The atmosphere [is 21% oxygen](http://teachertech.rice.edu/Participants/louviere/comp.html). I assume that there must be an equilibrium between processes that produce oxygen (e.g. [photosynthesis](http://www.rsc.org/Education/Teachers/Resources/cfb/Photosynthesis.htm)) and those that consume oxygen (e.g. [aerobic respiration](http://www.rsc.org/Education/Teachers/Resources/cfb/Respiration.htm)).

Moreover, there should be a self-regulatory negative feedback which restores the equilibrium if the system is driven away from it. If the amount of oxygen were increased, something that would subsequently cause it to decrease over time, and if the amount of oxygen were decreased, something to cause it to be replenished.

*What processes are responsible for this negative feedback?*