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Archaea are a group of prokaryotes, not bacteria, that date back to the Archean era and are extremophiles, thriving in harsh conditions. They include halophiles, thermophiles, and acidophiles.
Archaea are a major group of prokaryotes, single-celled organisms without nuclei. In the three-domain classification system introduced by Carl Woese in 1990, they are one of three groups, along with Bacteria and Eukaryota. When they were first discovered, in extreme environments like the hot springs of Yellowstone Park, Archaea were misclassified as bacteria and were called Archaebacteria. Organisms in this group are sometimes still referred to as archaea, although that term has fallen out of favor, as they are not bacteria. Despite this, many have the suffix -bacteria included in their species name, a holdover from the time they were thought to be bacteria.
Possibly among the earliest living beings on Earth, the Archaea appear to date back to the Archean era, 3800 – 2500 million years ago. Their name means “old men” in Greek. Although they are prokaryotes like bacteria, they are more closely related to eukaryotes like amoeba. Archaea were initially found only in small numbers in extreme environments, but have since been found in many other locations and may make up 20% of the planetary biomass.
Archaea are well known for being extremophiles and would have thrived in the harsh conditions found on early Earth billions of years ago. It is difficult to conceive of a planetary disaster that could destroy all these organisms. Three main groups are the halophiles (salt lovers), thermophiles (heat lovers), and acidophiles (acid lovers).
Halophiles, such as Halobacterium, can survive in water five times as salty as the ocean and are found in large numbers in places such as the Great Salt Lake, Dead Sea and Lake Magadi in Kenya. Thermophiles, such as Thermus aquaticus, thrive in temperatures above 113°F (45°C), making use of unique enzymes that only operate at such high temperatures. Hyperthermophiles are a type of archaea that need even hotter temperatures to reproduce. The infamous Strain 121, extracted from a deep-sea hydrothermal vent off the coast of Washington state, can survive and reproduce in temperatures of 250°F (121°C), the temperature of an autoclave. Acidophiles such as Acidianus infernus survive in water with a pH below 2, about as acidic as stomach acid.
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