Most extremophiles are archaea.

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Extremophiles are organisms that live in extreme environments, such as high temperatures, acidic or alkaline conditions, high pressure, or high levels of radiation. While there are some extremophiles that are bacteria or eukaryotes, the majority of them are in fact archaea. Archaea are a type of single-celled organism that are genetically distinct from bacteria and eukaryotes, and are known for their ability to thrive in extreme environments.

Most Extremophiles Are Archaea