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It is said ,

Evolution prefers stability to an organism for survival

But it chose protium over deuterium , which would have been great choice for stability of enzymes and nucleic acid in thermoacidophilic bacteria. While everyone would argue that the main reason would be ¹H abundance or metabolism rate or flexibility. But come on! A life produces enzymes as per its need(so nothing to do with ¹H abundance) . Also deuterium doesnt itself alter too much metabolism or flexibility.

Rather it would have only benefited.

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    $\begingroup$ Evolution does not prefer the best choice - it goes with whatever is good enough. Evidently the meagre increase in deuterium bond strength was not a sufficient benefit over the difficulty of performing isotopic enrichment. Alternatively, even if there were some net advantage (highly doubtful), it may just have been inaccessible, hindered by a fitness barrier. $\endgroup$
    – Nicolau Saker Neto
    Commented Sep 5 at 5:32
  • $\begingroup$ @NicolauSakerNeto that's where my question titles what is the preference for hydrogen over deuterium for thermoacidophiles. What mechanism would have impacted? $\endgroup$
    – Unknown
    Commented Sep 5 at 5:34
  • $\begingroup$ It’s important to consider that in extreme environments, even minor improvements in molecular stability could be highly advantageous.Though be true incorporating deuterium might be metabolically costly, but would remove chances of denaturation especially during spore formation say for example Thermoplasma acidophilum. @NicolauSakerNeto $\endgroup$
    – Unknown
    Commented Sep 5 at 5:42
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    $\begingroup$ There are much simpler methods to achieve a similar enough outcome. An organism's evolutionary history can converge on proteins which have a higher average number of regular hydrogen bonds, instead of improving each one by moving to deuterium. Another clear route is to increase the amount of disulfide bonds relative to non-thermophiles, which has actually been experimentally verified. Evolutionary barriers to these outcomes are certainly far smaller. Again, this turned out to be enough, and that's all it takes. $\endgroup$
    – Nicolau Saker Neto
    Commented Sep 5 at 6:06
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    $\begingroup$ I think one can assume that the first organisms would have selected building materials based on their abundance and ease of incorporation. It does seem worth asking however whether some advantage from biotic isotopic enrichment was not later exploited by some organisms, particularly under extreme conditions where the advantages might be emphasized. $\endgroup$
    – Buck Thorn ♦
    Commented Sep 6 at 7:41

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Those microorganisms did not choose $\ce{^1H}$ (protium) to $\ce{^2H}$/$\ce{D}$ (deuterium). They treat them the same according to abundance, with just slight shift due slight shifts in the kinetic and thermodynamic constants.

The complex system that would be needed to isolate $\ce{^2H}$ would not be evolutionary justified, as using $\ce{^2H}$ would cause also many disadvantages.

Aside of that, the cells would need to speed up their water metabolism by about 10000 times, plus with the need for very intense and energy demanding $\ce{^2H}$ separation.

Such cells would be literally dying due thirst and hunger in the middle of water full of nutrients.

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Echoing @Poutnik's answer, life did not choose deuterium over protium (or the reverse either).

Perhaps you could infer from this fact that the evolutionary "cost" of developing enzymatic systems to selectively concentrate and isolate deuterium for incorporation into enzymes were too high.

I can't point to data specifically for thermoacidophiles, but there is a deep literature on microbial fractionation of hydrogen isotopes. They mostly prefer protium over deuterium. I suspect this is because most chemotrophs and chemoautotrophs use hydrogen containing substrates like $\ce{H2}$, $\ce{H2O}$, $\ce{CH4}$, and $\ce{CH3COOH}$ to not only obtain material for protein synthesis, but also to obtain energy. Breaking deuterium-carbon or deuterium-oxygen bonds is slightly harder than breaking the corresponding protium bonds, and therefore most microbes don't find the potential investment worth it.

For further reading:

  • The USGS has very good intro pages to isotopic ratio terminology and quantification.
  • John Hayes' 2001 review article remains a great place to learn the basics of isotopic fractionation in biochemical pathways.
  • This 2009 PNAS article from Alex Session's lab is a great single paper that examines a variety of different modes of metabolism and the D/H fractionation between water in the growth medium and biosynthesized microbial lipids exhibited for each. Chemoautotrophs most strongly prefer H over D. There are some heterotrophic types of metabolism that enrich D (relative to growth water) in lipids, but they are also consuming hydrogen atoms from their substrate.

In any case, the strongest differentiation between isotopes is about -400 ‰ in the negative direction or + 200 ‰ in the positive direction. That's the same as a -40% relative decrease or a 20% relative increase. So if you start with deuterium at an abundance of 150 ppm, you'll have deuterium at 90 ppm or 180 ppm in the bio-modified materials. Still fantastically dilute.

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There was no choice; the preponderance of H meant it had to be used. The differences between H and D are small and there seems to be no real advantage to choosing D over H. The small increases in bond strengths do not overcome the deficiencies in rates. It would be interesting were H and D in different proportions, say 25-75. Would life have used each to its advantages or would two different strains have evolved?

This has not happened in the selection of elements. C definitely won out over Si, but N and P, S and O, Cl- and HCO3- have all reached detente and found their functions. The alkalines do replace each other, Sr will replace Ca but Mg found a niche. The alkalis do a similar dance let your Na+/K+ get out of balance and see what happens. The transition elements seem undecided, were the amounts different, different enzymes might have evolved, Fe, Cu, Mn, Co, and possibly others seem to have found their niche. Al seems to have no function probably because of nonavailability as an ion or possibly it is deadly. F- is again not very available, possibly a good thing, but I found a use in the thyroid.

This study partially deuterated some bacteria possibly suggesting research into completely deuterated species: https://www.pnas.org/doi/10.1073/pnas.1420406112

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  • $\begingroup$ People (mainly protein NMR people) routinely grow bacteria in 100% deuterated media, to express proteins for esoteric (to me) NMR characterization. $\endgroup$
    – Curt F.
    Commented Sep 6 at 3:29
  • $\begingroup$ I was thinking [or not..] more 100% D including lipids, cell walls etc. were H replaced by D in atomic stability and amounts. Would we have evolved if so perhaps have a need for higher T and global warming, and operate at a higher body T. Of course with a D2O standard 100 Celsius would be higher etc and nothing would change. Maybe we are lucky that protium prevailed. $\endgroup$
    – jimchmst
    Commented Sep 6 at 21:53

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