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In my story, Saturn’s moon Titan’s methane lakes are inhabited by microorganisms that photosynthesize, converting hydrogen into methane (A process thought possible on Titan after some study). This helps supply the methane continuously cycled through Titan’s atmosphere.

Enter my question. A horse-sized artificial life-form is set loose on Titan that is capable of “breathing” methane in the air (or drinking liquid methane and storing it) and producing hydrogen. Methane isn’t a substitute for oxygen, so the “breathing” process would probably be more similar to eating, but it would be nice if it still “looked” like breathing (for visual effect).

Is this feasible, and if so, how could this process occur? My first thought was combustion, but there’s nothing to combust with (Titan has little oxygen). Could it still be used alone to fuel a physically active animal? Most non-combustion processes I find for methane seem to involve oxygen and carbon dioxide anyway. Complex life without oxygen seems like a tall order, but if there’s an easy approach I’m open to it. Some other questions in this SE mention single-celled life, but I’m not sure if those solutions would apply to a much larger organism.

(If combustion is actually feasible, that would be ideal. Probably too extreme to be true, but I have a dream of the reaction making the creature exhale flames in oxygen-rich environments.)

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You've got it backwards. Autotrophs (plant-equivalents) would not produce methane; they would consume methane (and probably nitrogen) and use the carbon to construct more complex organic molecules, releasing hydrogen as a byproduct.

Heterotrophs / animals would breathe in hydrogen, use it to reduce complex organic molecules, producing energy in the process, and breathe out methane (and possible nitrogen as well).

Hydrogen-breathing produces somewhere between a quarter and a third as much energy as aerobic respiration with oxygen, but hydrogenic photosynthesis is correspondingly easier (so there's more food available) and colder temperatures make metabolic processes potentially more efficient; plants can use a larger portion of the solar spectrum (so, again, more food than would be available in a terrestrial ecosystem with the same solar power input), and fermentation and reduction reactions have higher thermal efficiency, so animals don't actually have to eat 3-4 times as much after all. Even if they did, though, it is entirely plausible to have complex animals that merely have to consume 4 times as much food as their terrestrial equivalents; that rules out some of the less efficient types of creatures that have to eat nearly constantly just to survive on Earth, but plenty of creatures (including strict herbivores) eat relatively slowly, and could certainly handle a 3x increase in dietary needs.

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  • $\begingroup$ To make sure I get it, it would be more likely that the animal would eat the micro-organisms (or more easily edible alternative), using hydrogen they breathe with it to generate methane. That could certainly work. $\endgroup$ Dec 21, 2021 at 5:41
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    $\begingroup$ intelligence would be much more difficult to develop under these conditions. just because of a lack of energy. since intelligence/thinking requires a lot of it. $\endgroup$ Dec 21, 2021 at 9:18
  • $\begingroup$ @PostlimFort Eh, maybe? On the other hand, I spend a very small fraction of my day eating, and quadrupling the mass & volume of my food intake would not seriously impact my lifestyle. I suspect the biggest hurdle to developing intelligence would still be just finding a niche in which it is actually advantageous. $\endgroup$ Dec 23, 2021 at 20:45
  • $\begingroup$ @LoganR.Kearsley the time it takes to actually put the food in your mouth is not all there is. how long dous it take for your stomach to process the food. and how long and how much effort must it have taken to get that amount of food. remember our ancestors didn't have tractors and ploughs and fridges. or even the fast growing plants and animals we eat today. having to get and process even 20% more won't help things. during the dark ages that was more than the margin people had. 6 to 9 farmers were needed just to support 1 that wasn't doing anything related to food production. $\endgroup$ Dec 23, 2021 at 22:31
  • $\begingroup$ @PostlimFort Yes, but remember that food production is also 4x easier with hydrogenic photosynthesis, so it balances out. $\endgroup$ Dec 23, 2021 at 23:22
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The process of breaking methane into hydrogen to get energy is energetically possible: the standard enthalpy of formation for hydrogen is 0, while for methane is -74.9 kJ/mol

The problem is the upstream process that you call photosynthesis: the organism doing it would take hydrogen from the atmosphere, add carbon taken from somewhere else, add energy taken from the sun, and then throw everything away? It makes little sense. On Earth oxygen is released as byproduct of the conversion of water and CO2 into sugar. But in the way you depict it, the organism would do it for nothing in return.

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  • $\begingroup$ As detailed in Logan R Kearsley’s answer, I had the process backwards. $\endgroup$ Dec 21, 2021 at 5:43
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    $\begingroup$ You have enthalpy backward, forming methane releases energy (this is why some archea release methane: they methabolize free hydrogen and carbon dioxide), splitting methane into hydrogen would cost energy. $\endgroup$
    – mart
    Dec 21, 2021 at 15:29

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