A domain of prokaryotic organisms containing the archaea (or archaebacteria), including the methanogens, which produce methane; the thermoacidophilic bacteria, which live in extremely hot and acidic environments (such as hot springs; see thermophilic); and the halophilic bacteria, which can only function at high salt concentrations and are abundant in the world’s oceans. Archaea are distinguished from the Eubacteria in that their membrane lipids are ether-linked as opposed to being ester-linked, and they lack peptidoglycan in their cell walls. However, the archaea are grouped together principally on the basis of molecular systematics, particularly similarities in the base sequence of their ribosomal RNA. This molecular evidence shows the archaea to be phylogenetically distinct and closer to the eukaryotes than the eubacteria. Multiple groups of archaea are now recognized, including the euryarchaeotes, the TACK supergroup, and the nanoarchaea and other minute organisms constituting the DPANN supergroup.