Transcriptionally active ammonia-oxidizing nitrifiers in a surface flow constructed wetland

Ammonia-oxidizing nitrifiers (AON) are essential for nitrogen removal in constructed wetlands (CWs). With increasing recognition of the diversity of AON, a reappraisal of the AON community and its relationship with environmental factors in CWs is needed. Here, we investigated the active AON community in inflow and outflow sediments of a landscape CW using amoA (encoding a subunit of ammonia monooxygenase) transcript analysis. Diverse amoA transcript phylotypes of ammonia-oxidizing archaea (AOA), bacteria (AOB), and comammox bacteria (CMX) were identified. Quantitative analysis revealed shifts in the abundances of AOA, AOB, and CMX amoA transcripts between inflow and outflow sediments. AOA amoA transcripts were primarily affiliated with the Nitrosocosmicus, Nitrososphaera, Nitrosotalea, and Nitrosotenuis clusters; AOB amoA transcripts grouped within the Nitrosospira and Nitrosomonas clusters; and CMX amoA transcripts fell into Nitrospira Clade A and B. The amoA transcript abundances of the three AON groups were differentially correlated with potential nitrification rates, ammonia, total nitrogen, pH, organic matter, and nitrate. Canonical correspondence analysis identified ammonia as a factor consistently associated with the community structures of all three AON groups, whereas pH was additionally associated with those of AOB and CMX. This study provides a snapshot of the active AON community in a CW, highlights spatial heterogeneity in abundance and composition, and serves as a reference for future hypothesis-driven investigations into nitrogen removal in wetland systems.