bacteria, they survive in environments similar to those found on the young earth: hot springs, sea vents releasing sulfide-rich gases, boiling muds around volcanos, that sort of place. They differ from other bacteria (called Eubacteria) because:
Methane FormingThe methanogenic bacteria create ATP by reducing carbon dioxide from the atmosphere using hydrogen, formate, methanol or other reduced molecules. As a result methane is liberated. This can only be done in the absence of free oxygen. So they are poisoned by normal air.
These bacteria live in marshes, when we call the methane they produce 'marsh gas'. They also thrive in sewage treatment plants and the intestines of animals.
Salt LovingBefore the invention of refrigeration and freezing, people used to preserve their food by salting it. This killed the bacteria. However there are some bacteria, the Halophilic bacteria, which can only live in water with a high salt concentration! They produce a pink scum on salt flats. They are aerobic.
Heat LovingAnother way to kill normal bacteria is to cook food. But heat does not kill some archaebacteria. Thermoacidophilic bacteria can only live in water which is very hot (70 - 75° C) and as acidic as concentrated sulphuric acid. They are found in hot springs.