Traditionally, Archaea were seen as the third domain of life, besides bacteria and eukaryotes. Recently, new genetic result suggest that Eukarya may be a branch within the Archaea (Hug et al 2016, Castelle & Banfield 2016). They are single-celled organisms without nucleus or organelles, and have a similar size and shape as bacteria, but differ from them biochemically. Their membrane is made of a unique type of lipids and most archaea have a cell wall. Many Archaea are extremophiles, thriving at high temperature and pressure, but also cold, sub-zero environments.