Methane is a powerful greenhouse gas that plays a central role in the global carbon cycle. At the same time, it is an important energy source for us humans. About half of its annual production is made by microorganisms known as methanogens that decompose organic material such as dead plants. This normally takes place in a habitat without oxygen as this gas is lethal to methanogens. But even in actually oxygen-free habitats, oxygen molecules occasionally appear. To render these intruders harmless, methanogens possess a special enzyme that is able to convert oxygen into water.
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Source: Phys.org