Super- heat -loving (“hyperthermophilic”) bacteria and archaea are thriving within high temperature environments, representing the upper temperature border of life. They grow optimally (=fastest) between 80°C and 106 °C, depending on the strains. Members of the genera Pyrodictium and Pyrolobus even survive for hours autoclaving at 121 °C. In their basically anaerobic environments, hyperthermophiles gain energy by inorganic redox reactions employing compounds like molecular hydrogen, carbon dioxide, sulphur and ferric and ferrous iron. Based on their growth requirements, hyperthermophiles could have existed already at the Early Earth at the end of the great meteorite bombardement (about 3.8 Gyr ago). In impact ejecta, they could have been successfully transferred in between the planets and moons of our early solar system. Within the ss rRNA-based phylogenetic tree of life hyperthermophiles occupy all the short deep branches closest to the root. The deepest archaeal phylogenetic lineages are represented by the Nanoarchaeota and the Korarchaeota. Cells of the Nanoarchaeota consist of minicocci, only 0.4 µm in diameter. Cultivation of Nanoarchaeum equitans requires cell-cell-contact with Ignicoccus hospitalis, possibly a very early type of symbiosis. The second lineage is represented by Korarchaeum cryptofilum which consists of ultrathin needle- shaped cells. Its genome revealed an unprecedented heterogeneous gene complement suggesting that the Korarchaeota had diverged very early in the archaeal lineage.
