Oxygenic photosynthesis...
    provides the energy to run the 'factory' that process carbon dioxide in order to provide oxygen for the atmosphere.

Photosynthesis-1

Ponder exercise... when did the chloriphyl plants show up? Where did the Carbon dioxide come from.  Where did the water get the oxygen to becom H2O? 
Astron0my Magazine June 2014

 Photosynthesis-1