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For a while now, I've been wanting to design a world where the average temperature is very high (compared to Earth, but nowhere near as high as Venus) but the water does not boil because the atmospheric pressure is several times as high as it is on Earth.

In this scenario, we would have oceans of a solvent which we know life can work in, but the temperatures may well exceed the point at which proteins denature.

So, what biochemistry would work here?

Ideally, the biochemistry on such a planet should be primarily of carbon, hydrogen, and oxygen, similarly to Earth biochemistry, but I do not care if the biochemistry on this alien planet could not incorporate nitrogen, phosphorus, etc.

For the purposes of this question, we will assume an average temperature of 70°C and an atmospheric pressure of 3.5 atmospheres.

I will only accept answers saying that Earth biochemistry would work on such a planet after all if at least one organism has been found not only to survive but to reproduce with its whole body under such conditions.

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  • $\begingroup$ At 70°C water would not boil even on Earth at sea level. $\endgroup$
    – L.Dutch ♦
    Aug 3, 2022 at 11:28
  • $\begingroup$ I'm aware of that, L.Dutch, but 70°C is only the average temperature; there will be areas on the planet well above boiling point at standard pressure, even if most of the planet is well below it. $\endgroup$
    – Choroflorocarbon
    Aug 3, 2022 at 11:32
  • $\begingroup$ This question gets much more interesting and speculative as you near the critical point of water (374C, 218 atm). $\endgroup$
    – Kevin Kostlan
    Aug 4, 2022 at 2:45

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Earthly extremophile bacteria, like Pyrolobus fumarii and Pyrococcus furiosus, have been found living between 110 °C and 121 °C, in Submarine hydrothermal vents and oceanic crust, where the pressure is way higher than 3.5 atmosphere (anywhere deeper than 35 meter underwater has pressure higher than that), therefore Earth biochemistry can already work in the conditions you want.

Pyrococcus furiosus is an extremophilic species of Archaea. It can be classified as a hyperthermophile because it thrives best under extremely high temperatures—higher than those preferred of a thermophile. It is notable for having an optimum growth temperature of 100 °C (a temperature that would destroy most living organisms), and for being one of the few organisms identified as possessing aldehyde ferredoxin oxidoreductase enzymes containing tungsten, an element rarely found in biological molecules.

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Protein thermostability can be increased by the formation of salt bridges within parts of the protein structure. Denaturing as a result of pressure would begin to occur at about 2000 atm. I don't see the conditions on your planet creating a need for some exotic biochemistry besides making salt bridges more common within protein structures.

Information from Life at High Temperatures and Pressure as a Limiting Factor for Life

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