How much of the world's oxygen comes from the ocean?

The answer

Ocean phytoplankton produce approximately 50 to 80 percent of Earth's atmospheric oxygen through photosynthesis, generating an estimated 270 billion tonnes of oxygen annually. The dominant producers are cyanobacteria (especially Prochlorococcus, the most abundant photosynthetic organism on Earth, discovered only in 1986), diatoms, and dinoflagellates. Prochlorococcus alone is responsible for roughly 20% of global oxygen production - more than all tropical rainforests combined. Despite their microscopic size, phytoplankton collectively contain 1 to 2 billion tonnes of carbon. However, phytoplankton populations have declined an estimated 40% since 1950 due to ocean warming and stratification reducing nutrient mixing. This decline threatens not just oxygen production but the entire marine food web that depends on these microscopic organisms.