Nearly all of Earth’s atmospheric oxygen comes from photosynthesis, the process by which organisms use sunlight to split water molecules and release oxygen gas as a byproduct. The ocean is the single largest source, with marine phytoplankton generating roughly half of the oxygen produced each year, while land plants account for the other half. But the full story is richer than that split suggests. The balance between oxygen production and consumption involves geology, chemistry, microbial metabolism in places sunlight never reaches, and a deep history stretching back billions of years.
The Ocean’s Tiny Powerhouses
When people think about oxygen, they tend to picture forests. Rainforests get called “the lungs of the Earth” in popular shorthand. But the ocean’s contribution is at least as large, and the organisms responsible are mostly invisible to the naked eye. Marine phytoplankton, single-celled photosynthetic organisms drifting in the sunlit surface layer of the ocean, collectively produce an enormous share of global oxygen. Among these, one genus stands out for punching far above its weight.
Prochlorococcus marinus is the smallest known photosynthetic organism and the most abundant on the planet. Despite each cell being less than a micrometer across, Prochlorococcus contributes roughly 13 to 48 percent of net primary production in the nutrient-poor open ocean, which corresponds to about 30 percent of global oxygen production.1PubMed Central. Prochlorococcus marinus responses to light and oxygen That range is wide because its contribution varies with season, latitude, and nutrient availability, but even the conservative end of the estimate is staggering for a single genus of bacteria.
Diatoms are another major group. These are eukaryotic phytoplankton encased in intricate glass-like shells made of silica, and they tend to dominate in nutrient-rich upwelling zones and at high latitudes. Diatoms are especially important during seasonal blooms, when their populations explode and they become the primary engine of both oxygen production and carbon export in their region.2Philosophical Transactions of the Royal Society B: Biological Sciences. The evolution of diatoms and their biogeochemical functions: Evolution and impact of ocean diatoms Beyond diatoms and Prochlorococcus, a diverse cast of other cyanobacteria, dinoflagellates, and coccolithophores contribute to the ocean’s total photosynthetic output.
Land Plants and the Other Half
Terrestrial vegetation accounts for roughly the other half of global photosynthetic oxygen. Tropical forests, boreal forests, grasslands, and croplands all contribute. But land-based oxygen production comes with an important caveat that trips people up: most of the oxygen a mature forest produces during the day is consumed at night and during decomposition. When a tree dies and its wood rots, microbes break down the organic carbon and use up oxygen in the process. The net oxygen contribution from a mature, stable forest is close to zero over long timescales.
This does not mean forests are unimportant for atmospheric oxygen. What it means is that the oxygen currently in the atmosphere was not produced last year; it accumulated over geological time because a fraction of organic carbon was buried in sediments before it could decompose. That burial is the key mechanism that keeps free oxygen in the atmosphere, and it connects the ocean and land in a shared geochemical cycle that matters far more than any single growing season.
Why Burial Matters More Than Production
Here is the part that surprises most people. The total amount of oxygen produced by photosynthesis each year is enormous, but almost all of it is consumed again. Animals breathe it. Bacteria use it to decompose organic matter. Fires burn it. The net oxygen that stays in the atmosphere over geological time depends not on how fast organisms produce it but on how much organic carbon gets buried in sediments and locked away from decomposition. Every atom of carbon that sinks to the seafloor and gets trapped in rock is one molecule of oxygen that never gets used up.
The long-term history of atmospheric oxygen tracks closely with the history of organic carbon burial in sedimentary deposits.3Earth and Planetary Science Letters. Atmospheric oxygenation driven by unsteady growth of the continental sedimentary reservoir When burial rates increase, oxygen accumulates. When burial slows or tectonic activity exposes old carbon-rich rocks to weathering, oxygen gets consumed. This is why the oxygen content of the atmosphere has fluctuated substantially over Earth’s history rather than following a steady climb.
Pyrite burial provides a parallel mechanism. When iron reacts with sulfur in oxygen-poor sediments to form pyrite (iron sulfide), that process also effectively removes reducing agents from the system, allowing oxygen to persist in the atmosphere.4PubMed Central. Sedimentary conditions drive modern pyrite burial flux to exceed oxidation Carbon burial and pyrite burial together are the two main geological brakes that keep the atmosphere oxygenated.
The ocean plays a central role in carbon burial because its biological carbon pump transfers organic matter from the sunlit surface to the deep ocean through sinking particles, vertical mixing, and the daily migrations of zooplankton. Carbon sequestered in deep water can remain there for centuries before returning to the surface.5Global Biogeochemical Cycles. Quantifying the Carbon Export and Sequestration Pathways of the Ocean’s Biological Carbon Pump When some of that carbon ends up permanently buried in seafloor sediment, the oxygen it once paired with stays free in the atmosphere.
Cyanobacteria and the Great Oxidation Event
The oxygen in our atmosphere today exists because of an event that unfolded over hundreds of millions of years, beginning around 2.4 billion years ago. Before that, Earth’s atmosphere contained essentially no free oxygen. The organisms that changed everything were cyanobacteria, the ancestors of modern blue-green algae and the distant relatives of the chloroplasts inside every plant cell.
Molecular clock studies estimate that oxygenic photosynthesis originated around 3.6 billion years ago, roughly a billion years before oxygen finally began accumulating in the atmosphere.6PubMed Central. Timing the Evolutionary Advent of Cyanobacteria and the Later Great Oxidation Event Using Gene Phylogenies of a Sunscreen During that billion-year gap, cyanobacteria were producing oxygen, but it was immediately consumed by reactions with dissolved iron, volcanic gases, and other reducing agents. Oxygen existed only in scattered local pockets, so-called “oxygen oases,” rather than as a global atmospheric gas.
The shift to a permanently oxygenated atmosphere, known as the Great Oxidation Event, was likely driven in part by cyanobacteria evolving multicellularity. Genetic analyses suggest that key multicellularity genes appeared in cyanobacteria around 2.6 to 2.7 billion years ago, just before the onset of the Great Oxidation Event.7PubMed Central. Evolution of multicellularity genes in Cyanobacteria in the lead up to the great oxidation event Multicellular cyanobacteria can form mats, filaments, and specialized cells, which likely increased their ecological dominance and overall oxygen output.8PubMed Central. Cyanobacteria and the Great Oxidation Event: evidence from genes and fossils Once cyanobacterial oxygen production finally outpaced the planet’s capacity to soak it up through chemical reactions, free oxygen began accumulating in the atmosphere and transformed Earth’s chemistry forever.
How Oxygen Levels Have Swung Over Time
After the Great Oxidation Event, oxygen did not simply rise to modern levels and hold steady. The atmosphere went through dramatic swings. For much of the Proterozoic eon (roughly 2.4 billion to 540 million years ago), oxygen concentrations were likely only a small fraction of current levels. The deep ocean remained anoxic for most of that period. Evidence from deep-sea sediments shows the deep ocean was still oxygen-free and iron-rich right up until about 580 million years ago, just before the first large, complex animals appeared in the fossil record.9PubMed. Late-Neoproterozoic deep-ocean oxygenation and the rise of animal life The timing suggests that the oxygenation of the deep ocean may have been a prerequisite for animal evolution.
Later, during the Carboniferous period (about 360 to 300 million years ago), atmospheric oxygen likely peaked well above today’s roughly 21 percent, perhaps reaching 30 percent or higher. This hyperoxic environment appears to have enabled the evolution of giant insects, including dragonfly relatives with wingspans approaching 70 centimeters.10PubMed Central. Environmental and biotic controls on the evolutionary history of insect body size Insects deliver oxygen through a network of tiny tubes called tracheae, and higher ambient oxygen would have allowed those tubes to supply larger bodies without running into physical limits.11PubMed Central. Increase in tracheal investment with beetle size supports hypothesis of oxygen limitation on insect gigantism
The Carboniferous oxygen spike resulted from a period of unusually high organic carbon burial. Vast coastal swamp forests grew, died, and were buried in waterlogged sediments before they could fully decompose, eventually forming coal deposits. With so much carbon locked underground, the oxygen released during photosynthesis accumulated. Oxygen levels subsequently declined during the Mesozoic, and modeling suggests that the spread of flowering plants and their interactions with fire played a role. Increased wildfire activity suppressed large land biomass and reduced the amount of phosphorus washing into the ocean, which lowered global carbon burial and pulled oxygen levels down from their Cretaceous highs.12Nature Communications. The rise of angiosperms strengthened fire feedbacks and improved the regulation of atmospheric oxygen
Oxygen Without Sunlight
Photosynthesis dominates global oxygen production, but it is not the only game in town. Researchers have recently documented “dark oxygen” production in environments where no light penetrates, challenging the assumption that oxygen is always a product of photosynthesis.
In deep groundwater systems in Alberta, Canada, geochemical and metagenomic analyses revealed that oxygen is being produced underground by microbial processes, specifically through the dismutation of compounds like chlorite, nitric oxide, and hydrogen peroxide. The oxygen isotope signatures of dissolved gas in these old, deep groundwaters were too light to have originated from the atmosphere, pointing to in-situ production from local water molecules.13Nature Communications. Hydrogen and dark oxygen drive microbial productivity in diverse groundwater ecosystems Genes for chlorite dismutase and nitric oxide dismutase were found across multiple microbial species in these aquifers.
Separate laboratory work has shown that dark oxygen production through nitric oxide dismutation is common among ammonia-oxidizing archaea and bacteria, organisms that are among the most abundant microbes on Earth. When oxygen runs out, these organisms can generate their own supply internally, which may explain why they thrive in oxygen-depleted environments where you would not expect them to survive.14PubMed Central. Oxygen production via nitric oxide dismutation in diverse ammonia oxidizers The quantities of dark oxygen produced are tiny compared to photosynthesis at a global scale, but the discovery has broad implications. It means oxygen can exist in environments cut off from sunlight entirely, which matters for understanding subsurface ecosystems on Earth and for interpreting signs of oxygen on other worlds.
Water radiolysis is another abiotic oxygen source. When naturally occurring radioactive elements like potassium-40 emit radiation into surrounding water, they split water molecules into hydrogen and oxygen. This process has been occurring since Earth’s earliest oceans and also takes place in underground water near radioactive minerals.15Radiation Physics and Chemistry. Radiolysis of water: a look at its origin and occurrence in the nature On modern Earth, radiolytic oxygen production is negligible compared to biological sources, but it may have been one of the few oxygen sources available in the planet’s first billion years.
The Photosynthetic Machinery Itself
The molecular machine that splits water is called Photosystem II, and it sits in the membranes of every photosynthetic organism from cyanobacteria to oak trees. At its core is a cluster of four manganese atoms, one calcium atom, and five oxygen atoms. This metal cluster strips electrons from water molecules in a four-step cycle, releasing protons and molecular oxygen as byproducts.
The water-splitting step is remarkably specific. Quantum mechanical and molecular dynamics studies show that two substrate water molecules coordinate to the manganese-calcium cluster as terminal ligands. One becomes an oxygen radical, and the other, activated by nearby amino acids, attacks it in a concerted reaction that forms the oxygen-oxygen bond.16PubMed. Quantum mechanics/molecular mechanics study of the catalytic cycle of water splitting in photosystem II The protons released during this cycle are shuttled away through a specific channel involving conserved amino acid residues that act as molecular gates and local proton storage sites.17PubMed Central. Mechanism of proton release during water oxidation in Photosystem II This is one of the most energetically demanding reactions in biology, which is why it requires the energy of sunlight to drive it.
Every molecule of oxygen you breathe was once part of a water molecule that passed through a Photosystem II complex somewhere on Earth. The fact that one molecular machine, conserved across billions of years, is responsible for essentially all biological oxygen production makes it one of the most consequential innovations in the history of life.
Oxygen on Other Planets and False Positives for Life
Earth’s oxygen-rich atmosphere is one of the most detectable features that an alien astronomer could spot from light-years away, and the search for similar oxygen signatures on exoplanets is a major goal of astrobiology. But the discovery of abiotic oxygen sources has complicated the picture. If oxygen can build up without biology, then detecting oxygen in an exoplanet’s atmosphere is not, by itself, proof of life.
Modeling work shows that planets orbiting dim red dwarf stars can accumulate large amounts of abiotic oxygen. During a star’s early, more luminous phase, intense ultraviolet radiation can photolyze water vapor in a planet’s atmosphere, splitting it into hydrogen and oxygen. The lightweight hydrogen escapes to space, leaving oxygen behind. Under certain conditions, planets with weak surface oxygen sinks could build up hundreds to thousands of times more atmospheric oxygen than Earth has, all without a single living cell.18PubMed Central. Extreme Water Loss and Abiotic O2 Buildup on Planets Throughout the Habitable Zones of M Dwarfs Similar abiotic oxygen accumulation can occur on planets with pure water-vapor atmospheres, where water cannot be cold-trapped and is freely photolyzed by stellar radiation.19The Astrophysical Journal Letters. ABIOTIC OXYGEN-DOMINATED ATMOSPHERES ON TERRESTRIAL HABITABLE ZONE PLANETS
This means astronomers searching for biosignatures need to look at oxygen in context. A comprehensive framework for evaluating oxygen as a biosignature emphasizes that the planetary environment matters: the star’s spectrum, the planet’s atmospheric composition, and the presence or absence of other gases like methane can help distinguish biological oxygen from abiotic accumulation.20PubMed Central. Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment On Earth, the coexistence of oxygen and methane in the atmosphere is a strong thermodynamic disequilibrium that is hard to explain without biology. Finding that same pairing on an exoplanet would be far more compelling than finding oxygen alone.
Venus and Mars offer cautionary tales closer to home. Both planets likely had early oceans that underwent photolysis and hydrogen escape. Modeling estimates that Venus may have lost an ocean equivalent to about 450 meters of average depth, while Mars lost one equivalent to roughly 200 meters, with a substantial fraction of the freed oxygen escaping to space along with the hydrogen or reacting with surface rocks.21Icarus. Hydrodynamic Escape of Oxygen from Primitive Atmospheres: Applications to the Cases of Venus and Mars Neither planet retained enough oxygen to maintain an oxygenated atmosphere, illustrating how planetary size, volcanic activity, and surface chemistry determine whether oxygen sticks around.
Earth’s Mantle and the Slow Oxidation of the Planet
The story of oxygen is not just atmospheric. Earth’s interior has its own oxidation state, and the relationship between mantle chemistry and surface oxygen is an active area of research. Recent work analyzing the oxygen fugacity (a measure of how oxidizing or reducing a melt is) of mantle-derived rocks going back to the Hadean eon found that the mantle’s intrinsic oxidation state has been essentially constant for over four billion years.22PubMed Central. The constant oxidation state of Earth’s mantle since the Hadean What changed over time was not the mantle itself but the depth at which melting occurred. As the mantle cooled, melting happened at shallower depths, which increased the apparent oxidation of the lavas that reached the surface. This gradual shift in volcanic gas composition may have contributed to the conditions that allowed atmospheric oxygenation.
In other words, the rise of atmospheric oxygen was not only about biology producing more oxygen. It also depended on the planet’s deep interior slowly changing the chemistry of the gases it exhaled through volcanoes. A planet with different mantle properties or a different cooling history might never have accumulated a breathable atmosphere, even with photosynthetic life on its surface.
The Future of Earth’s Oxygen
Earth’s oxygenated atmosphere is not permanent. As the Sun slowly brightens over the next billion years, rising temperatures will accelerate the weathering of silicate rocks, which draws down carbon dioxide. With less carbon dioxide, photosynthetic organisms will eventually be starved of the raw material they need. Modeling using stochastic simulations projects that Earth’s atmosphere will remain above one percent of its current oxygen level for roughly another 1.08 billion years, after which oxygen will drop sharply to levels resembling the Archean eon, before life had oxygenated the planet.23Nature Geoscience. The future lifespan of Earth’s oxygenated atmosphere
That collapse is projected to happen before the planet loses its surface water through a runaway greenhouse effect, meaning Earth will likely become uninhabitable for complex aerobic life well before it becomes a scorched, waterless world. The trigger is not a failure of biology per se but the inevitable increase in solar luminosity disrupting the carbonate-silicate cycle that sustains carbon dioxide availability. For astrobiologists, this timeline highlights an uncomfortable reality: oxygen-based biosignatures may be detectable on an Earth-like planet for only a fraction of its total habitable lifetime, making them potentially easy to miss in surveys of exoplanet atmospheres.
Tracking Atmospheric Oxygen in Real Time
Given that oxygen makes up about 20.95 percent of the atmosphere, you might assume changes would be easy to spot. They are not. The variations scientists care about are tiny, on the order of parts per million against that enormous background. Detecting them requires instruments of extraordinary precision. A methodology using modified paramagnetic oxygen analyzers achieved an effective precision of 0.2 parts per million over one-hour measurement intervals, enabling researchers to track seasonal and interannual fluctuations in atmospheric oxygen for the first time.24Global Biogeochemical Cycles. Precise atmospheric oxygen measurements with a paramagnetic oxygen analyzer
These measurements reveal a subtle annual breathing pattern: oxygen rises during the growing season of each hemisphere as photosynthesis dominates, then dips when decomposition and respiration take over. They also show a very slow, long-term decline in atmospheric oxygen, driven primarily by fossil fuel combustion. Burning coal, oil, and gas consumes oxygen and releases carbon dioxide. The decline is far too small to pose any risk to human breathing in relevant timescales, but it provides a precise, independent check on how much fossil carbon humans are burning each year. In a way, the oxygen record is the other side of the carbon dioxide coin: every molecule of carbon dioxide we add to the atmosphere is paired with the disappearance of oxygen molecules that once floated freely in the air.