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

Shelley, Felicity, Abdullahi, Frah, Grey, Jonathan, Trimmer, Mark
Freshwater biology 2015 v.60 no.1 pp. 150-160
biological control, biological production, carbon dioxide, gravel, greenhouse gases, methane, methane production, oxidation, rivers, stems, stream channels, temperature
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. We characterise the potential response to temperature in the biological production of CO₂and CH₄and the subsequent microbial oxidation of that CH₄, that is the sink and source components of the CH₄cycle, in contrasting river bed sediments: fine sediments, which are largely anoxic, and oxic, coarse gravels. In the fine sediments, anaerobic production of both CH₄and CO₂increased 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:CO₂production for a 1 °C increase in temperature. In the coarse gravels, microbial CH₄oxidation showed no response to temperature at CH₄concentrations characteristic of these gravel beds (30–200 nmol CH₄ L⁻¹), due to strong substrate limitation. In contrast, at higher (although still rate limiting) CH₄concentrations, more characteristic of the fine sediment patches (2–4 μmol CH₄ L⁻¹), CH₄oxidation exhibited an increasingly strong response to temperature, eventually exceeding that for CH₄production. In the fine sediment, the surface layers had a CH₄oxidation capacity over 100 times greater than the gravels and the kinetic response to differing pore water CH₄concentrations meant CH₄was oxidised some 2000 times faster in the fine sediment patches compared with the coarse gravels. The calculated kinetic and temperature responses showed that with warming, methanogenesis is unlikely to outstrip methanotrophy and the ratio of CO₂to CH₄emitted could be conserved. Consequently, any changes in the efflux ratio of CH₄to CO₂are unlikely to be due to the incapacity of methanotrophy to respond to CH₄production, 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.