Most of Earth’s oxygen comes from photosynthetic organisms, and roughly half of it is produced not on land but in the ocean. Tiny marine organisms called phytoplankton generate as much oxygen as all terrestrial plants combined, making the ocean the single largest oxygen-producing system on the planet.
The Ocean Produces Half of Earth’s Oxygen
When most people picture oxygen production, they think of trees and forests. But scientists estimate that at least 50% of all oxygen produced on Earth comes from the ocean. The organisms responsible aren’t seaweed or kelp (though those contribute). The heavy lifting is done by phytoplankton: microscopic, single-celled organisms that float near the ocean surface and photosynthesize just like plants do. They use sunlight, water, and carbon dioxide to build energy for themselves, releasing oxygen as a byproduct.
One species stands out. Prochlorococcus is the smallest photosynthetic organism on Earth, a bacterium so tiny that a single drop of seawater can contain thousands of them. Despite its size, it’s estimated to be the most abundant photosynthesizer on the planet and is responsible for producing roughly 20% of all the oxygen in the atmosphere. That’s more than all the tropical rainforests on land combined.
Diatoms, another major group of phytoplankton, also contribute significantly. These glass-shelled algae are found across every ocean and are particularly productive in nutrient-rich coastal and polar waters. Together with Prochlorococcus and other marine microorganisms, they form the foundation of ocean oxygen production.
What Land Plants Actually Contribute
The other half of global oxygen production comes from terrestrial plants. Forests, grasslands, and agricultural crops all photosynthesize, pulling in carbon dioxide and releasing oxygen through their leaves. But the net contribution of any given ecosystem is more complicated than it first appears.
The Amazon rainforest is often called “the lungs of the Earth,” credited with producing 20% of the world’s oxygen. That figure is wrong on two levels. First, the actual gross oxygen production of the Amazon is closer to 6 to 9% of global photosynthetic output. Second, and more importantly, the Amazon consumes nearly all the oxygen it produces. Plants themselves use about half their oxygen output through respiration, breaking down carbohydrates to fuel their own growth. The remaining oxygen gets consumed by microorganisms in the soil that decompose fallen leaves, dead wood, and other organic debris. The net contribution of the entire Amazonian ecosystem to the world’s oxygen supply is, as Oxford ecologist Yadvinder Malhi has explained, effectively zero.
This doesn’t mean the Amazon is unimportant. It stores enormous amounts of carbon, supports biodiversity, and regulates regional climate. But its role as an oxygen supplier is vastly overstated. The same basic math applies to most mature forests: they produce and consume oxygen in roughly equal measure.
How Photosynthesis Creates Oxygen
Whether it happens in a redwood tree or a single-celled ocean bacterium, the core chemistry is the same. During photosynthesis, organisms capture light energy and use it to split water molecules. The hydrogen from those water molecules is used to convert carbon dioxide into sugars, while the oxygen atoms are released as a gas. This is a key detail: the oxygen you breathe originated from water, not from carbon dioxide.
This process requires light, which is why phytoplankton live near the ocean surface and why dense forest canopies are so productive. At night, or in deep water where light can’t reach, photosynthesis stops. The organisms continue to respire, consuming oxygen just like animals do. The balance between daytime production and nighttime consumption determines how much net oxygen an ecosystem actually adds to the atmosphere.
How Oxygen First Built Up in the Atmosphere
Earth’s atmosphere wasn’t always rich in oxygen. For the first two billion years of the planet’s history, oxygen was virtually absent from the air. That changed around 2.4 billion years ago during what geologists call the Great Oxygenation Event. The organisms responsible were cyanobacteria, ancient microbes that evolved the ability to photosynthesize using water as an energy source, releasing oxygen as waste.
Before cyanobacteria took over, Earth’s surface was dominated by other photosynthetic bacteria that didn’t produce oxygen. These older microbes relied on chemical compounds like hydrogen sulfide instead of water. Cyanobacteria gradually outcompeted them, and as oxygen accumulated faster than geological processes could absorb it, the atmosphere permanently shifted. Every oxygen-producing organism alive today, from Prochlorococcus in the ocean to the oak tree in your yard, descended from or acquired the photosynthetic machinery that cyanobacteria pioneered billions of years ago.
Why Ocean Oxygen Production Is Vulnerable
The fact that half of Earth’s oxygen depends on microscopic ocean life makes it sensitive to environmental change. Phytoplankton oxygen production depends heavily on water temperature. Research has shown that some plankton species produce less oxygen as water warms, and their output can drop to zero, or even reverse, when temperatures rise by about 5 to 6°C. At that point, phytoplankton switch from producing oxygen to consuming it.
There’s an important nuance to the ocean’s contribution, though. While marine organisms produce at least 50% of Earth’s oxygen, roughly the same amount is consumed by marine life through respiration and decomposition. So like the Amazon, the ocean’s net oxygen contribution to the atmosphere is relatively small in the short term. The atmospheric oxygen reservoir is enormous and has been built up over billions of years. Even significant disruptions to photosynthesis wouldn’t cause oxygen levels to drop noticeably within a human lifetime.
The more immediate risk from declining phytoplankton isn’t suffocation. It’s the collapse of marine food webs that depend on these organisms, and the loss of a major carbon dioxide sink that helps regulate global climate. Phytoplankton are the base of nearly every ocean food chain, so a sustained decline ripples upward through fish, marine mammals, and ultimately the fishing economies that feed billions of people.