Single-celled organisms which are bounded by ether-linked lipid membranes which contain isoprinoid side chains instead of fatty acids. The most famous of the Crenarchaeota are the hyperthermophiles, organisms which thrive at temperatures close to the boiling point of water. Members of the crenarchaeal order Sulfolobales also live in extremely acidic waters. However, further research has shown that these organisms are also common in moderate-temperature water and soil.