Thermophilic and hyperthermophilic eubacteria and archaea not only survive hot temperatures but need them for their growth and reproduction. The optimum growth temperature of thermophiles is above 40°C and that of hyperthermophiles is between 80 °C and 100 °C. Hyperthermophiles generally do not grow below 60 °C. They are thus using capacity adaptation and their biological systems are adapted to operate at these high temperatures. How can these organisms live and survive at temperatures which are fatal to most others on Earth?
Eubacteria and archaea are small and lack insulation. The inside of their cells are thus at the same temperature as that of their surroundings. Since many proteins denature and cease to function at temperatures above about 50 °C, the proteins involved in the structure and metabolism of thermophiles and hyperthermophiles must either be exceptionally stable at high temperatures or are resynthesised at the same rate as they are degraded. To resynthesise their components continually would be very expensive for a thermophile, in terms of using their resources, and so producing thermostable enzymes, and other proteins, is likely to be the favoured strategy. The other types of molecules, and structures such as membranes, that make up the cells of thermophiles also need to be thermostable.
The composition of the membranes of archaeal cells is fundamen-
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