The Archaea (/ɑrˈkiːə/ or /ɑrˈkeɪə/ ar-KEE-ə or ar-KAY-ə; singular archaeon) constitute a domain or kingdom of single-celled microorganisms. These microbes are prokaryotes, meaning that they have no cell nucleus or any other membrane-bound organelles in their cells.rchaea are a major part of Earth's life and may play roles in both the carbon cycle and the nitrogen cycle. No clear examples of archaeal pathogens or parasites are known, but they are often mutualists or commensals.