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.
Archaea is a domain of microorganisms that are mostly extremophiles, meaning they live in environments with extreme temperatures. Archaea are found often in geysers, such as those in Yellowstone National Park, and in the deep basins of the ocean.