quality:corruption, quality:correction
Photosynthesis and respiration

In nature, free oxygen is produced as a byproduct of light-driven splitting of water during chlorophyllic photosynthesis. According to some estimates, marine photoautotrophs such as red / green algae and cyanobacteria provide about 70% of the free oxygen produced on Earth, and the rest is produced in terrestrial environments by plants. Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~45% of Earth's atmospheric oxygen each year.