Solution: Methanogens belong to archaebactera. It contains three mahor classes of primitive bacteria, i.e. methanogens, halophilic and thermoacidophilic. Methanogens are strict anaerobes, present in the gut, of serveral ruminant animals (e.g. cows, etc.) and are responsible for production of mthane gas from the dung of these animals. Concept Enhancer Halophilic bacteria usually occur in salt rich substrate like salt marshes, etc. and are aerobic chemoheterotrophs. Thermophilic bacteria have the dual ability to tolerate high temperature as well as high acidity. These are basically chemosynthetic.