Final answer:
Methanogens are a type of archaea that thrive in anoxic environments and produce methane, belonging to the phylum Euryarchaeota, and are important for the carbon cycle on Earth.
Step-by-step explanation:
The type of archaean that lives in oxygen-free habitats like swamps and releases biogas is known as a methanogen. These organisms are part of the phylum Euryarchaeota, which also includes halobacteria that live in extremely saline environments.
Methanogens are unique because they reduce carbon dioxide with hydrogen to produce methane as a metabolic waste product. They thrive in a variety of extreme environments and are also found in more common settings, such as soil and the digestive tracts of animals, including humans. Due to their production of methane, they play a vital role in the carbon cycle and could potentially be analogs for life on other planets, like Mars.