Question

Where would you find methanogens, which can function only in the absence of oxygen?

Answer 1

"They are common in wetlands, where they are responsible for marsh gas, and in the digestive tracts of animals such as ruminants and humans, where they are responsible for the methane content of belching in ruminants and flatulence in humans.[1] In marine sediments biomethanation is generally confined to where sulfates are depleted, below the top layers.[2] Moreover, the methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments.[3] Others are extremophiles, found in environments such as hot springs and submarine hydrothermal vents as well as in the "solid" rock of the Earth's crust, kilometers below the surface."

http://en.wikipedia.org/wiki/Methanogen