Methane oxidation kinetics in northern freshwater lakes

Understanding the drivers of aerobic methane (CH4) oxidation (MOX) is paramount in assessing the current and potential future CH4 emissions from freshwater aquatic systems. Regulation of MOX kinetics is a complex function of CH4 and oxygen (O-2) concentrations. While MOX activity is usually proportional to the concentration of CH4 itself, the effects of O-2 have been more conflicting, with maximum MOX rates often restricted to low O-2 concentrations. Despite the complexity involved, MOX kinetics are often modelled as monotonic positive functions of both CH4 and O-2 concentrations. We conducted a series of incubation experiments using natural and unamended water samples obtained from multiple depths in northern temperate lakes that vary widely and independently in their CH4 and O-2 concentrations. Our results showed the expected positive effect of CH4 concentration and temperature but also demonstrated the strong inhibitory effects of O-2 at high concentration. We then developed a general model describing the kinetics of MOX, simultaneously integrating the effects of CH4 concentration, temperature as well as the non-linear effect O-2 on MOX activity. The model revealed an overall temperature dependency (activation energy=0.49 +/- 0.06eV) much lower than reported for methanogenesis and an optimal O-2 level around 15mol O-2 L-1 where maximum MOX activity occurs, regardless of CH4 concentration and temperature. We further show that ignoring the inhibitory effect of O-2 can lead to significant bias in calculating the expected MOX rates in different portions of the water column.