Effects of anaerobic oxidation of methane (AOM) driven by iron and manganese oxides on methane emissions in constructed wetlands and underlying mechanisms

In recent years, anaerobic oxidation of methane with iron and manganese oxides as electron acceptors has been confirmed to exist in electrochemical constructed wetlands (CW), and this process is an important way to reduce methane emissions. However, the internal mechanisms of emission reduction, microbial ecological networks, and electron transfer mechanisms remain unclear. In this study, we found that AOM driven by iron and manganese oxides (Fe/Mn-AOM) reduced methane emissions by more than 50.86% in constructed wetland – microbial fuel cell (CW-MFC) systems. In iron and manganese systems, the relative abundance of microorganisms associated with methane metabolism (methanotrophic bacteria) increased significantly. AOM-related microorganisms cooperated with other microorganisms to form a robust and stable microbial ecological network. In the process of methane metabolism, there was an increase in the proportion of reverse methanogenic genes in iron and manganese systems, with the proportion of mcr genes being 1.75 and 1.79 times that of control systems. The key genes (cyc1 and cox11) that influence cytochrome C synthesis in AOM process were significantly enriched, resulting in enhanced electron transfer process. The coexistence of iron and manganese oxides enabled the simultaneous biochemical processes of CH4 oxidation and CO2 reduction within a single system. Additionally, the increase in humic acid also facilitated electron transfer, thereby significantly enhancing the removal efficiency of COD, TN, and TP in the CW-MFC systems. This study reveals the potential mechanism of Fe/Mn-AOM for CW-MFC methane emission reduction, which can provide a theoretical basis for the collaborative treatment of sewage and methane control in CWs, so as to achieve the maximum environmental benefits of CW.