Functional Dominance and Competitive Strategy of Comammox Bacteria among Ammonia Oxidizers in Urban Secondary Effluent-Constructed Wetlands

Amid growing concerns over water pollution, the secondary effluent from wastewater treatment plants poses significant threats to aquatic ecosystems with limited self-purification capacity. Secondary effluent-constructed wetlands (SECWs) offer a sustainable solution for advanced nitrogen removal from this low-ammonia effluent, yet the functional role of comammox bacteria remains largely unexplored in such habitats. This study investigated the abundance, activity, kinetics, and ecological adaptations of comammox in typical SECWs. Quantitative PCR and amplicon sequencing revealed that comammox Nitrospira ubiquitously presented across all samples, even numerically dominated over ammonia-oxidizing bacteria/archaea (AOB/AOA). Consistent results from double-inhibition assays and DNA-stable isotope probing microcosm experiments indicated that comammox actively participated in nitrification, contributing 2.03-3.89 times those of canonical nitrifiers. Substrate kinetic and metagenomic analyses identified the Nitrospira nitrosa cluster as the sole active comammox population in SECWs, which exhibited relatively lower ammonia affinity (Km(app) = 0.055 ± 0.007 mg N/L) than other comammox species and distinct genomic adaptations to SECW-specific stressors, potentially explaining its dominance. Compared to AOB, comammox combines high substrate affinity with environmental resilience, aligning with K-strategist traits that enable it to outperform r-strategist AOB in SECWs. Overall, it is within this niche differentiation among comammox species and nitrifiers that the N. nitrosa cluster numerically and functionally dominated the nitrification process in SECWs, positioning comammox Nitrospira as pivotal biocatalysts for advanced nitrogen removal in engineered ecosystems.