methanogen

noun

1. any of a diverse group of widely distributed archaebacteria that occur in anaerobic environments, as the intestinal tracts of animals, freshwater and marine sediments, and sewage, and are capable of producing methane from a limited number of substrates, including carbon dioxide and hydrogen, acetate, and methylamines: an important source of natural gas.

n.

1. Any of various anaerobic methane-producing bacteria belonging to the family Methanobacteriaceae.