Editing Three-domain system (section)

== Classification ==
{{multiple images
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| image1            = RT8-4 scale.jpg
| alt1              = Electron micrograph of Sulfolobus infected with Sulfolobus virus STSV1.
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| image2            = Staphylococcus aureus VISA 2.jpg
| alt2              = Scanning electron micrograph of S. aureus; false-color added
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| image3            = Australia green tree frog (Litoria caerulea) crop.jpg
| alt3              = Australian green tree frog (Litoria caerulea)
| footer            = The three-domain system includes the [[Archaea]] (represented by ''[[Sulfolobus]]'', left), [[Bacteria]] (represented by ''[[Staphylococcus aureus]]'', middle) and [[eukaryote]]s (represented by the [[Australian green tree frog]], right).
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The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or [[six-kingdom system]]s. This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to eukaryotes than they are to other [[prokaryotes]] – bacteria-like organisms with no [[cell nucleus]]. The three-domain system sorts the previously known kingdoms into these three domains: [[Archaea]], [[Bacteria]], and [[Eukaryote|Eukarya]].<ref name="Gabaldón"/>

===Domain Archaea===
The [[Archaea]] are [[prokaryotic]], with no nuclear membrane, but with biochemistry and RNA markers that are distinct from bacteria. The archaeans possess unique, ancient evolutionary history for which they are considered some of the oldest species of organisms on Earth, most notably their diverse, exotic metabolisms.

Some examples of archaeal organisms are:
* [[methanogen]]s – which produce the gas [[methane]]
* [[halophile]]s – which live in very salty water
* [[thermoacidophile]]s – which thrive in acidic high-temperature water

===Domain Bacteria===
The [[Bacteria]] are also [[prokaryotic]]; their domain consists of cells with bacterial rRNA, no nuclear membrane, and whose membranes possess primarily ''diacyl glycerol diester lipids''. Traditionally classified as bacteria, many thrive in the same environments favored by humans, and were the first prokaryotes discovered; they were briefly called the '''Eubacteria''' or "true" bacteria when the Archaea were first recognized as a distinct [[clade]].

Most known pathogenic prokaryotic organisms belong to bacteria (see<ref name="Eckburg">{{cite journal |last1=Eckburg |first1=Paul B. |last2=Lepp |first2=Paul W. |last3=Relman |first3=David A. |year=2003 |title=Archaea and their potential role in human disease |journal=Infection and Immunity |volume=71 |issue=2 |pages=591–596 |doi=10.1128/IAI.71.2.591-596.2003 |pmid=12540534 |pmc=145348}}</ref> for exceptions). For that reason, and because the Archaea are typically difficult to grow in laboratories, Bacteria are currently studied more extensively than Archaea.

Some examples of bacteria include:
* "[[Cyanobacteria]]" – photosynthesizing bacteria that are related to the chloroplasts of eukaryotic plants and algae
* [[Spirochaetota]] – [[Gram-negative]] bacteria that include those causing syphilis and Lyme disease
* [[Actinomycetota]] – [[Gram-positive]] bacteria including ''[[Bifidobacterium animalis]]'' which is present in the human large intestine

===Domain Eukarya===
[[Eukaryota]] are organisms whose cells contain a membrane-bound nucleus. They include many large single-celled organisms and all known non-[[microscopic organism]]s. The domain contains, for example:

* [[Holomycota]] – mushrooms and allies
* [[Viridiplantae]] – green plants
* [[Holozoa]] – animals and allies
* [[Stramenopile]]s – includes brown algae
* [[Amoebozoa]] – solitary and social amoebae
* [[Discoba]] – includes euglenoids