Iron modulation of sulfur-mediated autotrophic denitrification: denitrification efficiency, microbial succession, and metabolic pathways

• 1 mM of Fe 2+ enhanced sulfur-mediated denitrification efficiency to 91.1 %. • Denitrification inhibition from cell encrustation did not occur at low Fe 2+ levels. • Iron regulation increased abundances of denitrification genes napA , napB and nosZ . • Iron regulation promoted sulfur metabolism through the SOX complex. • Campylobacterota and Sulfurimonas were enriched under iron regulation. The application of iron sulfide-mediated autotrophic denitrification is promising for nitrate reduction in carbon-deficient wastewater and polluted groundwater. Previous studies have shown distinct functional microbial communities in different iron sulfide-mediated autotrophic denitrification systems, obscuring how iron modulates their composition and activity. In this study, iron-modulated sulfur autotrophic denitrification efficiency, microbial succession, and key pathways were investigated at different iron levels. Results showed that 1 mM Fe 2+ enhanced denitrification efficiency (91.1 %) and prevented cell encrustation. Metagenomic analysis indicated that phylum Campylobacterota (16.0 %) and genus Sulfurimonas (14.4 %) were enriched under iron-modulated conditions. Iron modulated nitrate reduction by improving the relative abundance of complete denitrification genes ( napA , napB , and nosZ ) and stimulating sulfur metabolism through the SOX complex pathway ( soxZ and soxY ). These findings reveal the role of iron in modulating sulfur-mediated autotrophic denitrification and provide new insights into the microbial mechanisms involved in iron–sulfur coupling systems.