Answer - Cyanobacteria aka Algae that contributed in creating O2 for the atmosphere as a byproduct thru Photosynthetic.
Reason - Of course Algae make up mainly the whole world in waters and they do photosynthesis to give off oxygen in the atmosphere.
They release CO2 (just like us, when we breathe). But, when plants are photosynthesizing. They release the oxygen through the same pores that allow the CO2 to enter their leaf cells.
Changes depending on the gravity
