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.
Like the methanogens and sulfur reducers, these archaebacteria live near volcanic vents and pools and thus are adapted to high temperatures (65 to 80 degrees Celsius). Unlike the other two classes, though, thermoacidophiles also prefer extremely acidic conditions, living in environments with a pH as low as 1.0.