archea

Archaea are a domain of single-celled microorganisms that, although prokaryotic in organization, are distinct from Bacteria in their genetic and biochemical traits. They were defined as a separate domain in 1977 by Carl Woese and George Fox after comparing 16S ribosomal RNA genes, which revealed a lineage separate from Bacteria. Along with Bacteria and Eukarya, Archaea form the three-domain model of life.

Many archaea resemble bacteria in being unicellular and lacking a nucleus, but they have several distinctive features. Archaea typically possess cell membranes built from ether-linked isoprenoid lipids, and many archaeal cell walls lack peptidoglycan, instead using pseudopeptidoglycan or S-layer proteins. Some lineages have a rare monolayer membrane formed by tetraether lipids. The genetic machinery of transcription and translation contains similarities to eukaryotes: multiple-subunit RNA polymerase, and in some groups histone-like proteins that package DNA. Introns occur in some archaeal genes, though not universally.

Archaea occupy a wide range of habitats, including oceans, soils, and animal guts, and many thrive in extreme conditions such as high temperature, salinity, or acidity. Metabolic groups include methanogens that produce methane in anaerobic environments, halophiles that require high salt, and thermophiles and acidophiles. Archaea contribute to global cycles of carbon and nitrogen and often form symbiotic communities with other organisms, including ruminant mammals and marine animals.

Traditionally, archaea were divided into several phyla such as Euryarchaeota and Crenarchaeota, with Thaumarchaeota recognized more recently. Genomic studies have revealed new lineages, including the Asgard superphylum, which has implications for understanding the evolutionary origin of eukaryotes. Ongoing research continues to refine archaeal taxonomy and illuminate the deep evolutionary relationships among the domains of life.