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this post was submitted on 04 Dec 2023
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As someone else pointed out, nitrogen is non-reactive. Almost any gas would work, as long as it was plentiful enough to maintain the necessary air pressure, and non-reactive. You don't need nitrogen to live; you just need oxygen. Just, not so much that you get acute oxygen toxicity, which mainly happens with pure oxygen at regular atmospheric pressure for extended periods of time. There are even applications where pure oxygen is administered to people, usually at lower than atmospheric pressure.
Nitrogen is a filler gas. It's there to take up space and keep the air molecules bouncing around at the appropriate pressure. (Or perhaps it would be more accurate to say our lungs require a certain pressure because this is where we evolved; that pressure happens to be maintained mostly by nitrogen.)
We aren't exploring other planets in person yet, but if we were, we'd need to filter out all the bad shit in the air, keep the oxygen, and maintain the normal pressure. If we were lucky enough to encounter an atmosphere with oxygen, a non-reactive filler gas, and no toxins, we might be able to just breathe it; or to breathe it after compressing it to the appropriate pressure. Nitrogen wouldn't need to be there at all.
The confusing thing about the scuba application is that nitrogen isn't in the mix because you need the nitrogen. It's there because it reduces the pressure of toxic gases to a threshhold you can survive.
Thank you for your detailed response. That explains things very well. I don’t know a lot about chemistry, but is oxygen specifically required for cell metabolism or could that be replaced with a similarly reactive gas, too?
We're pretty hyper-specialized to use it, but there are organisms on earth that don't need it and in fact find oxygen deadly; they are called anaerobic. They still need chemical energy, it's just not provided by oxygen. (As I was looking this up I discovered there's even a creature in the animal kingdom that doesn't breathe oxygen.) Some gases, like carbon monoxide, will actually participate in gas exchange in your lungs and react with your body chemistry, but in a way that rapidly breaks down cell functioning.
So, yes, there are definitely other forms of biochemistry that can process non-oxygenated environments and extract energy from them, just not us, not by a long shot.
This is why:
A) in spaceships, you can have 100% oxygen environments, at low pressures
B) scuba divers replace nitrogen with helium for deep dives (trimix) - and reduce oxygen.
As for replace oxygen: yes, but that would kill us very quickly.