Would A Biological "Cold Haber" Process Deplete an H2/N2 World of its H2 or N2?

Question

Based on my reading on some science papers, large, wet terrestrial planets likely exist that are swathed in a thin atmosphere of nitrogen/hydrogen ($H_2$ & $N_2$). If life evolved on such worlds, they may very well develop a biological version of the "Haber Process" that is used to industrially manufacture ammonia ($NH_3$). Since the biological version of the process would need to run at much cooler temperatures than the kiln-hot industrial Haber Process, the biological version can be called a "Cold Haber" process.

It would look like this:

$${3H_2 + N_2 \rightarrow 2NH_3}$$

At least that's the contention of some papers I've read (see references below).

Question in Full:

On an ${N_2}$/${H_2}$ atmosphere world would a Cold Haber process utilized by organisms run until it completely sequestered the non-dominant major gas (either ${N_2}$ or ${H_2}$) as ammonia? Or would something intervene to set some type of equilibrium long before the atmosphere was appreciably depleted of either ${N_2}$ or ${H_2}$?

Basically...what might be the equilibrium atmosphere and why?

You may postulate the evolution of a biological process to utilize the ammonia, but need not. If you think the evolution of organisms using a complementary process is likely or feasible and want to include that by all means include it. That certainly would effect the answer to the question!

The answer is the difference between an atmosphere of ≈99% Hydrogen/Nitrogen with traces of ammonia in both air and water, and oceans saturated with ammonia with an atmosphere chock full of it.

If needed here are parameters to run with:

PRE-COLD-HABER ATMOSPHERE

• ${H_2}$ & ${N_2}$ (90%+ of atmosphere in any ratio of 10:1 hydrogen:nitrogen all the way to 4:1 nitrogen to hydrogen)
• ${H_2O}$ Vapor (≈1%)
• ${CH_4}$ (0.01 - 5%)
• Other trace to minimally-present compounds may include ${CO_2}$, ${Ar}$, etc.

PLANET

• Oceans, continents, and some volcanic activity, much like earth
• Less UV received than Earth (1/3 at most, probably much less)
• Significant magnetic field
• Temperature: (I'd like the answer to account for more than a single planet's likely temperature range, but if needed let's go with a mean temperature between ${-40°C}$ and ${20°C}$ (your choice).
• Atmosphere between ${1bar}$ and ${20bar}$ (your choice)

OTHER BIOLOGICAL PROCESSES

• Such a world may evolve methanogenesis, converting plentiful hydrogen and outgassed $CO_2$ to Methane and Water (${4H_2 + CO_2 \rightarrow CH_4 + 2H_2O + 193 kJ}$ per mol at ${25°C}$), deriving easy energy. This would likely mean the atmosphere's supply of carbon dioxide would almost entirely convert to methane.
• Such a world may evolve photosynthesis utilizing the following chemical reaction: ${CH_4 + H_2O + y \rightarrow CH_2O + 2H_2}$, converting methane and hydrogen in the atmosphere to biomass and water.

In your answer please explain in detail your thought process. It should contain a discussion of the relevant chemical processes as well as what you suspect the new equilibrium atmosphere may be. If you can supply equations or calculations to back your answer, all the better!

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References:

Photosynthesis in Hydrogen-Dominated Atmospheres – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4284464/

BIOSIGNATURE GASES IN H2-DOMINATED ATMOSPHERES ON ROCKY EXOPLANETS – https://iopscience.iop.org/article/10.1088/0004-637X/777/2/95/meta

A BIOMASS-BASED MODEL TO ESTIMATE THE PLAUSIBILITY OF EXOPLANET BIOSIGNATURE GASES – https://iopscience.iop.org/article/10.1088/0004-637X/775/2/104#apj480437s4

Answer 1

Your "cold Haber process" already exists--it's what nitrogen-fixing bacteria do on Earth! The ammonia generated by that process is then further transformed into nitrites and nitrates, with all three forms of bound nitrogen being used in various ways in terrestrial biology to build more complex molecules. This is an energy-intensive process for Earthlings, because we have to split water to fix nitrogen (just like we have to split water to perform photosynthesis), but that bottleneck doesn't exist on your reducing-atmosphere world.

On Earth, we avoid depleting our atmosphere of nitrogen because denitrifying bacteria eventually release it again, by using nitrates and/or nitrates (rather than straight oxygen) as electron receptors for respiration, producing water and nitrogen gas as byproducts. On a chemically-reducing world, however, per your reference on hydrogenic photosynthesis, we should expect the average biomolecule to be less oxidized than the average terrestrial biomolecule. Thus, we should expect to see far fewer nitrite and nitrate groups in reducing biology, and a lot more amides and amines.

The reducing-world equivalent of denitrifying bacteria, then, would be organisms that use ammonia, rather than free hydrogen, as an electron donor to reduce biomass and generate energy--exactly the opposite of Earthling heterotrophs which oxidize biomass to generate energy, which in both cases undoes the work done by photosynthesis in each environment to bind that energy.

So, the question boils down to this: do such denitrifying organisms actually make sense? Where would they be needed?

Denitrifying organisms make sense on Earth because oxygen doesn't get everywhere. Denitrifying bacteria can engage in high-energy oxidizing respiration in anoxic environments by simply decomposing mixed biomass on its own. Is that true of hydrogen on a reducing world?

Surprisingly, the answer may be "yes". In one sense, hydrogen should be more readily available on a reducing world than oxygen is on an oxidizing world, because free hydrogen will be primordial, seeping out of crustal rocks, and also because it can diffuse more easily through smaller spaces and more quickly into areas that would otherwise be depleted by rapid "respiration". However, hydrogen has a much lower solubility in water than oxygen does--while ammonia is highly soluble.

Thus, once biological nitrogen fixation starts up (which it ought to do rather quickly), sea life on this world might be expected to fairly quickly learn to breathe ammonia, instead of or in addition to hydrogen, thus releasing nitrogen gas back into the environment.

So, you will have the following cycles:

CH4 + H2O -> CH2O + 2H2 via photosynthesis, restoring hydrogen to the atmosphere.

2N2 + 3H2 -> 2NH3 via exothermic nitrogen fixation, removing both nitrogen and hydrogen from the atmosphere but introducing ammonia to the atmosphere and ocean (and lakes and rivers and so on). Because this is an exothermic process, unlike Earthling nitrogen fixation, you can expect microorganisms to do it continuously, releasing ammonia as a byproduct, rather than having the rate limited to what is needed for the construction of biomolecules. Incidentally, ammonia will also spontaneously react with carbon dioxide, so, although that paper says CO2 ratios are fairly arbitrary and dependent on geological production, in fact you should expect the overproduction of ammonia to result in nearly all of whatever CO2 is available being sequestered in the oceans as ammonium carbamate. After the CO2 is gone, then ammonia will start to build up.

CH2O + 2H2 -> CH4 + H2O

This is the basic form of reducing respiration, consuming hydrogen and releasing methane back into the atmosphere, as an analogue to CO2 in our atmosphere.

CH2O + 2NH3 -> CH4 + H2O + H2 + N2

3CH2O + 4NH3 -> 3CH4 + 3H2O + 2N2

These are ammonia-consuming reducing respiration reactions, which replenish nitrogen in the atmosphere and may or may not release some excess hydrogen.

So, you have one process that removes both nitrogen and hydrogen from the atmosphere; one process that replenishes atmospheric hydrogen (photosynthesis) and one process that replenishes atmospheric nitrogen (ammonia-based respiration).

I have no clue how to determine what the final equilibrium concentrations would be, but it would appear that it is perfectly plausible for both H2 and N2 to remain as major components of the atmosphere indefinitely. Meanwhile, you will have sea creatures that can respire using free hydrogen or ammonia, expecting that their individual consumption of ammonia will not significantly impact the pH balance of the ocean and will be balanced by the activity of nitrogen-fixing microbes, and land creatures which would avoid ammonia-based respiration and instead exploit the more freely-available atmospheric hydrogen, both for better energetics and because they can't afford to screw with the pH of their isolated body fluids.