Manganese oxides in Phragmites rhizosphere accelerates ammonia oxidation in constructed wetlands

Phragmites reeds are widely used in constructed wetlands (CWs) for treating wastewater. The enrichment of microorganisms and Fe/Mn plaque in Phragmites rhizospheres largely contributes to pollutant removal. However, their interactions and potential synergistic roles in water purification are poorly understood. To address the issue, we first compared the microbial community traits in the Phragmites rhizosphere and adjacent bulk soil in six long-term operated CWs. Results showed that enriched microbes and functional genes in the Phragmites rhizosphere were largely involved in Mn oxidation, resulting in a two to three times enrichment of Mn oxides in the rhizosphere. In turn, the enriched Mn oxides played significant roles in driving microbial community composition and function. To further understand the biological manganese oxidation in the rhizosphere, we identified Mn-oxidizing bacteria using genome-centric analysis and found that 92% of identified Mn-oxidizing bacteria potentially participated in nitrogen cycling. We then conducted relationships between Mn-oxidizing genes and different nitrogen cycling genes and found Mn-oxidizing gene abundance was significantly correlated with ammonia oxidation gene amoA (R = 0.65). Remarkably, complete ammonia oxidation (comammox) Nitrospira, accounting for 39.11% of ammonia oxidizers, also positively correlated with Mn-oxidizing microbes. Based on the above observations, we inferred that the use of Mn oxides as a substrate in CWs may enhance ammonia oxidation. To apply this to actual engineering, we explored treatment performance in a pilot-scale Mn-amending CW. As expected, ammonia removal capacity improved by 23.34%, on average, in the Mn-amending CW. In addition, the abundance of amoA genes increased significantly in the Mn-amending CW, indicating improved biological processes rather than chemical reactions.