Sulfide‐driven mixotrophic denitrification: Mechanisms and microbial interactions for enhanced nitrogen removal

This study investigated sulfide‐driven mixotrophic denitrification (SDMD) for nitrogen (N) removal under low‐carbon (C) conditions. At a C/N ratio of 0.25 (with 80 mg S L −1 sulfide [S] and 50 mg N L −1 nitrate), SDMD achieved a total nitrogen removal efficiency of 76.14%, outperforming the combined efficiencies of sulfide‐driven autotrophic denitrification (58.34%) and heterotrophic denitrification (7.35%) by 10.45%. Increasing C/N to 0.75 maintained high denitrification efficiency, with organic C accelerating nitrite reduction and sulfide removal. Microbial analysis demonstrated heterotrophs metabolized organics via the tricarboxylic acid cycle, supplying inorganic C to autotrophs utilizing the reductive tricarboxylic acid cycle (rTCA) cycle, enabling mutualistic cooperation despite underlying competition. Core genera ( Thiobacillus , Rhodanobacter , Castellaniella , and Thauera ) formed a metabolically complementary network. Metagenomics showed organic C upregulated sulfur oxidation ( fccAB , sqr , sox , and s oeABC ), sulfate transport ( cysAPUW ), and dissimilatory sulfate reduction genes ( cysDN , sat , aprAB , cysIJ , and sir ), reinforcing sulfur cycling and N removal. These findings provide insights for optimizing SDMD‐based wastewater treatment.