Strain 121 (Geogemma barossii (name not recognized by the International List of Prokaryotic Names by Standing in Nomenclature (LPSN)) is a single-celled microbe of the domain Archaea. First discovered within a hydrothermal vent in the Northeast Pacific Ocean near the Endeavor segment of the Juan de Fuca Ridge, Strain 121 is a hyperthermophile capable of reproducing at , hence its name. Strain 121 is biostatic at , so while the archaeon is unable to reproduce until it has been transferred to a cooler medium, it remains viable at temperatures up to . At the time of its discovery, Strain 121 was the only known form of life that could tolerate such high temperatures, but in 2008 Methanopyrus kandleri was discovered to be able to reproduce at temperatures as high as 122 °C. Autoclaves, which are an important tool in sterilization, operate at temperatures of , marking Strain 121 a particularly notable discovery.
It appears highly improbable that Strain 121 marks the upper limit of viable growth temperature.