, including enhanced nitrogen removal and pH stabilization. Future research should focus on optimizing functional microbial consortia, developing novel sulfur cycle-based integrated processes, and applying intelligent control strategies to facilitate the practical application and scaling of SAD.