In taxonomy, the Thermococci are a class of microbes within the Euryarchaeota. They live in extremely hot environments, such as hydrothermal vents, and they have optimal growth temperatures above 80 °C. Thermococcus and Pyrococcus (literally "balls of fire") are both obligately anaerobic chemoorganotrophs. Thermococcus prefers 70–95 °C and Pyrococcus 70-100 °C. Palaeococcus helgesonii, recently discovered in the Tyrrhenian Sea, is an aerobic chemoheterotrophic that grows at temperatures of 45–85 °C with an optimal temperature of 80 °C. Thermococcus gammatolerans sp. nov. was recently discovered in the Guaymas Basin, and it grows at temperatures from 55–95 °C with an optimal temperature around 88 °C with an optimal pH of 6. It has pronounced radioresistance and can survive gamma radiation at 30 kGy. Thermococcus grows on organic substrates where there is a higher capacity of elemental sulfur. This archaeon mostly grows between temperatures 60–100 degrees Celsius. The average temperature where they thrive is around 85 degrees Celcius. The DNA structure has a cicular genome with around 2,353 coding sequence, and 2,306 are identified.