Environmental determinants of aerobic methane oxidation in a tropical river network

Aerobic methane oxidation (MOX) significantly reduces methane (CH₄) emissions from inland water bodies and is, therefore, an important determinant of global CH₄ budget. Yet, the magnitude and controls of MOX rates in rivers - a quantitatively significant natural source of atmospheric CH₄ - are poorly constrained. Here, we conducted a series of incubation experiments to understand the magnitude and environmental controls of MOX rates in tropical fluvial systems. We observed a large variability in MOX rate (0.03 - 3.45 μmol l^-1d^-1) shaped by a suit of environmental variables. Accordingly, we developed an empirical model for MOX that incorporate key environmental drivers, including temperature, CH₄, total phosphorus, and dissolved oxygen (O₂) concentrations, based on the results of our incubation experiments. We show that temperature dependency of MOX (activation energy: 0.66 ± 0.18 eV) is lower than that of sediment methanogenesis (0.71 ± 0.21 eV) in the studied tropical fluvial network. Furthermore, we observed a non-linear relationship between O₂ concentration and MOX, with the highest MOX rate occuring ∼135 μmol O₂l^-1, above or below this "optimal O₂" concentration, MOX rate shows a gradual decline. Together, our results suggest that the relatively lower temperature response of MOX compared to methanogenesis along with the projected decrease of O₂ concentration due to organic pollution may cause elevated CH₄ emission from tropical southeast Asian rivers. Since estimation of CH₄ oxidation is often neglected in routine CH₄ monitoring programs, the model developed here may help to integrate MOX rate into process-based models for fluvial CH₄ budget.