The Pod 4 simulation began with an atmosphere approximating Earth's: 21% oxygen, 0.036% carbon dioxide, and the rest nitrogen, except for trace gases.
Oxygen (O2)
As time goes on, photosynthesis in plants produces oxygen, which replaces
the oxygen consumed by Foster and other aerobic (oxygen-breathing) creatures
in the pod.
Carbon
Dioxide (CO2)
As time goes on, the pod's plants consume the carbon dioxide breathed
out by Foster and other aerobic (oxygen-breathing) creatures in the pod.
Nitrogen - essentially an inert gas.
Trace Gases - primarily methane. Living creatures produce methane as a byproduct of digestion, especially of cellulose. On Earth, ruminants such as cattle plus termites are responsible for about a quarter of the methane released each year.
Now, on Day 34, the carbon dioxide level is climbing dramatically and nears 0.5%. The oxygen level has dipped correspondingly. The methane level has also climbed steeply, almost parallel with the increase in carbon dioxide.