What do Aliens breathe?
Free oxygen is so reactive that it doesn’t last long in the atmosphere unless plants and other photosynthetic organisms continuously replenish it. This means that on any planet, including Earth, life must initially evolve without any oxygen to breathe.
There are lots of modern bacteria that ‘breathe’ many other elements and compounds, including sulphur, carbon dioxide, iron, manganese, cobalt and uranium. All of these are much less common in the Universe than oxygen though, which is the third most common element.
The reason we breathe in oxygen is because it’s a highly reactive molecule that can combine with the compounds in our food to build new molecules and release energy. Other metabolic pathways can achieve a similar result, albeit less efficiently, using nitrate or sulphate instead of oxygen, but they all share the chemical property of being oxidizing agents. Carbon dioxide (CO2) is already fully oxidized, so it can’t be used to power respiration in the same way.
Of course, plants take in CO2 for photosynthesis, but this is a completely separate metabolic process from breathing. Plants make their food using photosynthesis, but they still need to breathe in oxygen to ‘burn’ this food. So the CO2 they absorb doesn’t count as breathing, but even if it did, it’s doubtful that a photosynthetic organism would ever evolve intelligence. Brains are metabolically very costly organs and photosynthesis doesn’t generate energy fast enough to run on.
At the bottom of the Mediterranean Sea, in one of the most extreme environments on Earth, there is evidence of an animal able to live its entire life without oxygen.
Loriciferans are about the size of a large amoeba. They live in muddy sediments at the bottom of the seas. But supposedly, that mud should contain some oxygen to allow the animals to breathe. The mud in the L'Atalante basin at the bottom of the Mediterranean does not.
Also, creatures that can make their own food just by pointing at the Sun don’t need to hunt, plan or even move much. So there’s little selective pressure for them to evolve intelligence in the first place.
A concept presented about life that used different forms of respiration involved a world where you stored oxidizers for fuel, and breathed in reducers (like hydrogen) instead of our current process of storing reducers for fuel (like fats and sugars), and breathing in oxidizes. However, these aliens might combust when exposed to our atmosphere.
Also, rather than water, using Ammonia as a solvent would work, though if you want it to be pure ammonia, you'll have to keep your aliens quite cold, as it boils at -33°C. Aqueous ammonia solutions would work, but that does not seem to be what we're after. Ammonia isn't as broadly useful a solvent as water, but it does allow for lots of interesting chemistry to occur, including things like relatively stable solutions of solvated electrons, which are quite unstable in aqueous systems. Lots of biological reactions will produce water, so an anhydrous alien will need a way to excrete the water just as we have a way to excrete ammonia. Maybe our alien could excrete crystals of glycerol or urinate an alcohol or something like that. In any case, ammonia is a much better choice than a hydrocarbon.
Breathing hydrogen is a bit more problematic. The purpose of breathing oxygen is that we can break C-H bonds and combine oxygen with both carbon and hydrogen atoms to release energy in the metabolism of organic molecules. Oxygen is highly reactive stuff; we just don't realize it because we've evolved to walk around at the bottom of an ocean of it. If we breathed hydrogen instead, it would be far less reactive. Hydrogen will readily react with lots of things, but it typically doesn't release huge amounts of energy in the process.
One idea might be to have our aliens run a biological version of the Sabatier process. The reaction is spontaneous at low temperatures, though it would never actually occur without catalysis at liquid ammonia temperatures. This would give them a means of producing some basic metabolic building blocks from materials that are common in the universe as well as provide a reason for them to breathe hydrogen in the first place.
Water would be a waste product and not necessarily a toxin (although it might be a toxin). Sort of like how CO2 is for humans. If this was a low temperature environment, the critters would excrete solid water though - imagine pooping ice cubes! I was also thinking that it might make more sense for the critters to breath methane than hydrogen.
Ammonia can work as a solvent in place of water or in addition to water. Hydrogen is also very reactive so it could be a useful energy source. Also if a planet somehow had high concentrations of Boron then Boron Nitride could be a substitute for Carbon as together they form the same bonds as Carbon and life based on Boron and Nitrogen might use ammonia as a solvent as at the temperatures of liquid ammonia that reactions between Boron and Nitrogen could be more controlled. In this case the planet might have an atmosphere of Diborane, Hydrogen, Nitrogen, and Ammonia vapor. In this case the equivalent of plant life might get energy by reacting diborane and ammonia to produce sugar analogues and hydrogen while the equivalent of animals might react hydrogen and sugar analogues to produce diborane and ammonia. In this case the gasses that the organisms would breath would explode when in contact with air. Also liquid water in high concentrations would be too hot and too acidic for this type of life and if this life touched life on Earth both would mutually poison each other!
Chlorine: The problem with chlorine is that it is very reactive and nearly impossible to find freely in the atmosphere. However, you can have chlorine compounds (HCl, CFCs etc) in the atmosphere which the creature can inhale and absorb these compounds through lungs and then use solar-cell like organs to electrolyse them and release the chlorine. They would then use this chlorine and use it in their respiration.
Self sufficient salts: This could be an interesting choice, considering that you obtaining everything from one food source. Our creatures would be eating salts (NaCl, KCl etc) and then using the above solar-cell like organs to electrolyse them in water-solution and release hydrogen and chlorine. They would absorb these gases in different chambers and transport them to the organs in different channels. Within the organs, the hydrogen and chlorine would be allowed to mix, releasing energy. The waste product (aka HCl) would then be excreted out of the organism.
CH4 + H2O + light -> [sugar] + 2H2
is possible and requires 4 times less energy than normal photosynthesis.
Therefore, the reverse reaction of reducing sugars with hydrogen should be exothermic and yield 1/4 the energy - probably not enough for warm-blooded creatures, but surely enough for muscles and nerves to function.
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