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How would an heterotroph alien life form exist if it purely needed to inhale CO₂?

Would it be lazier and bulkier that carbon based life forms who require oxygen, like us humans?

I am trying to understand the chemical processes of a humanoid creature who requires CO₂ to survive - and would presumably exhale oxygen, as that’s only way to make sense of their respiration.

Would it being carbon-based make this impossible? For it to be a pure CO₂ breather would it need to be a non-carbon-based life form?

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  • $\begingroup$ Welcome to the site XRK. I think you're on to something, since such a thing (somewhat) exists on Earth. Could you tell us if you're looking for a "Life Form" as per the title, or the possibility of life as per the body of the question? If life in general, then perhaps edit the title to reflect the question in the body of the post. $\endgroup$ Commented Jun 16 at 2:22
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    $\begingroup$ Hello @XRK, welcome to Worldbuilding! For future reference, remember that you can ask one and only one question. Off the top of my head, the real-world example of a creature that "breathes" CO2 is a plant or tree. Why don't trees walk the Earth? Principally because life requires energy and dynamic life needs a lot of energy and that tends to come from O2 and when it's bound up with C it's not available to "burn" as energy. I'll wait for a better opinion from someone who understands chemistry better than I, but unfortunately, science is rigid. $\endgroup$
    – JBH
    Commented Jun 16 at 2:38
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    $\begingroup$ BTW, I hope you read the wiki for the hard-science tag. It serves a purpose, but it's also ruthless. Answers that can't be backed up with citations, mathematics, attributions, etc. are under threat of deletion. In other words, answers must prove they're correct or they're deleted. Was that what you intended? The tag won't necessarily give you a better answer, just a better annotated answer. Fair warning. $\endgroup$
    – JBH
    Commented Jun 16 at 2:40
  • $\begingroup$ Do you want an animal, or just life? AFAIK there are respiratory pathways from CO2+H2O + Various Phosphates + energy -> Various Phosphates + H2O + compound of H, C, O -> Various Phosphates + H2O + compound of H, C, O + energy, which might qualify, but the energy storage per molecule is way too low for an animal. $\endgroup$
    – g s
    Commented Jun 16 at 3:23
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    $\begingroup$ If they're not carbon based, it isn't obvious why they'd be obligate carbon-dioxide breathers. Can you clarify your question to clearly ask one thing? And consider removing hard-science for speculative biology questions, because there are unlikely to be many useful bits of research other than the obvious ones about plants, which presumably you're not interested in? $\endgroup$ Commented Jun 16 at 9:04

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Such organisms already exist here on earth, practising photosynthesis. For the most part, photosynthesising lifeforms are largely sessile since the photosynthetic efficiency of photosynthetic organisms on Earth is very low, on the order of 4.3% or less of the incoming sunlight energy being converted to biomass. Given such a low efficiency of photosynthesis, only the smallest, lightest photosynthetic organisms can afford to be motile, such as cyanobacteria. Euglenia is a motile, photosynthetic organism, but it fails the OP's criteria since it also obtains energy by heterotrophic means.

Moving on to macroscopic autotrophs, very few move at a rate perceptible to humans, and most are carnivorous, thus also failing the OP's criteria of requiring only carbon dioxide. One such organism is Mimosa pigra, a sensitive plant, which has leaves that fold up when touched. The motion of plants is typically slow, occuring in response to situations such as the direction of light or the amount of water.

To have a macroscopic obligate photosynthetic organism be truly motile would require that photosynthesis either be considerably more efficient than terrestrial photosynthesis, or for considerably more light energy to be available. The latter has the complication that with increased sunlight comes increased planetary temperatures, to the point where it is no longer possible to have liquid water.

So, to conclude, any life-form that requires CO2 that photosynthesises would most likely behave like a terrestrial plant. I have been unable to find any susggestion that CO2-uptaking respiration that does not rely upon photosynthesis might be possible or practical.

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    $\begingroup$ One adds that these plants actually gain energy by breathing back in their oxygen and burning the carbon-based molecules they generate through photosynthesis. They are net oxygen producers only because some molecules are used structurally and not for energy. $\endgroup$
    – Mary
    Commented Jun 16 at 13:08
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What you describe as inhaling carbon dioxide and exhaling oxygen is in fact what plants do as photosynthesis. This would require a source of energy and your creature would be classified as an autotroph - being able to synthesize its own food using simpler mineral components and producing more complex energy-rich materials via the process of photosynthesis or chemosynthesis. Let's call these creatures plant analogues.

However, if you want the creature to be a heterotroph (analogous to animals on Earth, consumes plants or other animals and breathes local "air" to power its metabolism), carbon dioxide must be used as a part of a fuel/oxidant mix, like respiration in animals. Obviously what you asked for is quite the opposite. To use carbon dioxide as a source of energy, you can use a material which combusts in carbon dioxide. One such material is magnesium. You can build the body chemistry around the use of magnesium as a source of energy in this reaction:

2 Mg + CO₂ -> 2 MgO + C

The downside of this reaction is that carbon (C) deposits as a solid material, known as soot. While an internal combustion engine can expel soot through the exhaust pipe, biological systems would have a hard time to do it. They must find a way to circulate it through the blood stream to whatever organ which can expel it. And carbon is a solid; I can't see how "plants" would absorb it unless they actively "eat through" the soil surface like earthworms do.

A better option is lithium:

2Li + 2CO₂ + 2H₂O → 2LiHCO₃ + H₂

This reaction produces lithium bicarbonate, which is water soluble, and hydrogen, which is a gas and exhaled through the lungs. The downside is the low abundance of lithium, but that's another story.

A third possibility is sodium - more abundant than lithium, giving

4Na + 3CO₂ → 2Na₂CO₃

This seems to be just one waste product, mainly expelled through the kidneys (or their analogs on this world) but without expelling any gas. Photosynthesizing plants on this world would rather absorb all nutrient matter through their roots and the biology on this world would be intriguing, if not impossible.

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    $\begingroup$ Where would these reduced reactive metals come from? Plants convert CO2 and H2O into O2 and carbohydrates because this is easily done biochemically with photosynthesis, and they and animals that eat them then reverse the process to get the energy to drive their metabolisms. Producing metallic sodium from sodium carbonate would require achieving biologically what we do with electrolysis of the molten salt at above 851°C, and the resulting sodium is highly reactive with water and easily self-ignites. Other reactive metals have similar problems. $\endgroup$ Commented Jun 17 at 0:22
  • $\begingroup$ This is the best direction for an answer. There are quite some metals that in pure form can react with CO2 to produce energy. Similar to how some organisms use chemicals from thermal vents deep in the ocean. It requires the existence of a whole ecosystem with 'plants' that produce those metals. But if those aliens bring blocks of pure sodium as meals ready to eat, they will be ok. Also, they could have gut microbes that extract the energy from the CO2 + metals and release other chemicals that are absorbed by the body like the ATP, ADP cycle. Then the soot will be expelled in the familiar way. $\endgroup$ Commented Jun 17 at 13:15
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Both other answers have pointed out plants and photosynthesis. I will point out that plants also take in oxygen and give off carbon dioxide. They just do this significantly less than they take in carbon dioxide and give off oxygen, provided it is daytime. At night, plants are seen to do the same respiration as humans.

Having said that, one could hypothesize an environment providing continual light, where the carbon dioxide intake would always dominate. Alternatively, the organism could go very dormant in the dark. (And this might not matter to you anyway.)

As for "could it be humanoid", the problem with photosynthesis is that it requires light. Lots of it, for most of the cells. This is why plants have thin leaves -- so all the cells get light. I would suspect that a humanoid form would be an outer shell of leaves, and a rather minimalist internals. It might weigh 5% to 10% of what a similar sized animal might weigh, and be correspondingly strong.

And of course, photosynthesis is not heterotrophic. One could postulate an organism that acts as a symbiote to photosynthetic organisms, and its respiration is to provide carbon dioxide for the photosynthsizers. Failing that, you might have to rely on higher energy reactions as suggested by @Christmas Snow.

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