Roughly 70% of the oxygen in Earth’s atmosphere comes from the ocean, produced by microscopic photosynthetic organisms collectively called phytoplankton. The remaining share comes from land plants, with tropical and boreal forests contributing the bulk of terrestrial production. That ratio surprises most people, who picture towering trees as the planet’s main oxygen factories, but the sheer scale of the ocean and the astronomical number of single-celled organisms living in its sunlit upper layers make the math lopsided in the sea’s favor.
Why Tiny Ocean Organisms Dominate
Phytoplankton are photosynthetic microbes that drift in the upper ocean wherever sunlight penetrates. They include cyanobacteria, diatoms, dinoflagellates, and a range of other single-celled or colonial organisms. Despite being individually invisible to the naked eye, they occupy a habitat that covers about 71% of Earth’s surface. That enormous area, combined with fast reproduction rates, means phytoplankton collectively fix carbon and release oxygen on a scale that dwarfs any single terrestrial ecosystem. Mathematical models of plankton-oxygen dynamics estimate that about 70% of atmospheric oxygen traces back to this marine photosynthesis.1PubMed. Mathematical Modelling of Plankton-Oxygen Dynamics Under the Climate Change
One way to appreciate the scale: a recent global budget of upper-ocean dissolved oxygen estimated that net biological oxygen production in the ocean runs about 889 trillion moles of O₂ per year.2Communications Earth & Environment. Global upper ocean dissolved oxygen budget for constraining the biological carbon pump That oxygen does not all end up in the air. Much of it is consumed by marine organisms and by the decomposition of sinking organic matter. But the gross output is staggering, and even after accounting for what gets used up underwater, the net contribution to the atmosphere remains the planet’s largest single source.
The Star Players Among Phytoplankton
Not all phytoplankton contribute equally. Among the most important are cyanobacteria in the genus Prochlorococcus, often called the smallest and most abundant photosynthetic organism on Earth. A single milliliter of surface seawater in the tropics can contain 100,000 or more of these cells. Different strains of Prochlorococcus are adapted to different light levels and oxygen concentrations: some thrive only in bright surface waters with longer day lengths, while others can photosynthesize under the dim light found deeper in the water column.3PubMed Central. Prochlorococcus marinus responses to light and oxygen This niche partitioning means that Prochlorococcus collectively covers a wide vertical range, capturing photons that other organisms miss.
Diatoms are another major group. These single-celled algae build intricate silica shells and tend to dominate in nutrient-rich waters, especially during seasonal blooms in temperate and polar oceans. Spring blooms of diatoms in the North Atlantic, for instance, drive massive pulses of oxygen production as nutrients mixed up by winter storms fuel explosive growth.4Deep Sea Research Part I: Oceanographic Research Papers. Estimates of net community production and export using high-resolution, Lagrangian measurements of O2, NO3−, and POC through the evolution of a spring diatom bloom in the North Atlantic These blooms are so large they are visible from space as green swirls in satellite imagery.
The ocean’s biological pump is not uniform, though. In situ measurements from profiling floats in the five subtropical gyres show clear autotrophy (net oxygen and carbon production) in the North Pacific and North Atlantic, while parts of the South Indian Ocean hover near zero or even tip toward net consumption.5Geophysical Research Letters. The Ocean’s Biological Pump: In Situ Oxygen Measurements in the Subtropical Oceans So “the ocean produces most of Earth’s oxygen” is true globally, but the contribution is patchy, concentrated in regions where nutrients, light, and circulation align.
What About Forests and Land Plants?
Terrestrial photosynthesis accounts for roughly the remaining 30% of global oxygen production. Tropical forests, boreal forests, grasslands, and agricultural crops all contribute. Trees photosynthesize at impressive rates during their growing seasons, and a single large tree can release enough oxygen for several people over the course of a year. But here is the catch that makes the popular framing misleading: mature ecosystems tend to consume nearly as much oxygen through respiration and decomposition as they produce through photosynthesis.
The Amazon rainforest is the case people bring up most often. It is frequently called “the lungs of the Earth,” implying it is a net oxygen supplier of outsized importance. In reality, modeling and field measurements of old-growth stands in the Amazon show that undisturbed forest stays within a narrow band of carbon neutrality, acting as either a small carbon sink or a small carbon source depending on the year and disturbance history.6Ecological Monographs. Modeling the carbon balance of Amazonian rain forests: resolving ecological controls on net ecosystem productivity Carbon neutrality means oxygen neutrality: if a forest absorbs about as much CO₂ as it releases, it also consumes about as much O₂ as it produces. The gross photosynthesis is enormous, but the net atmospheric contribution from a mature, undisturbed tropical forest is small.
Young, actively growing forests are different. A newly planted forest or one recovering from disturbance is building biomass, locking carbon into wood, and releasing surplus oxygen that is not immediately consumed. This is one reason reforestation programs genuinely do increase atmospheric oxygen over the medium term, even though mature forests eventually approach equilibrium.
Overlooked Land Contributors
Forests get most of the attention, but other land ecosystems also matter. Boreal mosses, for example, carpet vast stretches of the northern taiga and photosynthesize actively from spring through late summer, reaching peak rates in late August as young tissue accumulates.7PubMed. Moss functioning in different taiga ecosystems in interior Alaska: I. Seasonal, phenotypic, and drought effects on photosynthesis and response patterns Individually modest, mosses collectively cover enough area in northern latitudes to make a meaningful contribution. Grasslands, peatlands, and croplands round out the terrestrial picture. The takeaway is that no single land biome dominates in the way phytoplankton dominate the ocean; terrestrial oxygen production is spread across many ecosystem types.
Kelp Forests and Seagrass Meadows
Between the open ocean and the land sits a band of highly productive coastal ecosystems. Kelp forests, especially dense stands of large brown algae, produce substantial amounts of oxygen per unit area. In macro-tidal environments, in situ measurements of kelp productivity have recorded net production rates reaching about 9 g of carbon per square meter per hour at saturating light levels during late summer.8Limnology and Oceanography. In situ estimates of kelp forest productivity in macro‐tidal environments That is extraordinarily high compared to open-ocean averages, though kelp forests occupy a tiny fraction of total ocean area. Their oxygen contribution matters locally but is modest in the global budget.
Seagrass meadows also punch above their weight. Posidonia oceanica, the dominant seagrass in the Mediterranean, functions as a net autotrophic ecosystem year-round, with its highest oxygen productivity in spring driven mostly by the benthic (bottom-dwelling) compartment rather than the water column above it.9PubMed. Water column oxygenation by Posidonia oceanica seagrass meadows in coastal areas: A modelling approach Eelgrass (Zostera marina) meadows in temperate waters show a similar pattern, with above-ground tissues supplying oxygen to the water column while below-ground roots and rhizomes are net oxygen consumers.10PubMed. Oxygen allocation dynamics in a seagrass Zostera marina meadow These coastal habitats are more important for local water-column oxygenation and carbon sequestration than for the global atmospheric budget, but their loss through coastal development and warming would still have cascading ecological effects.
Where Oxygen Production Creates Its Own Problem
One of the more counterintuitive aspects of ocean oxygen is that areas with high productivity can end up oxygen-starved at depth. In upwelling zones where nutrient-rich deep water rises to the surface, phytoplankton bloom prolifically. The organic matter that sinks from these blooms is then broken down by bacteria, a process that consumes dissolved oxygen in deeper water. This microbial respiration drives the expansion of oxygen minimum zones (OMZs), bands of water at intermediate depths where oxygen levels drop so low that most marine animals cannot survive.11Geosystems and Geoenvironment. Is the expansion of oxygen minimum zones impacting the health of modern ocean basins? A review
Sediment records from the Arabian Sea illustrate this dynamic on long timescales: productivity was enhanced during glacial periods and suppressed during interstadials, and OMZ intensity tracked those shifts closely.12Earth and Planetary Science Letters. Variations of oxygen-minimum and primary productivity recorded in sediments of the Arabian Sea The same pattern plays out today off the coasts of Peru, West Africa, and the Arabian Peninsula. The surface waters there are among the most productive on Earth, but the depths below are some of the most oxygen-depleted. High oxygen production and low deep-water oxygen can coexist in the same water column.
A parallel effect shows up in shallow freshwater systems. In densely vegetated lakes, photosynthesis saturates surface waters with oxygen during daylight while dark bottom waters become anoxic from respiration, creating extreme daily swings in dissolved oxygen.13PubMed Central. Extreme diel dissolved oxygen and carbon cycles in shallow vegetated lakes These cycles show that oxygen production is always paired with oxygen consumption. What matters for the atmosphere is the net balance after all the respiration and decomposition are subtracted.
How Cyanobacteria Built the Atmosphere We Breathe
The dominance of photosynthetic microbes in oxygen production is not new. Cyanobacteria, the ancestors of today’s phytoplankton (and the chloroplasts inside every plant cell), were the first organisms to evolve oxygenic photosynthesis. Geological evidence suggests they were producing oxygen as early as about 2.7 billion years ago, based on stromatolites and biomarker molecules preserved in ancient lake sediments.14PubMed Central. When did oxygenic photosynthesis evolve? For hundreds of millions of years, that oxygen was absorbed by reactions with iron and other reduced minerals rather than accumulating in the air.
The Great Oxidation Event, roughly 2.4 billion years ago, marked the point when oxygen finally began to build up in the atmosphere. Molecular clock analyses suggest cyanobacteria had already evolved multicellularity before this event, which likely boosted their fitness and abundance enough to overwhelm the geological sinks that had been soaking up oxygen.15PubMed Central. Cyanobacteria and the Great Oxidation Event: evidence from genes and fossils The scale of oxygen production over geological time is hard to fathom: Earth’s sedimentary shell now contains enough buried organic carbon to account for the equivalent of about 33 modern atmospheres’ worth of oxygen.16Earth and Planetary Science Letters. Atmospheric oxygenation driven by unsteady growth of the continental sedimentary reservoir In other words, photosynthetic organisms have produced vastly more oxygen over Earth’s history than currently exists in the air; the rest was consumed by geological and biological processes along the way.
Climate Change and the Future of Ocean Oxygen
If the ocean is the planet’s primary oxygen factory, the question of whether climate change could slow that factory down matters a great deal. The short answer is that warming is projected to reduce marine net primary productivity, though the effect is moderate in most scenarios and geographically uneven.
Earth system model projections suggest that under high-emissions pathways, global marine net primary productivity could decline by roughly 3 to 4% by the end of this century.17Global Biogeochemical Cycles. Avoidable impacts of ocean warming on marine primary production: Insights from the CESM ensembles Under more aggressive mitigation scenarios, the decline drops to about 1 to 2%. The mechanism involves warmer surface waters forming a more stable layer that reduces the upward mixing of nutrients from deeper water. Less nutrient supply means less phytoplankton growth. The response is not uniform, though: polar regions and parts of the equatorial Pacific may see increases in productivity as ice retreats and circulation patterns shift, while the Atlantic, western Pacific, and Indian Ocean see declines.17Global Biogeochemical Cycles. Avoidable impacts of ocean warming on marine primary production: Insights from the CESM ensembles
Multi-model projections from the latest generation of climate models paint a broadly consistent picture. Under the highest emissions scenario, end-of-century ocean net primary production is projected to decline by about 3%, alongside a surface temperature rise of roughly 3.5°C, a substantial drop in surface pH, and a decline of about 13 micromoles per kilogram in subsurface dissolved oxygen.18Biogeosciences. Twenty-first century ocean warming, acidification, deoxygenation, and upper-ocean nutrient and primary production decline from CMIP6 model projections Under strong mitigation, the productivity decline shrinks to less than 1%. These numbers may sound small, but even a few percent reduction in the ocean’s productivity applies to a base that is enormous. And the combination of lower production, warmer water holding less dissolved oxygen, and expanding oxygen minimum zones creates compounding stress on marine ecosystems.
The composition of phytoplankton communities is shifting too. Models suggest diatoms, the larger and heavier cells that drive much of the biological carbon pump, are declining in productivity, while smaller phytoplankton groups are increasing.17Global Biogeochemical Cycles. Avoidable impacts of ocean warming on marine primary production: Insights from the CESM ensembles A shift toward smaller cells means organic carbon is recycled faster in surface waters rather than sinking to depth. That changes how much carbon, and by extension how much oxygen production, actually escapes the surface ocean’s recycling loop.
How Scientists Track Oxygen Production Across the Ocean
Measuring oxygen production in the ocean is harder than it sounds. You cannot simply stick a sensor in the water at one point and extrapolate to the whole planet. Traditionally, researchers incubated bottles of seawater on deck and measured how much oxygen accumulated in the light versus how much disappeared in the dark. This works but introduces artifacts: the organisms are confined, the light conditions are artificial, and the water stops mixing. Over the past decade, an incubation-free technique using the triple isotopic composition of dissolved oxygen has become a powerful complement. Because photosynthesis, respiration, and air-sea gas exchange leave different isotopic fingerprints on dissolved oxygen, measuring the ratios of oxygen-17 and oxygen-18 relative to oxygen-16 lets researchers estimate gross photosynthetic oxygen production without ever putting seawater in a bottle.19PubMed. Using triple isotopes of dissolved oxygen to evaluate global marine productivity
Autonomous profiling floats have expanded coverage further. Thousands of these robotic instruments now drift through the world’s oceans, rising and sinking every few days to measure temperature, salinity, and dissolved oxygen from the surface to depths of 2,000 meters. By tracking oxygen changes in the upper ocean and accounting for physical fluxes like air-sea gas exchange and mixing, researchers can estimate net biological oxygen production across entire ocean basins.2Communications Earth & Environment. Global upper ocean dissolved oxygen budget for constraining the biological carbon pump This combination of isotopic tracers, autonomous floats, and satellite observations of ocean color is what gives scientists confidence in the 70% figure, though the uncertainty bars remain wide enough that the true number could be somewhat higher or lower.
Kelp Productivity and the Challenge of Murky Water
Estimating productivity for coastal ecosystems like kelp forests introduces its own set of headaches. Turbid coastal waters mean that the amount of light reaching kelp blades varies dramatically over short distances and timescales. Researchers studying Laminaria hyperborea, the dominant kelp species in the northeastern Atlantic, found that variable underwater light and fluctuating photosynthetic parameters made it difficult to scale up from individual blade measurements to whole-forest productivity estimates.20PubMed. The challenge of estimating kelp production in a turbid marine environment Tidal cycles compound the problem: as the tide rises and falls, the depth of water above the kelp canopy changes, altering light availability on a schedule of hours. These challenges mean that global estimates of kelp forest oxygen production carry more uncertainty per unit area than open-ocean phytoplankton estimates, even though the per-area productivity of kelp can be an order of magnitude higher.
This uncertainty matters because kelp forests are declining in many regions due to warming, overgrazing by sea urchins, and pollution. If their true productivity is at the high end of current estimates, losses could have greater consequences for local oxygen budgets and carbon cycling than previously appreciated. Conversely, if kelp forests are less productive than peak measurements suggest, conservation arguments need to lean more on their role as habitat and biodiversity hotspots than on their oxygen output.