Methanogens are archaea that produce methane as a metabolic byproduct in anoxic conditions. They're common in wetlands, where they're responsible for marsh gas, and in the guts of animals such as ruminants and humans, where they're responsible for the methane content of flatulence. In marine sediments biomethanation's generally confined to where sulfates are depleted, below the top layers. Others are extremophiles, found in environments such as hot springs and submarine hydrothermal vents as wel… (More on Methanogen)