Could a planet with an atmosphere composed mostly of neon, with oxygen and carbon dioxide making most of the rest, support life as we know it (specifically on a low gravity planet)? If so, what impact would it have on the biota of the world?
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Different Nitrogen Cycle
The nitrogen cycle helps give plants (and the animals who eat those plants) the nitrogen they need to make all their compounds which help make their cells. With a decreased level of atmospheric nitrogen, how will these plants make the various amino acids they need? If you look at at the chemical composition of amino acids, you will quickly note that nitrogen is very common, and therefore very important, to life. Amino acids form the structures of proteins, which themselves are important to basic cellular functions and structure.
Life, if it is like life we know of, will need to develop some different techniques to even exist in this nitrogen poor atmosphere! Some bedrocks have nitrogen in them, and can serve as a source of nitrogen, this is not a property common to all bedrock. If nitrogen is too rare or uncommon, I do not know if life can even exist.
Different Atmosphere
Looking at a periodic table, you can see that Neon has an atomic number of 10, whereas Nitrogen is sitting at 7. Since Earth's atmosphere is made mostly of $N_2$, and Neon (as a noble gas) tends to exist as simple Ne, this means the atmosphere itself would be composed of lighter molecules.
This would result in less buoyant force for things which attempt to fly, as gaseous Neon is lighter and less dense than air. This will result in larger wingspans or more vigorous flapping for flying creatures in general.
It also means that creatures experience less drag, so I would expect larger creatures to be generally faster.
In Short: No Life
Nitrogen is a key element for all life. Although most forms of life rely on other forms of life to make atmospheric nitrogen available, removing those nitrogen fixers makes the nitrogen cycle crumble. No nitrogen cycle means no amino acids, and no amino acids means no life!
answered Aug 15, 2016 at 0:45
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$\begingroup$ I'd say that "no life" isn't an inevitability -- I wouldn't rule out an alternate nitrogen cycle that doesn't involve atmospheric N2. $\endgroup$ Aug 15, 2016 at 4:15
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1$\begingroup$ Agreed, IIRC, there are bacteria that can free nitrogen from molecules in dirt. Analogous creatures could presumably do the same elsewhere, getting it from the dirt or liquid compounds. Perhaps the plant analogues can do it themselves. They just haven't had to as often on Earth...But this is one of the reasons carnivorous plants exist. This world might have some aggressive plants! But what I really want to know is, how cool would lightning look on a world with a neon atmosphere?! $\endgroup$ Aug 15, 2016 at 4:28
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1$\begingroup$ Clearly, plants do not suck all the nitrogen out of the atmosphere. So, how high a percentage do they actually need? $\endgroup$ Aug 15, 2016 at 15:34
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$\begingroup$ So nitrogen is a necessity for traditional life; I can easily change the atmosphere to include it or create life forms that produce it. On a side note, could organisms use the neon atmosphere and biologically-produced electricity to create a light show? $\endgroup$– TwigAug 15, 2016 at 15:35
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$\begingroup$ @Shalvenay Read a little more closely, please. I said no nitrogen fixers would mean no life, not that these nitrogen fixers cannot get nitrogen from other sources. In fact, I even say bedrock has been used! $\endgroup$ Aug 16, 2016 at 2:34