Strain 121 (Geogemma barossii), a name not officially recognized by the International List of Prokaryotic Names, is a single-celled microbe in the domain Archaea. It was first found in a hydrothermal vent in the Northeast Pacific Ocean near the Endeavor segment of the Juan de Fuca Ridge. Strain 121 is a hyperthermophile, meaning it can reproduce at 121°C (250°F), which is why it was named. It becomes biostatic at 130°C (266°F), meaning it cannot reproduce at that temperature but remains alive up to 131°C (268°F). Strain 121 has a round shape and hair-like structures called flagella, and it is about 1 micrometer in diameter. It survives by breaking down iron oxide (Fe(III)), a substance found in sediment near hydrothermal vents.

The highest temperature at which Strain 121 can grow is 8°C higher than the previous record holder, Pyrolobus fumarii (maximum temperature = 113°C). When it was discovered, Strain 121 was the only known life form that could survive such extreme heat. However, in 2008, Methanopyrus kandleri was found to reproduce at temperatures up to 122°C. Autoclaves, tools used to sterilize equipment, operate at 121°C (250°F), making Strain 121 a significant discovery. Before 2003, scientists believed that a 15-minute exposure to autoclave temperatures would kill all living organisms. Strain 121, however, cannot reproduce below 85°C (185°F), so it cannot infect humans, whose average body temperature is about 37°C (99°F).

It is unlikely that Strain 121 represents the highest temperature at which life can survive. Scientists believe the true upper limit may be between 140°C and 150°C (284°F to 302°F), where molecules can no longer repair or rebuild themselves.