First time poster, please be gentle.

Long story short, my own alien worldbuilding/speculative biology project involves the idea that cyanobacteria and other oxygen producers never evolved and other bacteria took its place. Life on this planet has evolved to be both autotrophic and heterotrophic at the same time, with varying degrees one way or the other. To fill plant and animal type rolls.

I'm willing to change my world given new information but its planned to be a metal rich world with increased volcanism and similar temperatures to earth. The "increased volcanism" is an early idea to explain outgassing early microbes could exploit and how usually rare metals (Titanium, Platinum, Chromium, ect...) are commonly accessible. This is subject to change.

Is my proposed "Carbon" cycle theoretically sound? I'm not a chemistry major by any stretch. I believe the reactions are balanced throughout the plant/animal cycle, where the Plants come to a net positive enthalpy and animals come to a net negative. Could liquid water even exist on this planet?

The main proposed atmospheric gases are and their quantities:

  • Methane (4%)
  • Acetylene (8%)
  • Carbon Dioxide (10%)
  • Hydrogen (Trace gas)
  • Nitrogen (75%)

These atmospheric numbers don't come from any sources and are made up.

The "Plant" type roll: *

  • 3 CH4 + CO2 -> 2CH3CHO + 2H2
  • C2H2 + H2O -> CH3CHO
  • 2 CH3CHO + 5O2 -> 4CO2 + 4H2O
  • 4CO2 + 2H2O -> 2C2H2 + 5O2*

Note that each of these are reactions that take place separately within the organism. With Methane, Acetylene, and CO2 being broken down photosynthetically to provide energy in the form of Acetaldehyde. Releasing some Acetylene, CO2, and Hydrogen gas as waste.

The Animal type roll: *

  • CO2 + 4H2 -> CH4 + 2H2O
  • C2H2 + 2O2 -> H2 + CO2
  • CH3CHO + H2O -> C2H4O2 + H2
  • C2H4O2 + H2O -> 2CH4 + 2O2*

These organisms would breath CO2, Acetylene, and Hydrogen. Looking to intake the Acetaldehyde to be broken down into Methane waste and usable oxygen. Again these reactions would take place separately.

The Oxygen's in these reactions are produced and used internally, mediated throughout the organism through some "magical enzyme" means. This idea comes from earth plants, in photosynthesis. A reaction that normally would take more energy that is gains, but its ok because SUN. The oxygen is not supposed to be energy balanced, but so there is no need or excess of the molecule.

If this idea is completely wrong please tell me. Or if you have better suggestions on what to use as an energy source I'd be happy to hear. Discussion on how life would potentially evolve using this system are welcome too.

Thanks to any and all who reply - Austin

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    $\begingroup$ Your species producing oxygen, what prevent it to accumulate in the atmosphere? $\endgroup$
    – RomainL.
    May 24, 2022 at 1:48
  • 1
    $\begingroup$ Unless you as the worldbuilder simply "make it so" it's very hard for me to believe there won't be any free oxygen in the atmosphere - to combine with three highly combustible gases in large quantities. The only thing you're missing is gaseous gasoline and a match (and I doubt you need the gasoline!). But, if I remember my college chemistry correctly, it's not enough to be chemically balanced, you must also be chemically stable, which is why our atmosphere's major component (Nitrogen) is explosively non-reactive. (Someone correct me if I'm wrong!) How much of your atmosphere is CO2? $\endgroup$
    – JBH
    May 24, 2022 at 3:39
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    $\begingroup$ Now we have a question that doesn't involve the entire planet burning to a crisp when a volcano erupts! As a rule, never trust that people with read through comments. Always edit your question with answers to questions posed in comments. $\endgroup$
    – JBH
    May 24, 2022 at 4:00
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    $\begingroup$ Acetylene is pretty energetic with its triple bonded carbon. It could be used as storage, but, unlikely to be a waste product. Ethane C2H6 is much more likely. You need to consider whats more likely energy wise as well if you are going for hard science plausible. $\endgroup$ May 24, 2022 at 20:24
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    $\begingroup$ @Austinman42 broadly speaking waste products have less energy then the inputs. the whole metabolize things to get energy to do stuff. The triple carbon bond represents comparatively more energy then a singly bonded carbon. $\endgroup$ May 25, 2022 at 0:41

1 Answer 1


There are a few things that stand out as potentially problematic with your proposed carbon cycle.

First, the overall efficiency of converting methane and acetylene into useful energy is very low. This is because a lot of the energy that is released when these molecules are burned is in the form of heat, rather than light (which can be used to drive photosynthesis). As a result, it is unlikely that any organisms could use this carbon cycle as their primary source of energy.

Second, the presence of methane and acetylene in the atmosphere would make it very difficult for liquid water to exist on the surface of your planet. These molecules are both highly reactive, and would quickly react with water to form other compounds (such as carbon dioxide and water vapor). This would make it very difficult for life to exist on the surface of your planet.

Finally, it is worth noting that your proposed carbon cycle does not actually recycle any carbon. The net effect of the reactions you have listed is that methane and acetylene are converted into carbon dioxide and water vapor. While this might be fine for a planet with no life, it would not be sustainable over the long term if there was life present.

Overall, it seems like your proposed carbon cycle would not be able to support life on your planet. I would suggest reconsidering the atmospheric composition and/or the energy sources that you are using.


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