Effluent organic matter facilitates anaerobic methane oxidation coupled with nitrous oxide reduction in river...

Effluent organic matter (EfOM) from wastewater treatment plants (WWTPs) contains humic-like substances that function as electron shuttles, thereby facilitating microbially-mediated redox reactions. However, the mechanisms governing the coupled processes of anaerobic oxidation of methane (CH₄) (AOM) and nitrous oxide (N₂O) reduction in river sediments, which receive WWTPs effluents, remain poorly understood. In this study, an incubation experiment with anoxic river sediments was conducted to assess the impacts of EfOM on AOM and nitrous oxide reduction using different effluent dilution ratios. The results showed that EfOM significantly enhanced both processes. Specifically, the AOM rate increased from 8.1 to 14.3 μg g_dw^-1 d^-1, while the N₂O reduction rate increased from 29.2 to 56.5 μg g_dw^-1 d^-1. The results of batch tests demonstrated that AOM process enhanced N₂O reduction in the presence of EfOM, highlighting the critical role of EfOM in linking these processes. Nitrate-dependent anaerobic methane oxidation (n-DAMO) archaea and denitrifying bacteria dominated the sediment incubated with EfOM. Metagenomic and metatranscriptomic analyses revealed that the denitrifying bacteria exclusively reduce N₂O, confirming the role of EfOM in facilitating electron transfer between n-DAMO archaea and N₂O reducers. This indicates that effluent discharge could be a potential factor driving the concurrent sinks of methane and nitrous oxide, offering a perspective for investigating the impacts of WWTPs effluent on greenhouse gas sinks in freshwater ecosystems.