the hard, variously colored, calcareous skeleton secreted by certain marine polyps. 2. such skeletons collectively, forming reefs, islands, etc. 3. the solitary or colonial polyp that secretes this calcareous skeleton.
There are three major known groups of Archaebacteria: methanogens, halophiles, and thermophiles. The methanogens are anaerobic bacteria that produce methane. They are found in sewage treatment plants, bogs, and the intestinal tracts of ruminants.
Archaebacteria are found in very harsh conditions such as in the volcanic vents or at the bottom of the sea. They. are often called  extremophiles . They can easily survive in such extreme environment as sea vents releasing sulfideârich gases, hot springs, or boiling mud around volcanoes.