Most of the oxygen in Earth’s atmosphere comes from the ocean, not from forests. Marine organisms, primarily tiny photosynthetic bacteria and algae collectively called phytoplankton, produce somewhere between 50% and 70% of all the oxygen generated on Earth. The rest comes from land plants, including forests, grasslands, and other vegetation.
Phytoplankton: The Real Oxygen Factories
Phytoplankton are microscopic organisms that float in the sunlit upper layers of the ocean. Like plants on land, they use sunlight to convert carbon dioxide and water into sugar for energy, releasing oxygen as a byproduct. The sheer number of these organisms, spread across every ocean on the planet, makes them the dominant oxygen producers on Earth.
One species stands out. Prochlorococcus is the smallest photosynthetic organism on Earth, invisible to the naked eye, yet it alone is responsible for roughly 20% of all the oxygen in the biosphere. That single species of bacteria produces more oxygen than all the tropical rainforests on land combined. In nutrient-poor stretches of open ocean, Prochlorococcus and its relatives account for 13% to 48% of all photosynthetic production.
Other important marine oxygen producers include diatoms (single-celled algae with glass-like shells), dinoflagellates, and larger seaweeds and kelp forests in coastal waters. Together, these organisms form the base of the ocean food web while simultaneously keeping the atmosphere breathable.
What About Forests and Rainforests?
Land plants do produce enormous quantities of oxygen through photosynthesis. About one-third of all land-based photosynthesis happens in tropical forests, and the Amazon rainforest is the largest of these. This is why the Amazon is often called “the lungs of the Earth.”
That label is misleading, though. Almost all of the oxygen a forest produces gets consumed right back. Roughly half is used by the plants themselves as they burn sugar for energy (the same process your cells use, called respiration). The other half is consumed by insects, fungi, and microbes as they decompose fallen leaves, dead wood, and other organic material. Forest fires also burn through oxygen. When you account for all of this, the net oxygen contribution of land ecosystems is close to zero. Forests matter enormously for absorbing carbon dioxide and supporting biodiversity, but they’re not meaningfully adding new oxygen to the atmosphere on a year-to-year basis.
The same principle applies in the ocean. Most oxygen produced by phytoplankton gets consumed by marine life and decomposition. The atmosphere’s oxygen supply stays relatively stable because production and consumption roughly balance out over time.
Why the Atmosphere Stays at 21% Oxygen
Earth’s atmosphere is about 21% oxygen, and it has stayed in a broadly similar range for hundreds of millions of years, fluctuating between roughly 15% and 40% since forest ecosystems first appeared around 420 million years ago. This stability exists because oxygen production (photosynthesis) and oxygen consumption (respiration, volcanic gas reactions, mineral weathering) create large but roughly equal flows in and out of the atmosphere.
The small fraction of organic matter that gets buried in sediments before it can decompose is what actually adds oxygen to the atmosphere over geological time. When dead phytoplankton or plant material sinks to the ocean floor and gets locked into rock, the oxygen that was released during its growth stays in the air instead of being recaptured. This slow burial process, accumulated over billions of years, is what built the oxygen-rich atmosphere we have today.
How Oxygen Got Here in the First Place
For the first two billion years of Earth’s history, the atmosphere contained almost no free oxygen. That changed because of cyanobacteria, the ancient ancestors of today’s phytoplankton. Cyanobacteria are the only organisms that independently evolved the ability to perform oxygen-releasing photosynthesis. Around 2.5 to 2.3 billion years ago, their activity triggered what geologists call the Great Oxidation Event, a rapid (in geological terms) rise in atmospheric oxygen that fundamentally transformed the planet.
Before this event, oxygen-sensitive minerals sat exposed on Earth’s surface without rusting. Afterward, the chemistry of rocks, oceans, and the atmosphere all shifted. This oxygen accumulation eventually made it possible for complex, multicellular life to evolve. Every animal alive today, including humans, exists because cyanobacteria flooded the atmosphere with a gas that was originally toxic to most life on Earth.
Warming Oceans Threaten Oxygen Production
Rising ocean temperatures pose a direct threat to phytoplankton productivity. Warmer surface water creates stronger thermal layering in the ocean, which acts like a lid that prevents cooler, nutrient-rich water from mixing upward. Without that nutrient supply, phytoplankton in tropical and mid-latitude waters produce less oxygen. Research in the Pacific has documented this pattern: warming leads to stronger stratification, fewer nutrients reaching the surface, and declining phytoplankton production.
Some phytoplankton species are adapting by migrating deeper in the water column to find better light and nutrient conditions, or shifting their range toward the poles. Tropical species in particular have been observed diving deeper to avoid intense surface light that becomes damaging without adequate nutrients. But overall, the trend points in a concerning direction. Oceans are losing both phytoplankton abundance and dissolved oxygen under warming conditions, a process scientists call ocean deoxygenation.
Because phytoplankton produce the majority of Earth’s oxygen and form the foundation of marine food chains, changes to their productivity ripple through the entire planet’s life-support system. The oxygen currently in the atmosphere took billions of years to accumulate, so a near-term breathing crisis isn’t realistic. But declining phytoplankton populations affect ocean ecosystems, fisheries, and the carbon cycle long before they affect atmospheric oxygen levels.