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Chloros is a world with a huge amount of sulfur. Because sulfates displace halogens in alkali metal salts, fluorine and chlorine in Chloros aren't all locked up in rocks; instead, the oceans are a dilute mixture of hydrochloric and hydrofluoric acid.

The native organisms still rely mostly on water (it's dilute acid, after all), so water is available as a hydrogen source for photosynthesis. But with plenty of HCl around, would they actually use water for photosynthesis?

Early photosynthesizers on Earth used hydrogen sulfide, because it's less tightly bound than water, and only evolved oxygen, water-consuming photosynthesis to avoid the bottleneck of low hydrogen sulfide supply. Would Chlorosian organisms be likely to switch to HCl instead, producing chlorine as a byproduct? Or is that sufficiently difficult that they would just consume water to produce oxygen anyway, just like Earthlings?

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  • $\begingroup$ @John, no, it really isn't; which reaction is more energetically favorable is an objective fact. All else being equal, if stealing hydrogens from water is cheaper than stealing them from HCl, then organisms that release oxygen will outcompete organisms that release chlorine--and vice-versa. $\endgroup$
    – Logan R. Kearsley
    May 18, 2022 at 21:31
  • $\begingroup$ the reason oxygen is used is because it is far Far more common even on your planet, Chlorine is already abundant in plant chloroplasts and they don't use it. you drastically overestimate the importance of efficiency in photosynthesis, plant photosynthesis is rather poor compared other forms, it won because it was the from the lucked out to occur in a a eukaryote first. $\endgroup$
    – John
    May 18, 2022 at 21:37
  • $\begingroup$ @John The whole point of the question is that abundance isn't an issue--there is more than enough HCl to go around, if it makes energetic sense. Chloride ions are abundant in plants (and in us), but not in association with hydrogen. They aren't acidic, and just oxidizing chloride is pointless--the goal is to extract hydrogen. And all the other forms of photosynthesis on Earth use relatively rare substrates--plant photosynthesis won out because it can happen anywhere. If H2S were in unlimited supply, it would've won. $\endgroup$
    – Logan R. Kearsley
    May 18, 2022 at 21:42
  • $\begingroup$ No the point is to extract hydrogen ions, which are bound to oxygen, even in HCl, HCl acid is chlorine ions and hydronium ions. the hydrogen is bound to oxygen. Cl is just an ion in both cases. $\endgroup$
    – John
    May 18, 2022 at 21:46
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    $\begingroup$ photosynthesis does not pull one hydrogen ion off water it pulls all the hydrogen off of it, so you get oxygen from water or hydrochloric acid, chlorine can also act as an electron acceptor just like oxygen at the end this makes the pathway more complex but that is not really a hinderance, different forms of photosynthesis are kluge enough for it not to matter. $\endgroup$
    – John
    May 18, 2022 at 22:12

2 Answers 2

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Ideally, I would want to look at aqueous thermochemistry data for this, but all I can find references for is gas phase thermochemistry. However, if we run with that, we've got the following energy barriers to work with:

  • HCl -92.31kJ/mol
  • H2O -285.82kJ/mol
  • HClO -74.48kJ/mol

I have included HClO because gaseous chlorine spontaneously reacts with water to form HCl and HClO.

(Just for reference, the heat of formation of HF is -272.55kJ/mol--getting two hydrogens out of that is almost twice as expensive as getting it out of water, so that ain't happening! Nobody's gonna be breathing fluorine here.)

So, how much energy does it take to get 2 hydrogens from various reactions? As a baseline, splitting water and releasing oxygen requires 285.82kJ/mol, plus metabolic inefficiency:

H2O + 285.82kJ/mol -> $\frac{1}{2}$O2 + H2

What about exchanging a chlorine for a hydrogen, converting water and HCl into hypochlorous acid and hydrogen?

H2O + HCl + 303.65kJ/mol -> HClO + H2

Looks like that won't happen, because it costs more! And directly splitting HCl?

2HCl + 184.62kJ/mol -> CL2 + H2

That turns out to look considerably cheaper than splitting water! So yeah, Chlorosians should produce chlorine gas! Except... chlorine reacts with water. Based on the data I have, the reaction

H2O + Cl2 -> HClO + HCl

should consume 119.04kJ/mol, which explains why producing hypochlorous acid directly looks disfavorable here. But in reality, that reaction actually does occur spontaneously, so I have to assume that there are solvation effects or something that make this analysis incorrect.

So, it kinda looks like Chlorosians would really produce hypochlorous acid rather than chlorine gas or oxygen. But... exposure to sunlight causes hypochlorous acid to dissociate into HCl and free oxygen. So, they end up getting to an oxygen atmosphere in the end after all!

EDIT: According to Chloride Oxidation as an Alternative to the Oxygen Evolution Reaction on HxWO3 Photoelectrodes producing chlorite from aqueous Cl- and H+ ions requires a slightly higher electrode potential than electrolysing water--so it seems that Chlorosian plants would be better of just using water as their hydrogen source, and Chlorosian animals will therefore definitely end up breathing oxygen.

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It's practicality vs fun

The thermodynamic comparison you are looking for is here:

6CO2 + 6H2O -> C6H12O6 + 6O2 Vs 6CO2 + 12HCl -> C6H12O6 + 6Cl2

Note that while this is not the actual reaction in either system, it can be used due to Hess's Law (or whatever the free energy equivalent is called).

From here, you get that there is a difference of 12* 95 - 6 * 237 = 1140 - 1422 = 282 kJ/mol. The other species cancel out.

That's a trifling difference for a reaction involving 12 or 18 reactant species.

Photosynthesis, unlike every other biosynthetic reaction, is costless in energy, so given that you have a) more water, and b) known photosystems for oxygen, water is the choice that maximises the free energy of the author.

The limits in this case are scarcity of feedstock, and preventing unwanted side reactions in the starting reagents or waste products.

But either is credible in a sci fi book and chlorine is arguably more 'fun'.

PS Forget hypochlorous acid, it spontaneously disproportionates at anything except infinitesimal concentrations. It's only really a transitional species.

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