Nitrogen removal with the physiological stratification of cathodic biofilm in air-cathode single-chamber microbial fuel cell under different external resistances

•NH4+-N and TN removals were decreased with the external resistance decreasing.•NO2–-N accumulation of 4.4 ± 0.8 mg/L was observed within cycle 13 under 10 Ω.•The highest abundance of Glutamicibacter was 60.9% at layer 5 of cathodic biofilm.•AOB was closer to the cathode under lower external resistance.