Methanogens are a group of organisms classified under the archeae and they produce methane as their metabolic byproduct. They can function only in anoxic conditions. ie., environment devoid of oxygen.
Usually they are found in wetlands or in the digestive tracts of humans or ruminants. In wetlands, their produce the marsh gas or methane. In the digestive tracts where they are found, they are responsible for the presence of methane in the belching of ruminants or flatulence of humans.
They are very sensitive anaerobes and can be really affected by the presence of oxygen, even at trace levels. As such they play a vital role in strictly anaerobic conditions to remove excess hydrogen and other fermentation products by using them up along with CO2 and releasing methane.
Archaebacteria contain two sub-groups:
(1) Methanogens and (2) Thermoacidophiles
Methanogens are strict anaerobes which produce methane from carbon dioxide or formic acid. That's why these are called methane producers or methanogens. These are found in marshy areas and among the flora of cattle rumen. Rumen is the first stomach of cattles where cellulose present in their food material is fermented by micro-organisms.
Methanogens are used in biogas fermenters for the production of methanes. Some archaebacteria are halophiles or salt loving and can live in extremely strong solutions. In strong light, they develop a purple coloured pigmented membrane which can utilise light energy to run their metabolic machinery by producing ATP.