Methanogens: These archaebacteria are strict anaerobes. Nutritionally they are autotrophs which obtain both energy and carbon from decomposition products.
Example of Methanogens: Methanococcus, Methanobacterium
They occur in marshy areas where they convert formic acid and carbon dioxide into methane. This capability is commercially exploited in the production of methane and fuel gas inside gobar gas plants.
Some of the methanogenic archaebacteria live as symbionts (Methanobacterium) inside rumen cow, buffalo and helpful to the ruminants in fermentation of cellulose.