Thermococcus gammatolerans

Thermococcus gammatolerans is a gram-negative archaeon extremophile and the most radiation-resistant organism known to exist.

As reported in 2003 the type strain EJ3^T was taken from a submarine hydrothermal vent in the Guaymas Basin off the coast of Baja California at a depth of about 2,600 m by submersible Nautile during the 1991 Guaynaut cruise. Thermococcus gammatolerans thrives in temperatures between 55 and 95 °C with an optimum development around 88 °C. Its optimal growth pH is 6, favoring the presence of sulfur (S), which is reduced to hydrogen sulfide (H
₂S). It is the organism with the strongest known resistance to radiation, surviving a dose of 30,000 gray (Gy) of gamma rays.

Along with the genera Palaeococcus and Pyrococcus, Thermococcus belongs to the Thermococcaceae family, sole family of the Thermococci (called "Protoarchaea" by Cavalier-Smith), a class in the phylum Euryarchaeota of Archaea. Thermococcus species live in extremely hot environments such as hydrothermal vents with an optimum growth temperature above 80 °C. Thermococcus and Pyrococcus (literally "ball of fire") are both chemoorganotrophic and obligately anaerobic. Thermococcus spp. prefer 70–95 °C, whereas Pyrococcus species prefer 70–100 °C.

The resistance to ionizing radiation of T. gammatolerans is enormous. While a dose of 5 Gy is sufficient to kill a human, and a dose of 60 Gy is able to kill all cells in a colony of E. coli, Thermococcus gammatolerans can withstand doses up to 30,000 Gy, and an instantaneous dose up to 5,000 Gy with no loss of viability.