Archaea are a major evolutionary group of microorganisms distinct from bacteria. Their cells lack a membrane-bound nucleus, so they are described as prokaryotic, but many components of their genetic machinery resemble those of eukaryotes more closely than bacterial equivalents. Traditionally recognized as one of three domains of cellular life, archaea include methane-producing organisms, salt-loving microbes, and numerous inhabitants of ordinary soils, waters, and sediments. They are therefore not simply bacteria adapted to extreme environments. (pmc.ncbi.nlm.nih.gov)
Discovery and classification
Archaea were initially classified with bacteria because both groups consist predominantly of small cells without nuclei. In 1977, Carl Woese and George Fox compared ribosomal RNA and identified methane-producing microorganisms as a deeply distinct lineage, which they called “archaebacteria.” Their work demonstrated that outward cellular similarity could conceal fundamental differences in evolutionary ancestry. (pmc.ncbi.nlm.nih.gov)
In 1990, Woese, Otto Kandler, and Mark Wheelis proposed the three-domain system, separating cellular organisms into Bacteria, Archaea, and Eucarya, now commonly written Eukarya. This introduced “domain” as a taxonomic rank above kingdom and replaced the implication that archaea were merely an unusual bacterial subgroup. (pmc.ncbi.nlm.nih.gov)
Archaeal taxonomy has expanded substantially through environmental sequencing. Historically prominent groups include Euryarchaeota and Crenarchaeota, while subsequent discoveries revealed additional lineages, including Asgard archaea and the groups collectively termed DPANN. Names, ranks, and relationships vary among classification frameworks as new genomes become available. Many lineages remain known mainly from sequences rather than cultivated organisms. (pmc.ncbi.nlm.nih.gov)
Cell structure and molecular machinery
A defining distinction concerns the cell membrane. Typical archaeal membrane lipids contain branched isoprenoid chains attached through ether bonds to a glycerol backbone. Most bacterial and eukaryotic membrane lipids instead contain fatty-acid chains attached through ester bonds, with a different glycerol stereochemistry. Some archaeal lipids span the membrane, forming a monolayer rather than a conventional bilayer. These differences affect membrane properties, although ether lipids are not absolutely exclusive to archaea. (pmc.ncbi.nlm.nih.gov)
Archaeal cell envelopes lack bacterial peptidoglycan. Many have an external S-layer composed of protein or glycoprotein; others possess different wall materials, including pseudomurein in some methane-producing species. Cell shapes range from spheres and rods to irregular or flattened forms. Motile archaea may use rotating appendages called archaella, which are structurally and evolutionarily distinct from bacterial flagella. (nature.com)
Archaeal systems for copying DNA and expressing genes combine distinctive features with strong similarities to eukaryotic machinery. Their DNA replication, transcription, and translation systems generally show closer molecular relationships to eukaryotic counterparts than to bacterial ones. Different archaeal groups also use different mechanisms of cell division, emphasizing that archaea are not uniform in their basic cellular organization. (pmc.ncbi.nlm.nih.gov)
Habitats and metabolism
Some archaea are extremophiles, flourishing at high temperatures, high salinities, or unusual acidity. These organisms strongly influenced early descriptions of the group. Environmental studies subsequently demonstrated abundant archaeal populations under moderate conditions, including marine waters, freshwater, soils, estuaries, and subsurface sediments. Their distribution encompasses both oxygenated and oxygen-depleted habitats. (nature.com)
Archaeal metabolism is diverse. Different groups obtain energy by processing organic compounds or by using inorganic chemical reactions. Some fix carbon dioxide into cellular material, while others depend on organic carbon. These capacities cannot be inferred simply from whether an organism is archaeal: closely related lineages may occupy different metabolic niches. (pmc.ncbi.nlm.nih.gov)
Methanogenesis, the biological production of methane, is a characteristic archaeal metabolism. Depending on the organism, substrates include hydrogen and carbon dioxide, acetate, or methylated compounds. Methanogens contribute to the breakdown of organic matter in oxygen-depleted environments, including sediments and anaerobic digesters. Not all archaea produce methane, and methane production should not be treated as a definition of the group. (pmc.ncbi.nlm.nih.gov)
Roles in nutrient cycles
Archaea participate in the carbon cycle through organic-matter degradation, carbon fixation, methane production, and methane consumption. Anaerobic methane-oxidizing archaea occur widely in marine sediments, including methane seeps. Many associate with sulfate-reducing bacteria, coupling methane oxidation to sulfate reduction and limiting the amount of methane escaping from sediments. (pmc.ncbi.nlm.nih.gov)
Other archaea are important in nitrification, a process within the nitrogen cycle. In 2005, researchers isolated the marine archaeon Nitrosopumilus maritimus, demonstrating growth through aerobic oxidation of ammonia to nitrite while fixing inorganic carbon. This provided direct experimental evidence that archaeal activity could account for processes previously attributed primarily to bacteria. (nature.com)
Evolutionary relationships
Asgard archaea are particularly important to research on evolution because their genomes encode numerous proteins associated with eukaryotic cellular functions. Phylogenomic studies support a close relationship between Asgard lineages and eukaryotes, although the precise branching position has depended on the organisms, genes, and analytical methods examined. This evidence supports models in which eukaryotes arose from within archaeal diversity rather than as an entirely independent primary lineage. (nature.com)
Eukaryotic origins also involved a bacterial partner: the mitochondrion derives from an alphaproteobacterial ancestor through endosymbiosis. Archaeal ancestry and bacterial contributions therefore explain different components of eukaryotic cells. Research continues to reconstruct how these contributions produced complex cellular systems. (nature.com)
Research and biotechnology
Environmental genome analysis has exposed archaeal diversity inaccessible to traditional cultivation, but inferred metabolic abilities require experimental testing. Cultured representatives remain essential for connecting genome sequences with actual physiology. Archaeal enzymes also have practical uses: the thermostable DNA polymerase from Pyrococcus furiosus, known as Pfu polymerase, enabled high-fidelity polymerase chain reaction amplification through its proofreading activity. (pmc.ncbi.nlm.nih.gov)