Archaea lack a cell nucleus and other organelles with membranes, so they are prokaryotic. They were originally believed to be a type of bacteria, the other major group of prokaryotes.
Archaea are the oldest surviving organisms on Earth. They were first identified in extreme environments such as hot springs, but they have since been found everywhere. There are three major groups of archaea. The methanogens produce energy by converting hydrogen gas and carbon dioxide into methane. The halophiles live in environments with extremely high salt content that would kill other organisms. The thermoacidophiles live in areas with very high temperatures or high acidity.