# A reciprocal metabolism is needed on a world overrun by crazy plants

In this paper by Bains et al, an alternative to oxygenic photosynthesis is discussed. Apparently, 4 times the amount of biomass can be produced using hydrogenic over oxygenic photosynthesis.

Oxygenic photosynthesis is described by the equation:

$$n\, CO_2 + n\,H_2 O \rightarrow (CH_2 O)\,n + n\, O_2$$ And the hydrogenic process: $$CH_4 + H_2 O \rightarrow CH_2 O + 2H_2$$ Both processes are endothermic, but the bottom one requires only a quarter the sunlight as the one on the top.

This process of interest because, since organisms that use it would have so much expendable energy, it would give rise to a crazy plant world.

As this answer points out, you need some kind of reciprocal metabolism so that the super-plants don't eventually run out of methane and water. In addition to bringing an equilibrium to the planet, I would like this to be a heterotrophic metabolism, so that they can be herbivores that move around and do interesting things.

The only "solution" to this problem that's occurred to me is a metabolism relying on the combustion of hydrogen. But this is stupid because 1) it doesn't address the problem of methane 2) substantial amounts of hydrogen and oxygen in the same atmosphere is a recipe for disaster and 3) even if the combustion process was somehow perfectly contained and no lifeforms ever wanted to develop controlled use of fire, we would have to add another process which renews the oxygen content of the atmosphere.

The answer that I linked does speculate about the possibility of getting enough oxygen from terrestrial carbonate, is this feasible?

If not, what are some ways this could work? I'm open to adding any sort of compound to the atmosphere or crust of the world.

• And $CH_2 O$ is formaldehyde, which is also Not Good for life. – RonJohn Oct 14 '18 at 18:45
• @RonJohn Not good for Earthlike life. Also I'm pretty sure that the hydrogenic process is scalable, so it could just use 6 times the amount of reagents per mole reaction if the formaldehyde was a problem. – Eben Kadile Oct 14 '18 at 18:51
• You might want to swap out science-based for hard-science if you want specific calculations. – John Locke Oct 20 '18 at 18:01
• You should also be aware that methane is a ferocious greenhouse gas, being about 20 times more effective than CO2 (but oxidized much more quickly in our ecosystem). Since it must be a dominant gas in the proposed atmosphere, rather than a trace, you can imagine the consequences. Venus, anyone? – WhatRoughBeast Oct 21 '18 at 2:09

Your reciprocal metabolism will regenerate methane and water.

If your photosynthesizes generate carbohydrate and hydrogen from methane, water and solar energy, your heterotrophs will take the carbohydrate and hydrogen, liberate the energy and regenerate the methane and water.

I was interested to find in my reading that known methane-producing metabolic pathways all start with short carbons or CO2 and usually have a CO2 end product in addition to methane.

But methane production from CO2 and H2 yielding CH4 and H2O also happens. I don't understand why organisms don't skip over the CO2 making piece and just reduce carbohydrate right down to CH4 and H2O.

Ultimately, though, reduction of CHO to CH4 and H2O must happen all the time . Methane production starting with cellulose in biodigesters routinely make methane out of agricultural waste. Although I could not find a metabolic pathway described for a single organisms that did that, it is not hard to imagine.

Here too there is a CO2 step which must be necessary for reasons beyond my ken. If someone with strong chemistry can explain why methanogenesis must have a CO2 waste product, please do!

You have the novel photosynthesis dreamt up by Bains. You could dream up another organism which uses the above pathway in its own cells. Or if you want to stick to terrestrial metabolic pathways you could have your heterotrophic herbivores have a rumen full of mixed microbes which cooperate in hydrolytically breaking down cellulose - not that edgy.

They will need hydrogen which your primary producers are making as a waste product. Unlike oxygen, hydrogen might escape a gas atmosphere. There are workarounds you could use to keep the hydrogen from leaving.

1: Water environment and hydrogen is dissolved. Or a liquid alkane environment - CH4 lakes can exist where it is cold, like Titan. Pentane / hydrocarbon lakes could exist on earth,. Absent oxygen(which is the case!) this could work.

2: Hydrogen clathrates - the hydrogen equivalent to methane clathrates, which do exist in high pressure environments on earth.

Here is a very cool thing. For heterotrophs like us earth, water is available, oxygen is available and reduced carbon is what we want to eat. In this environment, it might not be the reduced carbon sugars heterotrophs that is scarce. It is the hydrogen product of the reaction. The reduced carbon is easy to find lying around but hydrogen not so much - if it is not in the atmosphere the heterotrophs will have to drink a lot of hydrocarbon with dissolved H2 or scavenge up the clathrates to get the hydrogen they need for that half of the equation.

Thinking about this, the "plants" will of course hang on to the hydrogen. They need to run the cycle backwards themselves, just as earthly plants oxidize carbohydrate for energy. Maybe these plants will have hydrogen clathrate "fruits".