Discovered in 1970, Archaea might be the least well-known of the three domains of life (the others being Bacteria and Eukaryota), but it is a fascinating and diverse group of organisms and quite possibly the first on Earth. Like bacteria, archaea are unicellular, prokaryotic organisms, meaning that they lack nuclei and other membrane-bound organelles in their cells. However, archaea are more closely related to eukaryotes (including us!), than to bacteria. According to the Theory of Endosymbiosis, the line of descent from the Last Universal Common Ancestor first split into Bacteria and Archaea.
Archaea are particularly interesting to astronomers because this domain contains a large portion of extremophiles, organisms that can thrive in extreme conditions. Some of the first archaea were discovered in the hot springs of Yellowstone National Park. Others have been discovered within the digestive tracts of various organisms, in underground petroleum deposits, or in waters with extreme pH (in either direction). It is notable that archaea developed in the much harsher conditions of the early Earth, which lacked “features” such as the ozone layer to keep out ultraviolet radiation.
One of the most interesting and best-studied groups of archaea is the Halobacterium (a bit of a misnomer, as it is not a bacterium at all!), which thrives in extremely salty water. This property opens up the possibility that they might survive on Mars, where liquid salt reservoirs have been discovered just last year. Experiments have already demonstrated that two types of halobacterium (Halococcus dombrowskii and Halobacterium sp. NRC-1) can survive conditions similar to those on Mars, including an atmosphere made almost completely (98%) of carbon dioxide, and an average temperature of -60˚C. In addition, Halobacterium salinarum NRC-1 can survive environments of high radiation because it has an unusual adaptation: multiple copies of its genes, spread out across different chromosomes. This way, if radiation damages one gene copy (or even two), the cell survives off the remaining copy(s), while it repairs the damage. Thus, the Halobacterium are a wonderful model for the types of organisms that might be found on Mars if we continue to explore!