The Archaea constitute a domain and kingdom of single-celled microorganisms. These microbes are prokaryotes, meaning that they have no cell nucleus or any other membrane-bound organelles in their cells. Archaeal cells have unique properties separating them from the other two domains of life, Bacteria and Eukaryota. The Archaea are further divided into multiple recognized phyla. Classification is difficult because the majority have not been isolated in the laboratory and have only been detected by analysis of their nucleic acids in samples from their environment.
Archaea and bacteria are generally similar in size and shape, although a few archaea have very strange shapes, such as the flat and square-shaped cells. Despite this morphological similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably the enzymes involved in transcription and translation. Other aspects of archaeal biochemistry are unique, such as their reliance on ether lipids in their cell membranes, including archaeols. Archaea use more energy sources than eukaryotes: these range from organic compounds, such as sugars, to ammonia, metal ions or even hydrogen gas.
For much of the 20th century, prokaryotes were regarded as a single group of organisms and classified based on their biochemistry, morphology and metabolism. Archaea were first classified as a separate group of prokaryotes in 1977 by Carl Woese and George E. Fox in phylogenetic trees based on the sequences of ribosomal RNA (rRNA) genes. These two groups were originally named the Archaebacteria and Eubacteria and treated as kingdoms or subkingdoms. Later Woese proposed a new natural system of organisms with three separate Domains: Eukarya, Bacteria and Archaea, in what is now known as “The Woesian Revolution”.
The classification of archaea, and of prokaryotes in general, is a rapidly moving and contentious field. Current classification systems aim to organize archaea into groups of organisms that share structural features and common ancestors. These classifications rely heavily on the use of the sequence of ribosomal RNA genes to reveal relationships between organisms (molecular phylogenetics).
Domain Archaea are separated into four phyla:
|Euryarchaeota||This phylum includes methanogens,which produce methane as a metabolic waste product, and halobacteria, which live in an extreme saline environment.||Methanogens: Methane production caused flatulence in humans and other animals.
Halobacteria: Large blooms of this salt-loving archaea appear reddish due to the presence of bacteriorhodopsin in the membrane. Bacteriorhodopsin is related to the retinal pigment rhodopsin.
|Crenarchaeota||Members of the ubiquitous phylum play an important role in the fixation of carbon. Many members of this group are sulfur-dependent extremophiles. Some are thermophilic or hyperthermophilic.||Sulfolobus: Members of this genus grow in volcanic springs at temperatures betweeen 75°C and 80°C and at a pH between 2 and 3|
|Nanoarchaeota||This group currently contains only one species, Nanoarchaeum equitans||Nanoarchaeum equitans : This species was isolated from the bottom of the Atlantic Ocean and from the hydrothermal vent at Yellow stone National Park. It is an obligate symbiont with Ignicoccus, another species of archaea.|
|Korarchaeota||Members of this phylum, considered to be one of the most primitive forms of life, have only been found in the Obsidian Pool, a hot spring at Yellowstone National Park.||No members of this species have been cultivated.|