Plankton produce roughly half of all the oxygen on Earth. That single fact surprises most people, who tend to credit forests and other land plants for the air we breathe. In reality, microscopic organisms floating in the sunlit layer of the ocean are doing just as much photosynthetic work as every tree, shrub, and blade of grass on land combined.
How Plankton Produce Oxygen
Phytoplankton, the plant-like members of the plankton family, generate oxygen the same way land plants do: through photosynthesis. They absorb sunlight and carbon dioxide, then use that energy to build sugars for growth. Oxygen is released as a byproduct. The process happens inside chloroplasts, the same tiny structures that make leaves green.
Recent work at the Scripps Institution of Oceanography has added new detail to this picture. Scientists identified a specific enzyme inside diatoms (a major group of phytoplankton) that acts like a pump, pushing more carbon dioxide toward the chloroplast. The more carbon dioxide the chloroplast receives, the more sugars it builds and the more oxygen it releases. This pumping mechanism helps explain why certain phytoplankton species are so extraordinarily productive relative to their size.
Where Ocean Oxygen Production Happens
Almost all of this oxygen production takes place in the euphotic zone, the uppermost layer of the ocean where sunlight can penetrate. In clear tropical waters, that zone can extend down to about 100 meters. In murky coastal waters, it may be only a few meters deep. Most primary production is concentrated in the upper half of this lit zone, meaning the bulk of the ocean’s oxygen comes from a remarkably thin skin of water at the surface.
Below a certain depth, called the compensation depth, phytoplankton consume more oxygen through their own respiration than they produce through photosynthesis. This depth shifts depending on water clarity and nutrient availability. In nutrient-poor tropical seas, the water is crystal clear because there are fewer phytoplankton to cloud it, and the compensation depth sits deeper. In nutrient-rich coastal areas, dense blooms make the water turbid but also churn out enormous quantities of oxygen near the surface.
One Tiny Bacterium, Outsized Impact
Not all phytoplankton contribute equally. Prochlorococcus, the smallest photosynthetic organism on Earth, is responsible for up to 20% of all oxygen produced in the biosphere. That single genus of bacteria, invisible to the naked eye, outperforms all the world’s tropical rainforests combined. Prochlorococcus thrives across vast stretches of the tropical and subtropical ocean, and its sheer abundance (estimated in the billions of cells per liter of seawater) compensates for its microscopic size.
Diatoms, which are larger and encased in glass-like silica shells, are another major contributor. They dominate in nutrient-rich waters, particularly in polar and coastal regions where upwelling currents bring iron, nitrogen, and other fertilizers to the surface.
Not All That Oxygen Reaches the Atmosphere
The ocean produces enormous amounts of oxygen, but a large share of it never leaves the water. Marine organisms consume it almost as fast as it’s made. Phytoplankton themselves use about a third of the oxygen consumed in the ocean through their own respiration. Bacteria breaking down dead organic matter account for another third. Single-celled predators called protozooplankton take roughly a quarter. Larger zooplankton, like copepods and krill, contribute a comparatively small slice.
This cycle of production and consumption is tightly coupled. When phytoplankton bloom in spring, oxygen levels spike in surface waters. When those blooms die and sink, bacteria at depth ramp up decomposition, pulling dissolved oxygen out of deeper water. The net oxygen that escapes into the atmosphere represents the small surplus left after the ocean’s own food web takes its cut.
The Biological Pump and Carbon Dioxide
Oxygen production and carbon removal are two sides of the same coin. When phytoplankton photosynthesize, they lock carbon from dissolved CO₂ into their cells. Some of that carbon gets eaten and recycled in the surface ocean, but a fraction sinks as dead cells, fecal pellets, and other organic debris. This downward flow of carbon is called the biological pump, and it effectively moves carbon from the atmosphere into the deep ocean, where it can remain for centuries.
The strength of this pump depends on how much phytoplankton grow in the first place, which comes down to nutrients and light. In vast regions of the Southern Ocean and equatorial Pacific, surface waters are rich in nitrogen but starved of iron, creating what scientists call high-nutrient, low-chlorophyll zones. Experiments adding tiny amounts of iron to these waters have consistently triggered surges in phytoplankton growth, biomass, and nutrient uptake. During ice ages, windblown dust delivered far more iron to the Southern Ocean, likely fueling stronger phytoplankton growth and drawing down atmospheric CO₂ to the low levels seen in ice core records.
When Plankton Remove Oxygen Instead
Plankton can also cause oxygen crises. When excess nutrients from agricultural runoff or sewage pour into coastal waters, they can trigger massive algal blooms. While the bloom is alive, oxygen levels may actually rise. The problem comes after. When the bloom collapses, the dead algae sink and bacteria consume them in a frenzy of decomposition that strips dissolved oxygen from the water. The result is hypoxia: oxygen levels too low to support fish, crabs, shrimp, and most other marine life.
These oxygen-depleted areas, often called dead zones, have been expanding worldwide. The Gulf of Mexico’s dead zone, fed by nutrient runoff from the Mississippi River basin, is one of the most studied examples. In this way, the same organisms that sustain the oxygen cycle can also disrupt it when nutrient pollution pushes the system out of balance.
Plankton Built the Atmosphere We Breathe
The relationship between plankton and oxygen stretches back billions of years. Cyanobacteria, the ancestors of modern phytoplankton, were among the first organisms on Earth capable of oxygenic photosynthesis. They likely originated more than 3 billion years ago, but for hundreds of millions of years, the oxygen they produced was absorbed by iron and other reactive minerals in the ocean and on land.
Then, between 2.5 and 2.3 billion years ago, cyanobacteria finally overwhelmed those chemical sinks. Oxygen flooded the atmosphere in what geologists call the Great Oxidation Event. Atmospheric oxygen rose from trace levels to concentrations that, while still far below today’s 21%, were enough to transform the planet’s chemistry. This shift made possible the eventual emergence of complex, oxygen-breathing life. Every animal that has ever lived owes its existence to that ancient microbial breakthrough.
Climate Change and Falling Production
Rising ocean temperatures are already reshaping phytoplankton communities, and the outlook for oxygen production is concerning. A 2025 study published in Nature Microbiology projected that warming tropical oceans could reduce Prochlorococcus production by 17 to 51% in the tropics. While warming may allow Prochlorococcus to expand into higher latitudes that were previously too cold, this habitat shift won’t compensate for tropical losses. Global Prochlorococcus production is projected to fall by 10 to 37%, depending on how aggressively greenhouse gas emissions continue.
Because Prochlorococcus alone accounts for up to a fifth of global oxygen production, declines of that magnitude would ripple through both the oxygen and carbon cycles. Warmer water also holds less dissolved oxygen to begin with, compounding the problem. The combination of reduced production and reduced oxygen solubility means the ocean’s role in the oxygen cycle could weaken substantially over the coming decades.