When first discovered in 1977, these microscopic single-celled organisms were classified as Bacteria. However, later genetic and biochemical analyses showed that they were more closely related to the Eukaryotes (which include humans). The researchers who discovered this, Woese and his colleagues, proposed the addition of the “domain” as a taxonomic level above that of kingdom, with all life classified within one of three domains, now called Bacteria, Eukaryotes, and Archaea. As of 2010 more than 250 species of Archaea had been described, most fitting into one of two groups, Euryarchaeota and Crenarchaeota. Many of these live in extreme environments – places where we once believed no life could exist.
Archaea are generally less than one micron long, may or may not have flagella, and cell-shapes include spherical, box-like, triangular, and slender filamentous forms (UCMP, accessed 2011).
Because Archaea can thrive in extreme environments, they occur in places we didn’t think were habitats – very low (4 °C) and very high (120 °C) temperatures of polar seas, hydrothermal vents and hot sulfur springs; high and very low pH; hypersaline water, and anaerobic environments like human colons and termite guts. They also live in moderate habitats in the oceans, soils, freshwater (Kimball, 2010).
Apparently nearly anywhere and eveywhere on Earth.
Evolution and Systematics
Systematics and Taxonomy
PHYLA / DIVISIONS
Euryarchaeota: methanogens, halophiles, thermophiles
Crenarchaeota: hyperthermophiles, cool-water marine plankton, nitrifying soil/sediment microbes
Korarchaeota (proposed): thermophiles
Thaumarchaeota (proposed): chemolithoautotrophic ammonia-oxidizers
(Hohn et al., 2002; Huber et al., 2002; Brochier-Armanet et al., 2008; Kimball, 2010; Spang et al., 2010; Auchtung et al., 2011)