Molecular and stable isotopic evidence for anaerobic ammonium-oxidizing bacteria in tidal flow constructed wetlands

The process of anaerobic ammonium oxidation (anammox) plays a significant part in global nitrogen cycling, yet its depth-specific distribution and importance in tidal flow constructed wetlands (TF CWs) remained unclear. Currently, the abundance, biodiversity, vertical distribution, rates and potential role of the anammox process in two TF CWs of the Yangtze Estuary, China, were explored. The results indicated that anammox bacteria were widely distributed in the TF CWs, and Candidatus Brocadia was the dominant group among the anammox genera. The widespread presence of the anammox bacteria was observed at different water depths of the soil cores, and the abundance of total bacterial 16S rRNA and hzsB genes was 4.47–20.08 × 10⁹ and 0.81–5.91 × 10⁵ copies per gram of dry soil (copies g⁻¹ ds), respectively. The homogenized soils were incubated with stable nitrogen isotope to confirm the rates of anammox in the TF CWs, and the anammox activities were 2.38–6.72 nmol of N₂ per gram of dry soil per day (nmol N₂ g⁻¹ dry soil d⁻¹), with contributions of 2.7–7.5% nitrogen gas production and anammox that was more active in the deeper layer of the two TF CWs.