Reducing　the　C/N　ratio　requirements　for　heterotrophic　nitrification-aerobic　denitrification　(HNAD)　is　crucial　for　its　practical　application;　however,　it　remains　underexplored.　In　this　study,　a　highly　efficient　HNAD　bacterium,　Paracoccus　denitrificans　XW11,　was　isolated.　The　HNAD　characteristics　of　XW11　were　studied,　and　the　redox　mediator　fulvic　acid　(FA)　was　used　to　reduce　the　C/N　requirements.　Whole-genome　sequencing　revealed　multiple　denitrification　genes　in　XW11;　however,　nitrification　genes　were　not　identified,　because　heterotrophic　nitrification-related　gene　sequences　were　not　included　in　the　database.　However,　the　nitrogen　removal　related　enzyme　activity　test　revealed　complete　nitrification　and　denitrification　pathways.　Reverse　transcription　PCR　showed　that　the　membrane-bound　nitrate　reductase　(NarG),　rather　than　the　periplasmic　nitrate　reductase,　was　responsible　for　aerobic　denitrification.　The　conventional　nitrite　reductase　(NirS)　also　does　not　mediate　nitrite　denitrification.　When　the　C/N　ratio　was　10,　the　ammonia　removal　efficiency　of　the　Control　was　71.71　%　and　the　addition　of　FA　increased　it　to　86.12　%.　Transcriptomic　analysis　indicated　electron　flow　from　the　carbon　source　to　FA　without　proton　transmembrane　transport,　and　the　presence　of　FA　constructs　another　electron　transfer　system.　The　redox　potential　of　oxidized　FA/reduced　FA　is　0.3679　V,　avoiding　competition　for　electrons　from　Complex　III.　Thus,　ammonia　monooxygenase　obtains　electrons　more　easily,　thereby　promoting　nitrification.　The　enzyme　activity　test　of　the　nitrification　process　confirmed　this　view.　In　addition,　NarG　expression　increased,　and　the　denitrification　process　was　enhanced.　Overall,　FA　improved　HNAD　efficiency　by　facilitating　electron　transfer　to　the　nitrogen　dissimilation　process,　offering　a　novel　approach　to　reduce　the　C/N　requirement　of　HNAD.　©　2024　Elsevier　Ltd