What is the main source of energy produced by microbes?

Methanogens produce methane (biogas) while degrading plant waste and biogas produced by microbes is used as a source of energy in rural areas.These bacteria are commonly found in the anaerobic sludge during sewage treatment. These bacteria are also present in the rumen (a part of stomach) of cattle. A lot of cellulosic material present in the food of cattle is also present in the rumen. In rumen, these bacteria help in the breakdown of cellulose and play an important role in the nutrition of cattle.