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 (O₂) concentrations. While MOX activity is usually proportional to the concentration of CH₄ itself, the effects of O₂ have been more conflicting, with maximum MOX rates often restricted to low O₂ concentrations. Despite the complexity involved, MOX kinetics are often modelled as monotonic positive functions of both CH₄ and O₂ 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 O₂ concentrations. Our results showed the expected positive effect of CH₄ concentration and temperature but also demonstrated the strong inhibitory effects of O₂ 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 O₂ on MOX activity. The model revealed an overall temperature dependency (activation energy = 0.49 ± 0.06 eV) much lower than reported for methanogenesis and an optimal O₂ level around 15 µmol O₂ L⁻¹ where maximum MOX activity occurs, regardless of CH₄ concentration and temperature. We further show that ignoring the inhibitory effect of O₂ can lead to significant bias in calculating the expected MOX rates in different portions of the water column.