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I am trying to build a truly alien form of life, and easily found several replacements for water, such as methane or ammonia, with there own unique advantages and disadvantages (Wikipedia has a wonderful article on this). But I am having difficulty finding a good atmosphere that a creature could breathe other than oxygen.

The one gas I have found is a compound of sulfur oxide, which is currently used by certain types of bacteria that live near underwater volcanoes instead of oxygen.

What other gasses would work?

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    $\begingroup$ Plants on earth "breath" carbon dioxide? $\endgroup$ – barney Oct 28 '16 at 4:41
  • $\begingroup$ Keep in mind that not all life on Earth breathes oxygen. Animals do, but plants excrete oxygen and consume CO2 an N2 instead. $\endgroup$ – ohwilleke Oct 28 '16 at 4:42
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    $\begingroup$ plants and animals use relatively the same reaction, just reversed with plants. Plants do still respire however (in oxygen - out CO2) usually at night, so only the anaerobic bacteria (mentioned by OC) in mud and the bottom of the ocean don't breath oxygen. $\endgroup$ – XenoDwarf Oct 28 '16 at 4:51
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    $\begingroup$ @barney no, plants eat carbon dioxide. It is a source of carbon and consumes power to do so. They still breathe oxygen (in the mitochondria), but they produce more than they use. $\endgroup$ – JDługosz Oct 28 '16 at 5:57
  • $\begingroup$ @JDługosz, in the sense that a large proportion of plants absorb carbon dioxide from the atmosphere around them, a very deliberately quoted "breath" seem perfectly analogous to our breathing of oxygen. $\endgroup$ – barney Oct 28 '16 at 6:13
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Oxygen, in our (aerobic) respiration system is used to burn up the ingested food and release the stored energy in it. If you want your aliens to operate on a similar respiration system (as in, they use a gas to burn up their food and release energy), then you have few choices other than oxygen.

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. Your 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.

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You don’t need a metabolism that is exactly analogous to what you know. For cells, like with combustion engines, it’s handy to half one of your reactants available always in the environment and not have to carry around that weight in fuel. But it would be more alien if the life-form did not behave in that manner at all.

If the life-form does use some atmospheric gas, it could very well be for a role that is not analogous to how we use oxygen. For example, plants use CO₂ but as a source of carbon atoms which eventually wind up forming tissue, but it’s not analogous to our food either because it requires energy rather than provides energy.

Gasses in the air are handy but not necessarily handy enough. Plants get carbon and oxygen using organells that have become part of the cell. But they never bothered taking that step with the bacteria that extract nitrogen from gas. Well actually, one organism did and others keep them at hand but with a less integrated relationship.

Whether use of a particular substance is analogous to breathing oxygen is a detail, and part of a far larger spectrum of possibilities.

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I won't find links now, but reaction

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.

Just a thought.

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Whatever gases you choose for your atmosphere will need to be stable at the ambient temperature.

Also, whatever reaction occurs from the gases reacting with solar radiation in the upper atmosphere will need to proceed at a low rate or be more or less reversible.

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There are actual anaerobic organisms on Earth, and before plants came along and spewed out toxic waste everywhere there were probably many more.

So, there are known working metabolisms you could crib.

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