Methanogenesis

Methanogenesis is the biological production of methane by microbes called methanogens, which are all archaea. It occurs where there is no oxygen, such as in wetland mud, rice paddies, landfills and the guts of animals.

Methanogens gain energy by converting simple molecules to methane. The main routes use carbon dioxide and hydrogen (CO₂ + 4H₂ → CH₄ + 2H₂O), acetate, or methyl compounds. The last step uses the enzyme methyl-coenzyme M reductase, which is unique to these archaea. The process produces a proton gradient used to make ATP, and it is a very old metabolism, possibly present near the root of the archaeal tree.

Wetlands are the largest natural source, and in rice fields, landfills and the rumen of cattle, methanogens turn the products of fermentation into methane. Termite guts and thawing permafrost are also sources. Together biological sources account for the majority of methane emissions, a main subject in the carbon cycle. Methanogens also live around hydrothermal vents and deep underground.

Methane is a potent greenhouse gas, with a global warming potential many times that of carbon dioxide over a century, so changes in methanogenesis feed back on climate. Methanogens are also used in biogas digesters that turn waste into fuel, and diets and feed additives for livestock are being tested to reduce emissions. Their geochemistry also matters to fossil fuels such as natural gas.

• Methane — the gas they make
• Archaea — the domain of methanogens
• Carbon Cycle — the cycle they take part in