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One of the three major classes major classes of living organisms; previously known as archaebacteria. They are unicellular mircoorganisms that are commonly found living in extreme abiotic conditions such as deep sea vents and extremely salty (hypersaline) waters. However, some species of archaea are also found living inside your gut, where they were first discovered in 1982[1].

The archaea was not recognised as a seperate domain of life until right up to the late 1970s when Dr. Carl Woese and his colleagues at the University of Illinois were studying relationships among the prokaryotes using DNA sequences[2]. When archaea were first founded they were often grouped as prokaryotes alongside bacteria under the organism classification system as they do not contain any membrane bound nuclei. Many archaea and bacteria are quite similar in size and shape, although a few archaea have very unusual shapes, for example the Haloquadratum ("salt square"), a genus of the family Halobacteriaceae, which has a very unique structure that looks like flattened square boxes. Nevertheless despite the 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.

The Archaea are very diverse in their metabolism and this allows them to thrive in a wide range of habitats.
Halophiles live in habitats with a very high salt content- at least 1.5 moldm-3 and often much higher. These concentrations are usually found in saline lakes such as the Dead Sea[3].
Thermophiles live in very hot habitats, the temperature of which sometimes reach 100°C. For example, hot springs in volcanic areas and geothermally heated regions of the sea floor, including hydrothermal vents known as black smokers[4].
Methanogens are spread out in anaerobic habitats where organic matter is available. Examples of these habitats are waterlogged soils, in the gut of cattle and in dumps of organic waste created by humans[5].

The archaea are very different to the bacteria and eukaryota in they neither require sunlight for photosynthesis as do plants, nor oxygen. Archaea absorbs CO2, N2, or H2S and give off methane gas as a waste product the same way humans breathe in oxygen and breathe out carbon dioxide[6].

Recently in 2011, researchers from Washington University and elsewhere have reported that they have sequenced the pan genome of the hydrogen-consuming gut archaea Methanobrevibacter smithiiM. smithii is an archaeal species that consumes hydrogen in the human gut. Since hydrogen produced during fermentation of food by bacteria in the gut affects the activity of some bacterial enzymes, this hydrogen use contributes to the energy produced from food[7].


  1. Purdy M. (2006). Gut microbes' partnership helps body extract energy from food, store it as fat. Available: Last accessed 1st Dec 2011
  2. Speer B.R., Waggoner B. (2001). Introduction to the Archaea Life's extremists. . . Available: Last accessed 1st Dec 2011
  3. Antonio Ventosa, Joaquín J. Nieto, and Aharon Oren. "Biology of Moderately Halophilic Aerobic Bacteria." Microbiology and Molecular Biology Reviews. N.p., 1998. Web. <>.
  4. Heather Beal. "Microbial Life in Extremely Hot Environments." Microbial Life in Extremely Hot Environments. Montana State University, n.d. Web. <>.
  5. Mark Pimentel MD, and Robert P Gunsalus PhD. "Methanogens in Human Health and Disease." Nature Publishing Group, 2012. Web. <>.
  6. Gardiner L.. (2004). Archaea. Available: Last accessed 1st Dec 2011.
  7. Unknown. (2011). Researchers Sequence Pan Genome of Gut Archaeal Species. Available: Last accessed 1st Dec 2011.
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