Doubt in atmospheric gaseous balance?

Right... lets see here.

Oxygen... pretty much 100% of it found on earth... comes from Photosynthesis.

Most photosynthesis is performed by plants, though quite a bit is done by phytoplankton (which literally means plant-plankton, though they aren't actually plants at all). They take in a bit of water and Carbon Dioxide, energise with sunlight, and they churn out oxygen and glucose. They then store away the glucose to use as a building material for growth and give off the oxygen as a waste product. Since there are photosynthetic organisms all over the place, phytoplankton in the water (along with quite a bit of macroscopic plantlife) and larger plants on the land, there tends to be a lot of oxygen given off.

During the night when there is no sunlight to be had, the process reverses somewhat. Photosynthesis can't take place, so the primary force taking place in plants becomes respiration... the same as we do... which is the exact reverse. It takes glucose and oxygen and combines the two to get energy out and produce water and carbon dioxide as waste products. Whats important to note here is that plants will tend to produce a LOT more oxygen during the day than they use up during the night.... so there is a net increase in oxygen. This is however dependant on how much of the necessary chemicals are in the air (or water, in the case of phytoplankton) in the first place.

And so why isn't the oxygen constantly building up?

Simple... because there are a lot of animals and smaller lifeforms around doing nothing but respiring, using up the extra oxygen and changing it back into carbon dioxide. This tends to keep things stable... and as an extra bonus, we have the world's oceans taking the excess oxygen and carbon dioxide into solution... buffering the whole process (to stop too much deviation).

It tends to take a LONG time for any significant chemical change to take place in the atmosphere between those naturally occurring substances.... though of course OTHER things that we humans churn into the atmosphere often can't sink anywhere and will tend to linger around and do all kinds of unpleasant stuff. ^_^;; ...

Oh.... and last I recalled.... Nitrogen is about 78% of the atmosphere, Oxygen 21% and Carbon Dioxide was 0.03% ... The rest is mostly water, along with a mix of noble gasses and other wazi stuff.

Everybody forgot Argon. It's almost 1%. 30 times as much as CO2.

Trees and other plants release a lot less CO2 at night than they take in during the day.

Did you know that when the big bang started, there was no such thing as gas, gaseous pressure, atmospheres, or electromagnetism? Until symmetry was broken, the fundamental forces of the universe were unified. And atoms didn't come into existence until 380,000 years after the start of the big bang.

Actually, we get most of our oxygen from the oceans. It covers most of the planet and comes from the phyla (sp?) that naturally exists in most warmer and moderate climates. They function like terresteral plants and convert CO2, sunlight, and sugars into O2 and starch.