A group of single-celled organisms, classified by some microbiologists as a type of bacteria and by others as a separate kingdom or domain of life. Unlike Eubacteria, they lack peptidoglycans in their cell walls. Their ribosomal RNA base sequences also differ from those found in Eubacteria. Many live in harsh environments, such as superheated steam vents or highly concentrated salt water. Some produce methane (natural gas).