The Upper Temperature Limit for Life Based on Hyperthermophile Culture Experiments and Field Observations

Substantial evidence suggests that microorganisms inhabit the subseafloor in association with hydrothermal systems, yet little is known about their ecology and physiology. They may also be useful for modeling temperature profiles, fluid flow rates and chemical fluxes within the seafloor. One key physical parameter necessary to model these is the ultimate upper temperature limit of life. Hyperthermophiles are microorganisms that grow optimally above 80°C and have the highest growth temperatures of any known life. Thirty-three genera of hyperthermophiles, primarily belonging to the Archaea, have been characterized and their maximum growth temperatures in culture are 108-113°C. Hyperthermophiles can enhance their tolerance of denaturing superoptimal temperatures for a few hours by producing enzymes, organic solutes and a biofilm that protect and repair vulnerable cellular components or degrade those damaged beyond repair (i.e., a heat-shock response). Furthermore, in situ pressure will raise the maximum growth and metabolism temperatures of some hyperthermophiles by 2-6°C and increase the optimum temperature growth rate of others. Laboratory studies of natural assemblages of microorganisms from hydrothermal environments and field observations suggest that life can exist at temperatures up to 120°C, though some circumstantial evidence suggests that life may exist above this temperature. Extracellular lytic enzymes produced by hyperthermophiles are active outside the cell at temperatures up to 140°C, which extends the biogenic impact temperature of life beyond the temperature range of the organisms. Zones of oceanic crust with hydrothermal fluid circulation under 120°C are extensive suggesting that organisms may populate vast regions of the crust and have a far-reaching effect on the chemistry and physics of these environments.