Microbial methane cycling in the bed of a chalk-river:oxidation has the potential to match warming enhanced Methanogenesis

1.Many rivers are oversaturated in methane (CH4) and carbon dioxide (CO2) relative to the atmosphere, but we know little about the biological controls on the balance between these two important greenhouse gases and how they might respond to warming. 2.We characterise the potential response to temperature in the biological production of CO2 and CH4 and the subsequent microbial oxidation of that CH4, that is the sink and source components of the CH4 cycle, in contrasting river bed sediments: fine sediments, which are largely anoxic, and oxic, coarse gravels. 3.In the fine sediments, anaerobic production of both CH4 and CO2 increased with temperature, with apparent activation energies for each being 0.51 eV and 0.24 eV, respectively. The difference between the two resulted in a 4% increase in the ratio of CH4:CO2 production for a 1 °C increase in temperature. 4.In the coarse gravels, microbial CH4 oxidation showed no response to temperature at CH4 concentrations characteristic of these gravel beds (30–200 nmol CH4 L−1), due to strong substrate limitation. In contrast, at higher (although still rate limiting) CH4 concentrations, more characteristic of the fine sediment patches (2–4 μmol CH4 L−1), CH4 oxidation exhibited an increasingly strong response to temperature, eventually exceeding that for CH4 production. 5.In the fine sediment, the surface layers had a CH4 oxidation capacity over 100 times greater than the gravels and the kinetic response to differing pore water CH4 concentrations meant CH4 was oxidised some 2000 times faster in the fine sediment patches compared with the coarse gravels. 6.The calculated kinetic and temperature responses showed that with warming, methanogenesis is unlikely to outstrip methanotrophy and the ratio of CO2 to CH4 emitted could be conserved. Consequently, any changes in the efflux ratio of CH4 to CO2 are unlikely to be due to the incapacity of methanotrophy to respond to CH4 production, but rather to a physical bypassing of the methanotrophic community (e.g. through ebullition or transport via plant stems) or contraction of the oxic layer