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Archaea are a distinct domain of life, separated from bacteria by fundamental genetic and biochemical differences. Once called archaebacteria, they now belong to the Archaea domain, where classification reflects ongoing debate about their evolutionary origins. These microorganisms thrive in environments that challenge most life forms—deep‑sea hydrothermal vents, high‑temperature hot springs, hypersaline lakes, and the human gut.
One well‑studied vent archaeon is Methanocaldococcus jannaschii, classified as Archaea: Euryarchaeota: Methanobacteria; Methanobacteriales: Methanobacteriaceae; Methanocaldococcus, species jannaschii. Isolated from a vent on the ocean floor, it endures pressures exceeding 200 atmospheres and temperatures above 85 °C. The organism is anaerobic and produces methane as a metabolic by‑product.
Methanobrevibacter smithii (Archaea; Euryarchaeota; Methanobacteria; Methanobacteriales; Methanobacteriaceae; Methanobrevibacter; species smithii) colonizes the human gastrointestinal tract. It functions without oxygen, converting CO₂ to methane and aiding in the digestion of complex nutrients.
The square‑shaped Haloquadratum walsbyi (Archaea; Euryarchaeota; Halobacteria; Halobacteriales; Halobacteriaceae; Haloquadratum; species walsbyi) thrives in hypersaline environments. Its photosynthetic process harnesses solar energy, and its gas‑filled sacs enable buoyancy. When conditions are right, these cells form extensive sheets.
Deep‑sea vent specialist Thermococcus litoralis (Archaea; Euryarchaeota; Thermococci; Thermococcaceae; Thermococcus; species litoralis) requires sulfur for growth and, unlike many archaea, does not produce methane. It thrives at high temperatures and represents a group of species that remain to be discovered.
The only known member of the Nanoarchaeota subclass, Nanoarchaeum equitans (Archaea; Nanoarchaeota), was found attached to the cell wall of Ignicoccus species near thermal vents and in Yellowstone’s hot springs. Measuring just 400 nm across, it appears to be a parasitic partner, surviving between 167 °F and 204 °F.
