<P> In nature , free oxygen is produced by the light - driven splitting of water during oxygenic photosynthesis . According to some estimates , green algae and cyanobacteria in marine environments provide about 70 % of the free oxygen produced on Earth , and the rest is produced by terrestrial 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 . </P>
<P> In nature , free oxygen is produced by the light - driven splitting of water during oxygenic photosynthesis . According to some estimates , green algae and cyanobacteria in marine environments provide about 70 % of the free oxygen produced on Earth , and the rest is produced by terrestrial 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 . </P>
oxygen ( o2 ) is formed in photosynthesis as a direct result of
oxygen ( o2 ) is formed in photosynthesis as a direct result of
the light - driven splitting of water
the light - driven splitting of water