Roughly half of the oxygen produced by photosynthesis each year comes from the ocean. The producers are not kelp forests or seagrass meadows alone, but an immense community of drifting algae and photosynthetic bacteria called phytoplankton.
There is a qualification hidden inside that clean statistic. The best global estimates put ocean and land production in the same broad range. Some agency summaries say the ocean produces at least half, while a landmark global estimate found roughly equal contributions from land and sea. “More than every tree and forest combined” is therefore the upper end of a moving estimate, not the result of a precise planetary contest.
I still find the comparison useful. It turns our attention from the most visible producers on Earth to organisms that are mostly invisible, dispersed through the light-filled surface ocean and constantly changing with the seasons.
The 50 percent figure describes a flow
NOAA estimates that roughly half of oxygen production on Earth comes from the ocean. Its wording matters. This is a flow of oxygen made through photosynthesis, not a claim that half of the oxygen presently stored in the atmosphere was manufactured by today’s plankton.
The atmosphere contains an enormous oxygen reservoir accumulated over hundreds of millions of years. A molecule in the breath you take now cannot be assigned neatly to a forest or plankton bloom operating this year.
The comparison is commonly estimated through primary production, the rate at which photosynthetic organisms turn carbon dioxide into organic matter. Oxygen is released during that chemistry, so estimates of carbon production help researchers calculate the associated oxygen flow. In a widely cited 1998 Science paper, Christopher Field, Michael Behrenfeld, James Randerson and Paul Falkowski estimated global net primary production at 104.9 petagrams of carbon per year, with land and ocean making roughly equal contributions. A petagram is one billion metric tons.
That is why “roughly half” is the sturdy number. Whether the ocean is slightly above or below half depends on the period, observations and model used.
Phytoplankton are not one kind of organism
The word phytoplankton sounds like a tidy biological group. It is closer to a description of how an organism lives. These are photosynthetic organisms carried by currents rather than swimming strongly against them. They include diatoms with glass-like silica shells, coccolithophores covered in calcium carbonate plates, other single-celled algae, and cyanobacteria.
They are small, but the ocean is large. Sunlight reaches only the upper ocean, yet that illuminated habitat covers most of the planet. Across it, uncountable cells use chlorophyll and other pigments to capture light, make organic molecules and release oxygen.
NASA’s guide to phytoplankton describes them as the base of the aquatic food web. Tiny grazers consume them, fish feed through that web, and some of the carbon fixed near the surface eventually sinks. The oxygen statistic is striking, but it is only one part of their ecological work.
Satellites see color, not oxygen molecules
No instrument hovers above Earth counting oxygen molecules as they leave plankton cells. Researchers combine shipboard samples, laboratory measurements, fixed observing stations, satellite data and models.
Phytoplankton contain pigments that alter how seawater absorbs and reflects light. When enough cells gather, a bloom can color the sea green, turquoise, brown or milky blue. Satellites measure that reflected light, allowing scientists to estimate chlorophyll concentration and plankton biomass across areas too large to sample from ships alone.
NASA’s PACE satellite, launched in February 2024, provides a more detailed view. Its Ocean Color Instrument measures a broad spectrum of light, helping researchers distinguish more of the pigments associated with different plankton communities. A 2025 NASA Earth Observatory view of the Norwegian Sea, for example, shows surface water streaked by blooms in green and milky blue.
Ocean color is still a proxy, not a direct oxygen meter. Clouds block the view. Some plankton live below the depth a satellite can see. The relationship between chlorophyll and photosynthetic output also changes with light, nutrients, temperature and the organisms present. Global percentages carry uncertainty even when the overall scale is well established.
The rainforest comparison needs care
Trees produce oxygen. A growing forest can also store carbon in wood and soils. Nothing about the plankton estimate makes a rainforest expendable or less ecologically important.
The complication is that gross production and net addition are different. Plants respire as well as photosynthesize. Animals, fungi and microbes consume organic matter, using oxygen and returning carbon dioxide. In a mature forest, a great deal of the oxygen released by leaves is eventually used again when organisms breathe and dead material decomposes.
The Science Panel for the Amazon’s review of the region’s biogeochemical cycles describes the forest carbon balance as the result of very large opposing flows. Photosynthesis brings carbon into the ecosystem, while plant metabolism and decomposition send much of it back. The report estimates that the Amazon accounts for around 16 percent of the terrestrial biosphere’s metabolism. It also emphasizes the forest’s huge carbon stores and its role in rainfall. “Lungs” is simply not a precise description of those functions.
The metaphor is anatomically odd, too. Lungs take oxygen from the air and release carbon dioxide. Forests and phytoplankton do both, depending on whether we are considering photosynthesis or respiration.
Most ocean-made oxygen is used again
The same accounting applies at sea. NOAA notes that roughly as much oxygen is consumed by marine life and decay as is produced in the ocean. Phytoplankton respire. Zooplankton eat them. Bacteria decompose dead cells. Each process uses oxygen.
A lasting addition to atmospheric oxygen requires some reduced material, particularly organic carbon, to escape oxidation for a long time. If a small fraction of dead plankton sinks and becomes buried in sediment before it is decomposed, the corresponding oxygen can remain behind. Over geological timescales, burial and chemical reactions helped build an oxygen-rich atmosphere. The annual production figure does not mean the ocean refills half of that atmospheric reservoir each year.
This distinction also explains why a large bloom is not automatically good news. When an algal bloom dies, rapid decomposition can remove oxygen from the surrounding water and create hypoxic conditions. The cells produced oxygen while growing, then left behind biomass whose breakdown created a large oxygen demand.
The cycle does not divide neatly
I recently wrote about how Earth’s five named oceans form one connected system. Oxygen offers another reason the divisions on a map can mislead. Currents move nutrients and organisms, winds carry material between land and sea, and the carbon and oxygen cycles connect forests, oceans, soils and the atmosphere.
Phytoplankton populations move as well. They rise and fall with light, mixing, temperature, grazing and supplies of nitrogen, phosphorus, iron and other nutrients. Productive coastal and upwelling waters can sit beside the nutrient-poor centers of subtropical oceans. The ocean is not one uniform green factory.
The narrow, defensible summary is that marine photosynthesis accounts for about half of Earth’s annual photosynthetic oxygen production, with estimates varying around that midpoint. The organisms responsible are easy to overlook because most are microscopic and their habitat can look empty from the shore.
Correcting the rainforest metaphor does not require replacing it with another metaphor. It requires noticing the scale of the living system spread through the sunlit sea.
