Methanogens are prokaryotic microorganisms that produce methane as an
end-product of a complex biochemical pathway. They are strictly
anaerobic archaea and occupy a wide variety of anoxic environments.
Methanogens also thrive in the cytoplasm of anaerobic unicellular
eukaryotes and in the gastrointestinal tracts of animals and humans. The
symbiotic methanogens in the gastrointestinal tracts of ruminants and
other "methanogenic" mammals contribute significantly to the global
methane budget. This monograph deals with methanogenic endosymbionts of
anaerobic protists, in particular ciliates and termite flagellates, and
with methanogens in the gastrointestinal tracts of vertebrates and
arthropods. Further reviews discuss the genomic consequences of living
together in symbiotic associations, the role of methanogens in
syntrophic degradation, and the function and evolution of
hydrogenosomes, hydrogen-producing organelles of certain anaerobic
protists.