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Strain 121
Scientific classification
Domain:	Archaea
Phylum:	Thermoproteota
Class:	Thermoprotei
Order:	Desulfurococcales
Family:	Pyrodictiaceae
Genus:	Geogemma
Species:	Geogemma barossii

Strain 121 (Geogemma barossii) is a single-celled microbe of the domain Archaea. First discovered 320 km (200 mi) off Puget Sound near a hydrothermal vent, it is a hyperthermophile, able to reproduce at 121 °C (250 °F), hence its name. It was (at the time of its discovery) the only known form of life that could tolerate such high temperatures. A temperature of 130 °C (266 °F) is biostatic for Strain 121, meaning that although growth is halted, the archaeon remains viable, and can resume reproducing once it has been transferred to a cooler medium. The ability to grow at 121 °C (250 °F) is significant because medical equipment is exposed to this temperature for sterilization in an autoclave. Prior to the 2003 discovery of Strain 121, a fifteen-minute exposure to autoclave temperatures was believed to kill all living organisms. However, Strain 121 is not infectious in humans, because it cannot grow at temperatures near 37 °C (99 °F). Strain 121 metabolizes by reducing iron oxide.

The maximum growth temperature of strain 121 is 8 °C higher than the previous record holder, Pyrolobus fumarii (T_max=113°C). However, it appears highly improbable that strain 121 marks the upper limit of viable growth temperature. It may very well be the case that the true upper limit lies somewhere in the vicinity of 140 to 150 °C (284 to 302 °F), the temperature range where molecular repair and resynthesis becomes unsustainable.

References

Kashefi, Kazem; Lovley, Derek R. (2003). "Extending the upper temperature limit for life". Science. 301 (5635): 934. doi:10.1126/science.1086823. PMID 12920290. S2CID 21189391.
Cowan, D. (2004). “The Upper Temperature for Life – Where Do We Draw the Line?” Trends in Microbiology (Regular Ed.), vol. 12, no. 2, Elsevier Ltd, pp. 58–60, https://doi.org/10.1016/j.tim.2003.12.002.