Euryarchaeotes include Earth’s only biological source of natural gas, or methane (CH₄). Known as methanogens, they generate methane by fermenting acetate (CH₃COO–) or by chemoautotrophic pathways in which hydrogen gas (H₂) reduces CO₂ to CH₄. Methanogenic Archaea play an important role in the carbon cycle, recycling organic molecules in environments where the supplies of electron acceptors for respiration are limited.

Most methanogens live in soil or sediments, and they are particularly prominent in peats and lake bottoms where sulfate levels are low and bacterial sulfate reduction is therefore limited. Some, however, live in the rumens (a specialized chamber in the gut) of cows (Case 5), helping to maximize the energy yield from ingested grass. Methane doesn’t last long in the atmosphere—it is quickly oxidized to carbon dioxide. However, the steady supply of methane generated by archaeons contributes significantly to the greenhouse properties of the atmosphere. For this reason, biologists are interested in how methanogenic Archaea will respond to 21st-century climate change (Chapter 49).