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There are three main types of archaea: the crenarchaeota (kren-are-key-oh-ta), which are characterized by their ability to tolerate extremes in temperature and acidity. The euryarchaeota (you-ree-are-key-oh-ta), which include methane-producers and salt-lovers; and the korarchaeota (core-are-key-oh-ta), a catch-all group for archaeans about which very little is known. Among these three main types of archaea are some subtypes, which include:
Methanogens (meth-an-oh-jins) — archaeans that produce methane gas as a waste product of their "digestion," or process of making energy.
Halophiles (hal-oh-files) — those archaeans that live in salty environments.
Thermophiles (ther-mo-files) — the archaeans that live at extremely hot temperatures.
Psychrophiles (sigh-crow-files) — those that live at unusually cold temperatures.
Then microbiologist Carl Woese devised an ingenious method of comparing genetic information showing that they could not rightly be called bacteria at all. Their genetic recipe is too different.
So different Woese decided they deserved their own special branch on the great family tree of life, a branch he dubbed the Archaea.