[Speculative Biology] Sugar/oxidizer reactions with components stable in (possibly aqueous) ammonia

Linguofreak

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In trying to do speculative biology for a sci-fi setting, I've run across the problem that nobody seems to have done a lot of previous well-thought-out speculation on alternatives to oxygen as a biological oxidizer (you'll occasionally see suggestions of fluorine or chlorine, but there seem to be a good number of problems with them, starting, particularly for fluorine, with availability). If you're trying to create a biology that can work in ammonia oceans, this is a problem, because ammonia reacts with oxygen to produce N2 and water, so before oxygen can start to collect in your atmosphere, your oceans are all water and your exotic alien planet looks a whole lot like Earth (it seems halfways possible that Earth's original oceans had a significant ammonia component until photosynthesis became a thing). So I've been looking for oxidizer/sugar reactions (with "sugar" here being any chemical on the reducing side of the equation that is plausible for use in a biological system) that would work in ammonia or ammonia/water solution without eventually eliminating all of the ammonia. I've tried some stab-in-the-dark research on Wikipedia and Google, but enough of my guesses turned out to be touchy explosives (because a lot of the things that don't react with ammonia contain nitrogen, and nitrogen compounds have a tendency to really want to become N2) that I didn't really want to keep guessing and racking up explosives in my search history. So, for anyone with better chemical knowledge than myself, I'm looking for:

1) Decent energy density.
2) At least one of the inputs (can be either the "sugar" or the oxidizer) should be a gas so it can accumulate in the atmosphere.
3) The other input should be a soluble solid or liquid, so it can be stored easily by organisms.
4) None of the inputs or outputs attack proteins, fats, other significant biological molecules (more than oxygen does, at least), or ammonia (at all).
5) None of the inputs or outputs explode.
 

Linguofreak

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I stumbled across the Alkali nitrates / nitrites, which seem to be good candidates for the oxidizer here. Wikipedia lists them as having moderate to good solubility in both water and ammonia, and at least for KNO3 and CaNO3, there are reactions that will produce them for which the other products are ammonia and water, which suggests that they wouldn't tend to attack ammonia or water (or else those reactions would produce something other than the nitrate and whichever liquid it broke down). The nitrates seem to be the stronger oxidizers, the nitrites seem to have better water solubility (unfortunately, Wikipedia doesn't have numbers for the ammonia solubility of any of them, just "moderately soluble", "soluble", or "very soluble").

So now there's just the question of finding a suitable fuel. Methane, maybe? It's non-polar, so it won't dissolve well in ammonia/water, but then, O2 is also non-polar, but enough manages to dissolve in water to support life.

I can see a few potential problems here: first of all, that alkaline metal (henceforth "x", since I haven't determined which alkaline metal to use) is likely to be trouble. If our fuel contains hydrogen, it seems likely that that will combine with the nitrogen and the oxygen in the xNO2/xNO3 to produce water and ammonia, leaving just the x, which will react with water to produce xOH + H2. The hydrogen is problematic because it's hard for any planet lighter than a gas giant to maintain, so a biosphere that produces a lot of hydrogen is likely not stable in the long term, and the xOH is problematic because yikes, that's going to make for a caustic environment inside your cells, and if your cells walls are lipids as on Earth, those will tend to react with the xOH and turn into soap. I suppose if we somehow introduce chlorine into the mix we can have the x go to xCl. So maybe our fuel is something like chloromethane? (but that is a bit too high-boiling to be an atmospheric gas if we take full advantage of the freezing point of the ammonia/water eutectic to allow a biosphere on a planet much colder than Earth.)

The second problem is that on Earth, photosynthesis achieves both energy collection and storage (by virtue of its products being exactly the inputs of respiration) and also carbon fixation (by virtue of turning atmospheric CO2 into sugar, which can then be further processed). But with our nitrate/nitrite metabolism, if CO2 is still an output of respiration, and our fuel is something like methane or chloromethane, then we're just shifting carbon around between two gasses, and not fixing it, so we need a separate carbon fixation process.
 
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