Explaining the coexistence of complete ammonium-oxidizing bacteria and canonical nitrite-oxidizing bacteria under...

The discovery of complete ammonium-oxidizing (comammox) bacteria has fundamentally transformed our perception of biological nitrogen removal (BNR). Although recent studies have shed light on the competitive interactions between comammox Nitrospira and ammonium-oxidizing bacteria, the mechanisms underlying the well-documented coexistence of comammox Nitrospira and nitrite-oxidizing bacteria (NOB) Nitrospira remain unresolved. To this end, we first enriched comammox Nitrospira in a lab-scale continuous-feeding reactor through 151-d operation under mainstream wastewater conditions and then performed comparative kinetic analyses between comammox Nitrospira-dominant enriched sludge and NOB Nitrospira-dominant inoculated sludge. The results proved that comammox Nitrospira-dominant sludge exhibited a statistically lower nitrite affinity constant (0.417 ± 0.015 mg-N/L) compared to NOB Nitrospira-dominant sludge (0.650 ± 0.018 mg-N/L), conferring a competitive advantage under nitrite-limited conditions. In contrast, NOB Nitrospira-dominant sludge displayed a lower oxygen affinity constant (0.009 ± 0.001 mg-O₂/L) than comammox Nitrospira-dominant sludge (0.019 ± 0.001 mg-O₂/L), allowing it to persist in oxygen-limited environments. This dual affinity-driven niche partitioning-comammox Nitrospira exploits nitrogen substrates, while NOB Nitrospira capitalizes on residual oxygen-might establish a kinetic equilibrium sustaining their stable coexistence despite metabolic competition. The findings not only further our understanding of ecological interactions between nitrifiers but also provide valuable insights for targeted microbial enrichment toward sustainable BNR.