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.
Archaea look and act a lot like bacteria. So much so that until the late 1970s, scientists assumed they were a kind of “weird” bacteria.
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.