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I'm trying to find out if it's possible that a mammal could orally ingest dissolved CO2 and convert it to energy for body heat, organ function, etc.

Unfortunately, most of the scientific sources I've been able to find talk instead of CO2 gas inhaled by mammals. All the sources speaking to orally ingested dissolved CO2 begin their argument with the assumption that all of it is converted to (and expelled as) gas in the digestive tract, but they never bother to prove that or cite anything.

It seems like this would rarely be observed in nature, and therefor not really be studied or documented, so I'm curious what degree of chemical or biological certainty there is about this, and what research has been done.

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  • $\begingroup$ As a new contrbutor you are welcome to SE Biology. However I would suggest that you be polite in your criticism of the sources you have read. It is fine to say "they do not prove…", but "they do not bother to prove…" is derogatory. Admitedly you do not name them so they won‘t be offended, but the principle of polite and objective scientific argument is the same. And people in glass houses shouldn‘t throw stones — you haven‘t cited your sources, which we might have expected and would have strengthened your question. $\endgroup$
    – David
    Nov 9, 2018 at 0:16

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There isn't any biologically useful energy in CO2. In fact, humans and all other animals produce CO2 as a waste product at the end of metabolism specifically because it is low-energy. You don't need to ingest it or breath it in as a gas, and if you didn't breathe it out you would die.

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    $\begingroup$ You're kind of just saying "no" here, and the statements about CO2 in waste and the necessity of ingestion/exhalation aren't really relevant to the question. I'm hoping for more specifics than this. $\endgroup$
    – Adelmar
    Nov 7, 2018 at 23:51
  • $\begingroup$ @Adelmar What sort of specifics? I included those other facts because it didn't seem from your question that you realized CO2 was a low-energy metabolic waste product, and because you were talking about the possibility of differences depending on ingestion versus inhalation of CO2. Those don't matter, because there is already dissolved CO2 being produced all over. $\endgroup$
    – Bryan Krause ♦
    Nov 7, 2018 at 23:54
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    $\begingroup$ @stormy That isn't relevant here; also, even photosynthetic organisms do not use CO2 as energy. $\endgroup$
    – Bryan Krause ♦
    Nov 8, 2018 at 0:03
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    $\begingroup$ @stormy Yes, petroleum is high energy. When you burn petroleum, you release energy by turning the hydrocarbons back to low-energy carbon dioxide. The reaction is "Hydrocarbons + O2 -> CO2 + Water + Energy". Plants do not use CO2 for energy, they use the sun for energy. They store that energy by taking low-energy CO2 and making it into high-energy molecules like carbohydrates. $\endgroup$
    – Bryan Krause ♦
    Nov 8, 2018 at 0:08
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    $\begingroup$ @stormy Did I say plants didn't need CO2? I said CO2 is not the source of energy. $\endgroup$
    – Bryan Krause ♦
    Nov 8, 2018 at 0:20
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No.

What is lacking from the question is a logical framework for the possibility suggested — that carbon dioxide can be a source of energy — and I suspect that this is because it lacks a clear chemical conception of what energy is and how it is formed in living organisms.

The generation of energy in a biological context can be considered as the consequence of a chemical reaction or physico-chemical process which involves a negative (Gibbs) free energy change. For use by the cell (except in exceptional circumstances like heat generateion) the energy released in this process must be used to drive the thermodynamically unfavourable formation of a molecule (e.g. ATP) in which this can be ‘stored’. The stored energy is used when the hydrolysis of ATP is coupled to a thermodynamically unfavourable biological reaction or change, hence providing the ‘energy’ for it. This topic is covered in most biochemical texts, e.g. Berg et al..

So if carbon dioxide were to be a source of energy, it would have to undergo some reaction with a negative (Gibbs)free energy change. Other carbon compounds that are sources of energy are so by virtue of their ability to be oxidized (again see Berg et al.). However, as has already been mentioned in a comment, carbon dioxide is the most oxidized form of carbon and so cannot generate energy this way:

Carbon oxidation state and energy generation

So unless the question could proposes another chemical or physico-chemical in which carbon dioxide could participate that has negative (Gibbs)free energy change, there is no reason to consider the postulate.

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  • $\begingroup$ Finally, some sense. The question as worded is just not going to be answered directly. Just a no suffices. $\endgroup$
    – stormy
    Nov 9, 2018 at 0:17
  • $\begingroup$ Thanks for your answer. One quick follow-up: Is it possible that the microbiome of a mammal's digestive tract could produce glucose if it comes into contact with dissolved CO2? Meaning the ingested CO2 contains no usable energy, but the act of ingesting it results in the production and consumption of usable energy inside the mammal? And is there anything other than the microbiome that might be able to do this (like something else being consumed together with the CO2)? $\endgroup$
    – Adelmar
    Nov 9, 2018 at 1:14
  • $\begingroup$ @Adelmar — No. As has been mentioned, photosynthetic organisms use carbon dioxide to make glucose, and carbon dioxide is fixed in gluconeogenesis, but these processes require energy in the form of ATP derived from other processes to do this (and I don't think there are any photosynthetic organisms in the human microbiome). You don't say why you have this idea. The only related thing I can think of is the way the ATP synthase of Ureaplasma urealyticum utilizes a pH gradient it generates from hydrolysing urea to ammonia in the bladder. But dissolved carbon dioxide is acidic. $\endgroup$
    – David
    Nov 9, 2018 at 13:55
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CO2 does not yield any biological energy.
Living things can oxidise compounds like glucose and drive ATP synthesis from it. This energy keeps us alive by not letting us fall into a state of equiibrium(death).
If mammals had any speacial complexes that converted light into chemical energy in their digestive systems they would have been able to derive energy from ingested CO2.
We don't even have any reactions in our metabolism that oxidise inorganic minerals(like chemosynthetic bacteria) to drive carbohydrate synthesis.
So I don't think mammals can convert ingested CO2 into useable energy.

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