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Archaea
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Although few people are aware of them, the Archaea are among the most fascinating organisms on earth. The Archaea have historically been considered part of the bacteria (they are sometimes called the Archaebacteria), but recent work has shown them to be very distinct. The Archaea are unique for two reasons. First, the Archaea represent the most primitive organisms yet discovered. In fact some living Archaea have a remarkable resemblance to some of the earliest known fossils dated at nearly 2 billion years old. One of the links below is an article discussing the ancestral position of the Archaea. Secondly, the Archaea live in some of the most extreme environments on earth. A common nickname for the Archaea is "Extremophiles". Although the Archaea break down into four distinct evolutionary lines, three major groups are recognized on the basis of their physiology. Methanogens. The methanogens live in anaerobic conditions (no free oxygen) such as the sediment at the bottom of a pond, sewage lagoons, or the intestinal tracts of animals. Methanogens use hydrogen and hydrogen rich compounds and carbon dioxide to produce energy for themselves and release methane (natural gas) as a waste product. The methanogens in the rumen of a cow are estimated to release about 50 liters of methane a day. Extreme thermophiles. The extreme thermophiles live in hot water (sometimes near the boiling point) and indeed often cannot live at lower temperatures. They are commonly found in fumaroles and geysers such as those found in Yellowstone Park or in the hot vents found on the ocean floor. These organisms oxidize sulfur as an energy source releasing sulphuric acid as a waste product. Extreme halophiles. Extreme halophiles live in water with extremely highly salt concentrations (often nearly 20 percent) such as the Great Salt Lake or the Dead Sea. They cannot live at low salt levels. They derive energy from a form of photosynthesis completely different from that used by algae and plants.
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