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Nicar is a carbon world (formed from a protoplanetary disk with more carbon than oxygen, so water is geologically unstable and the chemical environment is strongly reducing) with ammonia oceans and lots of atmospheric methane.

If it were bigger, it would be a perfect world for hydrogen breathers with ammonia-solvent biochemistry. But... it's too small to retain hydrogen. Like Mars, it will lose hydrogen over time, reducing the size of its oceans and making it less habitable. Unlike Mars, however, it can never develop an oxidizing environment, but it can develop a layer of less-hydrogenated, low-weight hydrocarbons, like propane and butane, that float on ammonia and retard further evaporation.

But if life keeps doing the obvious thing, and tearing apart ammonia and methane for building blocks, releasing excess hydrogen into the atmosphere, eventually everything will dry up and the world will die, just like Mars. So, given access to liquid ammonia and propane/butane, what are reasonable reactions that life could use to construct energy-storage molecules (like sugars) and structural molecules (like lipids and polysaccharides) which will not result in releasing excess hydrogen?

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  • $\begingroup$ You know that sugars are called carbo_hydrates_ for a reason, right? Maybe you may get around with polyethylene backbone, decorated with various groups (instead of just hydrogen - like polystyrene). Such polymers are quite stable in aqueous environ, you'll need oceans made of oil/petroleum fractions, but then you run into the problem of not enough polar molecules to create a dipoles that facilitates "bio"reactions. Carbon, as a reducing element, is quite good, you need strong oxidants to make it part with what it's attached to (try chlorine instead of oxygen?) Maybe more UV can help too? $\endgroup$
    – Adrian Colomitchi
    Nov 27, 2021 at 2:47
  • $\begingroup$ @AdrianColomitchi Yes, I do know that. Ergo, ammonia-solvated life will not use actual sugars, but some sort of nitrogenous functional equivalent. Chlorine is a kinda crappy oxidizer, low in abundance, and there's no really strong reason for autotrophs to liberate it, so that seems like a bigger stretch to me than C/N/H molecules that can release energy through decomposition. $\endgroup$
    – Logan R. Kearsley
    Nov 27, 2021 at 2:58
  • $\begingroup$ About the abundance of chlorine - not that much of a stretch to handwave a higher concentration, but I agree the Cl chemistry is a bitch (a single valence doesn't make it as versatile as the oxygen). Howevs, in the absence of water (or with a low availability of it), it makes quite interesting reagents that are highly active in organic chemistry - Gringard reagents (around magnesium), aluminium and zinc chlorides (Lewis acids), copper I chloride shows some interesting organic reaction too... $\endgroup$
    – Adrian Colomitchi
    Nov 27, 2021 at 3:14
  • $\begingroup$ Ammonia is a good complexing agent for transitional metals and you may need them in larger quantities in you biochemistries. Lack of oxygen is such a pain to get around, maybe if you add the sulfur something interesting may start to happen (but most of the transitional metals form strong/insoluble sulfides - so... I don't know, doesn't look like a question that is easy to answer). $\endgroup$
    – Adrian Colomitchi
    Nov 27, 2021 at 3:19
  • $\begingroup$ @AdrianColomitchi I guess I'll have to read up on the chlorine chemistry, and incorporating more metal complexes and sulfur is in the plan, but I don't see how metal complexes would be relevant to this specific issue. Nitrogen molecules tend to be very high energy to start with (e.g., azide explosives); I just need to figure out which ones to use without destroying the oceans, and metals don't tend to bond with hydrogen a whole lot. $\endgroup$
    – Logan R. Kearsley
    Nov 27, 2021 at 17:18

2 Answers 2

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Organometallic hydride hydrogen carriers.

Just as we have organometallic carriers of oxygen in our own biological systems (hemoglobin with the iron containing heme ring), in a world where hydrogen is energy your creatures will have organometallic metal hydrides.

From The Power of Hydrides

hydrides

Image cropped by me to emphasize biological organometal hydrides. https://ars.els-cdn.com/content/image/1-s2.0-S2542435120300866-sc1_lrg.jpg

I like that Iron's awkward cousin Nickel gets invited to this party. I here assert that Nickel is the Ni in NiCar.

A molecule of hemoglobin can pick up and drop off an oxygen many thousands of time over its working life. So too the valuable hydrogen used to power your creatures - in their circumstance arguably more valuable than freely available oxygen is to us.

Metal hydrides mostly get press lately because of interest in hydrogen-based energy systems, fuel cells and the like. But metal hydrides could work in your creature too.

If you are more interested in long term storage than in short term fungible hydrogen then you could just use long alkanes. You can make them out of methane, you get 2 hydrogens bonded for every carbon, and it is easy to desaturate the chain, breaking off a hydrogen and leaving a carbon-carbon double bond. Not as exotic as sweet heme ring analogs but it would do the job with the materials you have it your world.

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  • $\begingroup$ Well dang, now I just need a reference for what color they are, 'cause you know people are going to ask what color the aliens' blood is.... .... $\endgroup$
    – Logan R. Kearsley
    Nov 29, 2021 at 3:32
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I don't know why it took me this long to realize the answer, but it turns out I need to frame challenge myself:

When hydrogen is lost to space, there will be nitrogen left behind in the atmosphere

As a result, it doesn't actually matter if the reaction cycle that photosynthesizers use to produce carbohydrate-analogs releases hydrogen--because any hydrogen produced low to the ground can be immediately consumed, by the same organism or by others, to produce more ammonia, which life is incentivized to do because that reaction actually releases energy! Not a ton of energy, but enough to be of interest to anaerobic microbes.

As a result, supposing we use a direct glucose analog replacing all the oxygens with NH groups, the net equilibrium equation will end up looking something like this:

8 C4H10 (butane) + 4 NH3 + 16 N2 <=> 6 C6H18N6

Or possibly this:

6 CH3NH2 (methylamine) + 2 N2 <=> C6H18N6 + 4 NH3

Or probably a mixture of those and a few other similar equations.

Anyway, the end result is that energy is not, ultimately, stored in the biosphere through dehydrogenation, as it would be on a regular hydrogen-breathing world; rather, energy is stored, and structure built, by incorporating nitrogen, and energy is released by freeing nitrogen--in the process, some hydrogens get shuffled around between hydrocarbons and ammonia (and hydrocarbon amines), but it's really the nitrogen which is driving the energy metabolism.

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