Archaeen

Archaea constitute one of the three domains of life, alongside Bacteria and Eukarya. They are single-celled organisms that lack a nucleus and were historically grouped with bacteria as prokaryotes, but genetic and biochemical analyses show them to be a distinct lineage. Archaea inhabit a wide range of environments, from oceans and soils to the guts of animals and extreme settings such as hot springs, highly acidic or hypersaline habitats, and deep-sea vents.

A defining feature of archaea is their cell membranes and cell walls. Their membranes are composed of ether-linked lipids with isoprenoid chains, a structural difference from bacterial and eukaryotic membranes. Many archaea lack peptidoglycan in their cell walls, though some possess pseudopeptidoglycan. Archaeal genomes sometimes encode histone-like proteins, and their RNA polymerase is more similar to that of eukaryotes than to bacteria, reflecting a deep evolutionary split. Some archaea also harbor introns in certain genes.

Metabolically, archaea display diverse strategies. Methanogens produce methane in anaerobic environments and are important in wetlands, ruminant guts, and sediments. Halophiles thrive in very salty environments, while thermophiles and hyperthermophiles flourish at high temperatures. Ammonia-oxidizing archaea contribute to nitrogen cycling in soils and oceans. Ecologically, archaea are widespread and play critical roles in global biogeochemical processes, often forming the base of unique microbial communities in extreme ecosystems.

Taxonomically, archaea were traditionally divided into major groups such as Euryarchaeota and Crenarchaeota, with Thaumarchaeota and recently described lineages recognized in newer classifications. The discovery of archaea in the 1970s by Carl Woese and colleagues established the three-domain system and revealed that Archaea are more closely related to Eukarya than to Bacteria, highlighting a distinct domain of life.