The
Archaea (
[font="""]/ɑrˈkiːə/[/font] (
listen) [font="""]ar-kee-ə[/font]) are a group of
single-celled microorganisms. A single individual or
species from this
domain is called an
archaeon (sometimes spelled "archeon"). They have no
cell nucleus or any other membrane-bound
organelles within their cells.
In the past they had been classed with
bacteria as
prokaryotes (or
Kingdom Monera) and named
archaebacteria, but this classification is regarded as outdated.
[1] In fact, the Archaea have an independent
evolutionary history and show many differences in their biochemistry from other forms of life, and so they are now classified as a separate domain in the
three-domain system. In this system, the
phylogenetically distinct branches of
evolutionary descent are the Archaea,
Bacteria and
Eukaryota.
Archaea are divided into four recognized
phyla, but many more phyla may exist. Of these groups, the
Crenarchaeota and the
Euryarchaeota are the most intensively studied. Classification is still difficult, because the vast majority have never been studied in the laboratory and have only been detected by analysis of their
nucleic acids in samples from the environment.
Archaea and bacteria are quite similar in size and shape, although a few archaea have very unusual
shapes, such as the flat and square-shaped cells of
Haloquadratum walsbyi. Despite this visual 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 archaean biochemistry are unique, such as their reliance on
ether lipids in their cell membranes. Archaea use a much greater variety of sources of energy than eukaryotes: ranging from familiar
organic compounds such as
sugars, to
ammonia,
metal ions or even
hydrogen gas.
Salt-tolerant archaea (the
Haloarchaea) use sunlight as an energy source, and other species of archaea
fix carbon; however, unlike
plants and
cyanobacteria, no species of archaea is known to do both. Archaea
reproduce asexually by
binary fission, fragmentation, or
budding; unlike bacteria and eukaryotes,
no known species form spores.
Initially, archaea were seen as
extremophiles that lived in harsh environments, such as
hot springs and
salt lakes, but they have since been found in a broad range of
habitats, including
soils,
oceans,
marshlands and the human
colon. Archaea are particularly numerous in the oceans, and the archaea in
plankton may be one of the most abundant groups of organisms on the planet. Archaea are now recognized as a major part of Earth's life and may play roles in both the
carbon cycle and the
nitrogen cycle. No clear examples of archaeal
pathogens or
parasites are known, but they are often
mutualists or
commensals. One example is the
methanogens that inhabit the gut of humans and
ruminants, where their vast numbers aid
digestion. Methanogens are used in
biogas production and
sewage treatment, and enzymes from extremophile archaea that can endure high temperatures and
organic solvents are exploited in
biotechnology.
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