Where does most of the oxygen come from?

Most of Earth’s oxygen originates from photosynthetic organisms in the oceans, mainly microscopic marine plants known as phytoplankton. Although land plants like trees play an important role, the oceans generate roughly 50–70% of the planet’s atmospheric oxygen, with phytoplankton contributing the majority.

Marine photosynthesis: Phytoplankton comprising algae, bacteria, and other microscopic organisms carry out photosynthesis by using sunlight and carbon dioxide to create glucose and release oxygen.

Comparison with land plants: Phytoplankton outperform all land vegetation combined in oxygen production. Notably, the cyanobacterium Prochlorococcus alone is estimated to generate about 20% of Earth’s total oxygen.

Land-based oxygen: While terrestrial plants such as trees supply the remainder of atmospheric oxygen, the common idea that forests are the main source is misleading. Trees remain essential for maintaining ecosystems, absorbing carbon dioxide, and supporting life on land, but the true oxygen powerhouse lies within the oceans.