How do methane-producing archaea obtain energy in oxygen-free environments?

Methane-producing archaea, known as methanogens, obtain energy in oxygen-free environments through a specialized form of anaerobic respiration.1

In this process, these organisms typically use hydrogen gas ($H_2$) as an electron donor and carbon dioxide ($C{O}_2$) as their terminal electron acceptor to produce methane. While their metabolic range is narrow, they can also convert a few other substances into methane, including methanol, formic acid, acetic acid, and methylamines.12

Because methanogens are obligate anaerobes, they often function in communities with other anaerobic bacteria. These partner bacteria break down complex organic materials—such as cellulose, proteins, and fats—into simpler compounds like acetate, $C{O}_2$, and $H_2$, which the methanogens then utilize. By consuming the hydrogen gas produced by these other bacteria, methanogens effectively stimulate the metabolism of the entire microbial community, allowing for the continued decomposition of organic matter in environments like swamps, marshes, and the digestive tracts of ruminant animals.32