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සුළු r2.7.1) (රොබෝ එකතු කරමින්: frr:Uurbakteerien
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17 පේළිය:
[[Thaumarchaeota]]
}}
The '''Archaea''' {{Audio-IPA|en-us-Archaea.ogg|[ɑrˈkiə]}} areයනු a groupඒක ofෙෙසල single-celledක්ෂුදු [[microorganism]]sජීවීන් සමූහයකි. A single individual or species from this domain is called an ''archaeon'' (sometimes spelled "archeon"). They have no [[cell nucleus]] or any other [[organelle]]s within their cells. In the past they were viewed as an unusual group of bacteria and named '''archaebacteria''' but since the Archaea have an independent [[evolution|evolutionary history]] and show many differences in their biochemistry from other forms of life, they are now classified as a separate [[domain (biology)|domain]] in the [[three-domain system]]. In this system the three main branches of evolutionary descent are the Archaea, [[Eukarya]] and [[බැක්ටීරියා]]. Archaea are further divided into four recognized phyla, but many more phyla may exist. Of these groups the [[Crenarchaeota]] and the [[Euryarchaeota]] are most intensively studied. Classifying the Archaea is still difficult, since the vast majority have never been studied in the laboratory and have only been detected by analysis of their [[nucleic acid]]s in samples from the environment. Although archaea have, in the past, been classed with [[බැක්ටීරියා]] as ''[[prokaryote]]s'', this classification has been described as outdated, since it fails to distinguish between the three very distinct domains of life.<ref>>{{cite journal |author=Pace NR |title=Time for a change |journal=Nature |volume=441 |issue=7091 |pages=289 |year=2006 |month=May |pmid=16710401 |doi=10.1038/441289a}}</ref>
 
Generally, 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 ''[[Haloquadra|Haloquadra walsbyi]]''. Despite this visual similarity to bacteria, archaea possess genes and several [[metabolic pathway]]s that are more closely related to those of eukaryotes: notably the enzymes involved in [[transcription (genetics)|transcription]] and [[translation (biology)|translation]]. Other aspects of archaean biochemistry are unique, such as their reliance on [[ether lipid]]s in their [[cell membrane]]s. The archaea exploit a much greater variety of sources of energy than eukaryotes: ranging from familiar [[organic compounds]] such as [[sugar]]s, to using [[ammonia]], [[ion|metal ions]] or even [[hydrogen|hydrogen gas]] as nutrients. Salt-tolerant archaea (the [[Halobacteria]]) use sunlight as a source of energy, and other species of archaea [[carbon fixation|fix carbon]]; however, unlike [[plant]]s and [[cyanobacteria]], no species of archaea is known to do both. Archaea [[asexual reproduction|reproduce asexually]] and divide by [[binary fission]], fragmentation, or budding; in contrast to bacteria and eukaryotes, no species of archaea are known that form [[spore]]s.
86 පේළිය:
{{further|[[Microbial metabolism]]}}
 
Archaea exhibit a great variety of chemical reactions in their [[metabolism]] and use many different sources of energy. These forms of metabolism are classified into [[primary nutritional groups|nutritional groups]], depending on the source of energy and the source of carbon. Some archaea obtain their energy from [[inorganic compound]]s such as [[sulfur]] or [[ammonia]] (they are [[lithotroph]]s). These archaea include [[nitrifying bacteria|nitrifiers]], [[methanogen]]s and [[anaerobic]] [[methane]] [[oxidiser]]s.<ref name=valentine>{{cite journal |author=Valentine DL |title=Adaptations to energy stress dictate the ecology and evolution of the Archaea |journal=Nat. Rev. Microbiol. |volume=5 |issue=4 |pages=316–23 |year=2007 |pmid=17334387 |doi=10.1038/nrmicro1619}}</ref> In these reactions one compound passes electrons to another (in a [[redox]] reaction), releasing energy that is then used to fuel the cell's activities. One compound acts as an [[electron donor]] and one as an [[electron acceptor]]. A common feature of all these reactions is that the energy released is used to generate [[adenosine triphosphate]] (ATP) through [[chemiosmosis]], which is the same basic process that happens in the [[mitochondrion]] of animal cells.<ref name=Schafer>{{cite journal |author=Schäfer G, Engelhard M, Müller V |title=Bioenergetics of the Archaea |journal=Microbiol. Mol. Biol. Rev. |volume=63 |issue=3 |pages=570–620 |year=1999 |month=September |pmid=10477309 |pmc=103747 |url=http://mmbr.asm.org/cgi/pmidlookup?view=long&pmid=10477309 |day=01}}</ref>
 
Other groups of archaea use sunlight as a source of energy (they are [[phototroph]]s). However, oxygen-generating [[photosynthesis]] does not occur in any of these organisms.<ref name=Schafer/> Many basic [[metabolic pathway]]s are shared between all forms of life; for example, archaea use a modified form of [[glycolysis]] (the [[Entner–Doudoroff pathway]]) and either a complete or partial [[citric acid cycle]].<ref name=Zillig>{{cite journal |author=Zillig W |title=Comparative biochemistry of Archaea and Bacteria |journal=Curr. Opin. Genet. Dev. |volume=1 |issue=4 |pages=544–51 |year=1991 |month=December |pmid=1822288 |doi=10.1016/S0959-437X(05)80206-0}}</ref> These similarities with other organisms probably reflect both the early evolution of these parts of metabolism in the history of life and their high level of efficiency.<ref>{{cite journal |author=Romano A, Conway T |title=Evolution of carbohydrate metabolic pathways |journal=Res Microbiol |volume=147 |issue=6–7 |pages=448–55 |year=1996 |pmid=9084754 |doi=10.1016/0923-2508(96)83998-2}}</ref>
160 පේළිය:
Archaea carry out many steps in the [[nitrogen cycle]], this includes both dissimilatory reactions that remove nitrogen from ecosystems, such as [[nitrate]]-based respiration and [[denitrification]]: as well as assimilatory processes that introduce nitrogen, such as nitrate assimilation and [[nitrogen fixation]].<ref>{{cite journal |author=Cabello P, Roldán MD, Moreno-Vivián C |title=Nitrate reduction and the nitrogen cycle in archaea |journal=Microbiology (Reading, Engl.) |volume=150 |issue=Pt 11 |pages=3527–46 |year=2004 |month=November |pmid=15528644 |doi=10.1099/mic.0.27303-0 |url=http://mic.sgmjournals.org/cgi/content/full/150/11/3527?view=long&pmid=15528644}}</ref><ref>{{cite journal |author=Mehta MP, Baross JA |title=Nitrogen fixation at 92 degrees C by a hydrothermal vent archaeon |journal=Science (journal) |volume=314 |issue=5806 |pages=1783–6 |year=2006 |month=December |pmid=17170307 |doi=10.1126/science.1134772}}</ref> The involvement of archaea in [[ammonia]] oxidation reactions was recently discovered; these being particularly important in the oceans.<ref>{{cite journal |author=Francis CA, Beman JM, Kuypers MM |title=New processes and players in the nitrogen cycle: the microbial ecology of anaerobic and archaeal ammonia oxidation |journal=ISME J |volume=1 |issue=1 |pages=19–27 |year=2007 |month=May |pmid=18043610 |doi=10.1038/ismej.2007.8}}</ref><ref>{{cite journal |author=Coolen MJ, Abbas B, van Bleijswijk J, ''et al.'' |title=Putative ammonia-oxidizing Crenarchaeota in suboxic waters of the Black Sea: a basin-wide ecological study using 16S ribosomal and functional genes and membrane lipids |journal=Environ. Microbiol. |volume=9 |issue=4 |pages=1001–16 |year=2007 |month=April |pmid=17359272 |doi=10.1111/j.1462-2920.2006.01227.x}}</ref> The archaea also appear to be crucial for ammonia oxidation in soils, this produces [[nitrite]], which is then oxidized to [[nitrate]] by other microbes, and then taken up by plants and other organisms.<ref>{{cite journal |author=Leininger S, Urich T, Schloter M, ''et al.'' |title=Archaea predominate among ammonia-oxidizing prokaryotes in soils |journal=Nature |volume=442 |issue=7104 |pages=806–9 |year=2006 |month=August |pmid=16915287 |doi=10.1038/nature04983}}</ref>
 
In the [[sulfur cycle]], archaea that grow by oxidizing [[sulfur]] compounds are important as they release this element from rocks, making it available to other organisms. However, the archaea that do this, such as ''Sulfolobus'', can cause environmental damage since they produce [[sulfuric acid]] as a waste product, and the growth of these organisms in abandoned mines can contribute to [[acid mine drainage]].<ref name=Baker2003>{{Cite journal | year = 2003 | title = Microbial communities in acid mine drainage | journal = FEMS Microbiology Ecology | volume = 44 | issue = 2 | pages = 139–152 | doi = 10.1016/S0168-6496(03)00028-X | url = http://www.blackwell-synergy.com/doi/abs/10.1016/S0168-6496(03)00028-X | author1 = Baker, B. J | author2 = Banfield, J. F}}</ref>
 
In the [[carbon cycle]], methanogen archaea are significant as methane producers. The ability of these archaea to remove hydrogen is important in the degradation of organic matter by the populations of microorganisms that act as [[decomposer]]s in anaerobic ecosystems, such as sediments, marshes and [[sewage treatment]] works.<ref>{{cite journal |author=Schimel J |title=Playing scales in the methane cycle: from microbial ecology to the globe |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=101 |issue=34 |pages=12400–1 |year=2004 |month=August |pmid=15314221 |pmc=515073 |doi=10.1073/pnas.0405075101 |url=http://www.pnas.org/cgi/pmidlookup?view=long&pmid=15314221}}</ref> However, methane is one of the most abundant [[greenhouse gas]]es in Earth's atmosphere, constituting 18% of the global total.<ref>{{cite web | title= EDGAR 3.2 Fast Track 2000 | url= http://www.mnp.nl/edgar/model/v32ft2000edgar/ | accessdate= 2008-06-26 }}</ref> It is 25 times more potent as a greenhouse gas than carbon dioxide.<ref>{{cite web | date= 2008-04-23 | title = Annual Greenhouse Gas Index (AGGI) Indicates Sharp Rise in Carbon Dioxide and Methane in 2007 | url=http://www.esrl.noaa.gov/media/2008/aggi.html | accessdate= 2008-06-26 }}</ref> Methanogens are the primary source of [[atmospheric methane]], and are responsible for most of the world's yearly [[Methane#missions of methane|methane emissions]].<ref name="Trace Gases">{{cite web|url=http://www.grida.no/climate/ipcc_tar/wg1/134.htm#4211|title=Trace Gases: Current Observations, Trends, and Budgets|work=Climate Change 2001|publisher=United Nations Environment Programme}}</ref> As a consequence, these archaea contribute to global greenhouse gas emissions and [[global warming]].
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