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This is a sequel to my previous question in this vein, in which I handwaved away some issues related to Bob overheating.

As DWKraus pointed out when answering said previous question, cell membranes start to dissolve around 45°C, providing an upper limit to the temperature an organism can withstand even if it has plenty of heat shock proteins and topoisomerase V.

However, it's important to note that cell membranes are made of lipid bilayers. The longer a lipid's tail is, the more heat-resistant a bilayer made of that lipid is. Provided that you avoid carbon-to-carbon double bonds - i.e. avoid turning the lipids in the cell membranes into unsaturated fats (see here for the difference; basically, unsaturated fats are when fats have carbon-to-carbon double bonds) - they won't kink up and start behaving like a liquid.

As this source points out in this table:

enter image description here

...there are longer lipids out there, ones that dissolve at higher temperatures than 45°C. And if the diarachidoyl phosphatidylcholine/DAPC in the table, with a dissolution temperature of 64.1°C, isn't enough, you could probably base a lipid bilayer off of some other kind of fatty acid, of which there are many.

Tetracontanoic acid, for instance has a long alkane tail indeed - 40 carbons, well more than the 16 and 18 of dipalmitoyl phosphatidylcholine/DPPC and distearoyl phosphatidylcholine/DSPC. This suggests that a lipid bilayer made of an ester of tetracontanoic acid (all lipids are esters of carboxylic acids and alcohols) would be incredibly heat-resistant, and, moreover, that even longer (and therefore more heat-resistant) lipids could potentially be formed, meaning that cell membranes dissolving would be a non-problem.

Now, the hypothetical organism that has all these adaptions:

  • has proteins that can't denature
  • is immune to sepsis
  • can't have its cell membranes dissolve; if they aren't durable enough to avoid dissolving, just add more alkanes on the end until they are

The question: what's the next problem? After septic shock, proteins denaturing, and cell membranes dissolving are overcome, what's the next overheating-related thing to cause physical harm to/kill this organism? Last I checked, it was up to 45°C - can it get higher?

I considered the ether-based cell membranes of archaea for this, rather than the extra-long-lipided ester-based cell membranes I actually chose, but they have rigidity-related, permeability-related, and transmembrane protein-related issues.

Just for context: this is an eukaryotic, multicellular organism, not an archaea. I would also point out that it runs on Earth biology, and that 100°C is likely an upper limit here, considering water boils at that temperature under standard temperature and pressure.

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    $\begingroup$ Considering your first question was, "what's the next over-heating event" and this question is "what's the next over-heating event" I'm going to assume that you'll be content after this question. A third one would make me wonder why it's not a duplicate of the previous two (by definition) and further wonder why, having been shown twice how to discern an answer, you can't thereafter do it yourself. $\endgroup$
    – JBH
    Commented Jul 31, 2022 at 2:46
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    $\begingroup$ @AngryMuppet Atmospheric pressure, temperature, and composition is Earth standard at sea level, at least as far as "standard" can get you. Editing to clarify. $\endgroup$
    – KEY_ABRADE
    Commented Jul 31, 2022 at 4:57
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    $\begingroup$ Sepsis is not good but it is not related to heat stress. The other 2 on the list are fine. $\endgroup$
    – Willk
    Commented Jul 31, 2022 at 17:03
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    $\begingroup$ I suppose if you insist on going around with a giant tank of bacteria inside you when your gut proteins denature they will escape and cause sepsis. $\endgroup$
    – Willk
    Commented Jul 31, 2022 at 17:12
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    $\begingroup$ @Willk Unfortunately, I do need all those gut bacteria in order to digest things, at least, as far as I know. $\endgroup$
    – KEY_ABRADE
    Commented Jul 31, 2022 at 17:13

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It seems that there are eukaryotes that can live above 45 C. There is a nice review article life in extreme environments that has charts out the upper temperature limits for each taxa, at least as known in 2001. This paper claims ~60 $^o$C and a different paper without a reference for it claimed the world record was 62$^o$C. So it seems that even with real organisms there may be a little more wiggle room for higher temperatures for your story purposes.

enter image description here

reducing chemolithoautotroph, capable of growing at the highest temperatures of up to 113 $^o$C (ref. 15). Hyperthermophile enzymes can have an even higher temperature optimum; for example, activity up to 142 $^o$C for amylopullulanase 16. There are thermophiles among the phototrophic bacteria (cyanobacteria, purple and green bacteria), eubacteria (Bacillus, Clostridium, Thiobacillus, Desulfotomaculum, Thermus, lactic acid bacteria, actinomycetes, spirochetes and numerous other genera) and the archaea (Pyrococcus, Thermococcus, Thermoplasma, Sulfolobus and the methanogens). In contrast, the upper limit for eukaryotes is ~60 $^o$C, a temperature suitable for some protozoa, algae and fungi. The maximum temperature for mosses is lower by another 10 $^o$C, for vascular plants it is about 48 $^o$C, and for fish it is 40 $^o$C, possibly owing to the low solubility of oxygen at high temperatures (Fig. 3).

I think the bottom up approach to adaptations, is interesting. But it seems like there are adaptations before a sharp temperature cut off. For example, DNA denatures in the lab ~ 95 $^o$C when doing PCR. I would have assumed that would be a hard cutoff, but apparently DNA in a hyperthemophile grows optimally at 100$^o$C. So the conditions inside the cell are more important than the stability of the molecule in isolation.

It seems that after cell membranes that the flexibility and stability of proteins is probably really important. I think the protein structures would need to be stiffer and less floppy at higher temperatures.

If I read you original goal correctly, the goal was to have a large animal and you wanted to avoid overheating in the center of the animal. Making the cells survivable to temperature I think is one thing, but another think to think about is how oxygen gets transported to the cell. I think with increased temperature that oxygen binding to hemoglobin decreases. Also in general the percentage disolved gasses in a fluid decreases with increased temperature.

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