Autotrophic denitrification under anoxic conditions by newly discovered mixotrophic sulfide-oxidizing bacterium

Autotrophic denitrification (AutoDN) using sulfur represents a promising strategy for treating wastewater characterized by low carbon-to-nitrogen ratios (C/N). However, its widespread application is constrained by operational instability and the excessive sulfate accumulation. This study reports the isolation of Thauera sp. AutoDN2, a novel autotrophic denitrifier coupling nitrate reduction with sulfide oxidation while minizing sulfate production. AutoDN2 achieved nitrate removal of 99 ± 1 % at a sulfide-to-nitrogen ratio (S/N) of 4.8, primarily reducing nitrate to nitrite – a substrate for anaerobic ammonium oxidation (anammox), with minimal further reduction observed at S/N ratios ≥ 6.4. Unlike conventional sulfide-driven autotrophic denitrifiers, AutoDN2 predominantly generated elemental sulfur rather than sulfate, thereby mitigating secondary pollution. It also exhibited mixotrophic denitrification, indicating metabolic adaptability across a wide range of C/N ratios (0.3–5.0). These findings highlight AutoDN2′s capability for sustainable treatment of organic-deficient, nitrate-rich wastewater, contributing to an integrated carbon–nitrogen-sulfur (CNS) cycle with reduced sulfate release.