page_title: Microorganism
section_title: Extremophiles
hierarchical_section_title: Microorganism / Ecology / Extremophiles
caption_attribution_description: Transmission electron microgragh (TEM) of Deinococcus radiodurans
context_page_description: A microorganism, or microbe, is a microscopic organism, which may exist in its single-celled form or a colony of cells.
  The possible existence of unseen microbial life was suspected from ancient times, such as in Jain scriptures from 6th century BC India and the 1st century BC book On Agriculture by Marcus Terentius Varro. The scientific study of microorganisms began with their observation under the microscope in the 1670s by Antonie van Leeuwenhoek. In the 1850s, Louis Pasteur found that microorganisms caused food spoilage, debunking the theory of spontaneous generation. In the 1880s, Robert Koch discovered that microorganisms caused the diseases tuberculosis, cholera, and anthrax.
  Microorganisms include all unicellular organisms and so are extremely diverse. Of the three domains of life identified by Carl Woese, all of the Archaea and Bacteria are microorganisms. These were previously grouped in the two domain system as Prokaryotes, the other being the eukaryotes. The third domain Eukaryota includes all multicellular organisms and many unicellular protists and protozoans. Some protists are related to animals and some to green plants.
context_section_description: Extremophiles are microorganisms that have adapted so that they can survive and even thrive in extreme environments that are normally fatal to most life-forms. Thermophiles and hyperthermophiles thrive in high temperatures. Psychrophiles thrive in extremely low temperatures. – Temperatures as high as 130 °C (266 °F), as low as −17 °C (1 °F) Halophiles such as Halobacterium salinarum (an archaean) thrive in high salt conditions, up to saturation. Alkaliphiles thrive in an alkaline pH of about 8.5–11. Acidophiles can thrive in a pH of 2.0 or less. Piezophiles thrive at very high pressures: up to 1,000–2,000 atm, down to 0 atm as in a vacuum of space. A few extremophiles such as Deinococcus radiodurans are radioresistant, resisting radiation exposure of up to 5k Gy. Extremophiles are significant in different ways. They extend terrestrial life into much of the Earth's hydrosphere, crust and atmosphere, their specific evolutionary adaptation mechanisms to their extreme environment can be exploited in biotechnology, and their very existence under such extreme conditions increases the potential for extraterrestrial life.
caption: A tetrad of Deinococcus radiodurans, a radioresistant extremophile bacterium
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