Almost all the oxygen you breathe was made by living organisms splitting water molecules apart using sunlight, a process that has been running for billions of years. Photosynthesis, carried out by plants, algae, and cyanobacteria, is the dominant source of atmospheric oxygen on Earth. But nature has a few other tricks, and humans have developed several industrial methods to extract or generate pure oxygen for medicine, manufacturing, and even space exploration. The full picture involves deep-sea microbes, ultraviolet light, massive cryogenic towers, and plans to pull breathable air from Martian dirt.
Photosynthesis and the Machinery Behind It
Photosynthesis is, by a wide margin, the main way oxygen enters the atmosphere. Inside every green plant and photosynthetic microorganism, a molecular machine called photosystem II absorbs light energy and uses it to tear water molecules apart, releasing oxygen gas as a byproduct. This reaction solves what chemists consider a genuinely difficult problem: stripping four electrons from two water molecules cleanly enough to produce oxygen without generating dangerous reactive byproducts along the way.1PubMed Central. Water oxidation chemistry of photosystem II The oxygen you exhale carbon dioxide into was, in most cases, freed from a water molecule by this exact process somewhere on Earth.
At the heart of photosystem II sits a tiny cluster of four manganese atoms and one calcium atom. This cluster cycles through a series of energy states as it absorbs photons, ultimately reaching a point where it binds an additional water molecule and forces two oxygen atoms together to form the O₂ molecule that floats away as gas.2PubMed. Electronic structure of the oxygen-evolving complex in photosystem II prior to O-O bond formation This mechanism has been essentially unchanged for billions of years. Every leaf on every tree, every strand of kelp in the ocean, and every pond scum you have ever seen runs the same core chemistry.
Roughly half of global photosynthetic oxygen comes from land plants and half from marine organisms, mainly phytoplankton. The ocean contribution is easy to underestimate because you cannot see it the way you see a forest, but microscopic photosynthesizers floating in sunlit surface waters are pulling their weight.
How Earth Got an Oxygen-Rich Atmosphere
Earth did not always have breathable air. For the first couple billion years of the planet’s existence, the atmosphere contained almost no free oxygen. That changed because of cyanobacteria, single-celled organisms that evolved the ability to photosynthesize and began pumping oxygen into the environment. Genetic evidence suggests that key genes for cyanobacterial multicellularity and cellular differentiation appeared roughly 2.6 to 2.7 billion years ago, with additional complexity genes arriving around 2.5 billion years ago at the start of what geologists call the Great Oxidation Event.3PubMed Central. Evolution of multicellularity genes in Cyanobacteria in the lead up to the great oxidation event
The Great Oxidation Event, which took place around 2.4 to 2.3 billion years ago, marks the point where oxygen levels in the atmosphere rose high enough to leave a permanent chemical signature in the rock record. Analyses consistently point to cyanobacterial multicellularity originating before this event, with the majority of modern cyanobacterial groups diversifying afterward.4PubMed Central. Cyanobacteria and the Great Oxidation Event: evidence from genes and fossils Before oxygen accumulated in the air, much of what these early organisms produced was consumed by chemical reactions with iron and other elements dissolved in the ocean. Geological evidence from banded iron formations, the distinctive striped rocks from this era, indicates that iron oxidation was happening in deeper waters rather than at the surface, which tells us something about how slowly oxygen was spreading through the early ocean.5Global and Planetary Change. Aerobic carbon cycling and cerium oxidation: significance for Archean oxygen levels and banded iron-formation deposition
The transition was not smooth or fast. It took hundreds of millions of years for oxygen to build up to levels that could support complex animal life. But once cyanobacteria got going, they fundamentally and irreversibly changed the planet’s chemistry.
Oxygen Without Sunlight
Photosynthesis dominates global oxygen production so completely that other natural sources are often overlooked. They exist, though, and some of them are genuinely surprising.
In the deep ocean, far from any sunlight, certain microbes can produce oxygen in total darkness. Researchers have identified deep-sea bacteria from several different groups that generate oxygen through pathways linked to nitrate chemistry. In one well-studied case, bacteria use a process called dissimilatory nitrate reduction to ammonium, and oxygen appears as a byproduct. Other deep-sea organisms from entirely different bacterial lineages also produce what scientists call “dark oxygen” through nitrate-driven pathways that are not yet fully understood.6CrossRef / BioRxiv. Nitrate-driven dark oxygen production by diverse deep-sea microorganisms This does not meaningfully change atmospheric oxygen levels, but it matters for understanding how oxygen-dependent life can persist in environments that seem impossibly hostile.
A related phenomenon involves certain anaerobic methane-consuming microbes. Some of these organisms appear to generate small amounts of oxygen internally, which they then use to power what would normally be aerobic chemical reactions, even though they live in oxygen-free environments. The exact mechanism remains unclear, but evidence points to a role for nitric oxide or a related compound in forming the oxygen.7PubMed Central. Bacterial oxygen production in the dark The existence of these pathways hints that biological oxygen production may have predated photosynthesis in some form, though this is speculative.
Beyond biology entirely, oxygen can also be produced abiotically when ultraviolet light breaks apart water vapor molecules high in a planet’s atmosphere. This process, called water vapor photolysis, is relevant less for Earth and more for understanding other worlds. Researchers studying exoplanet atmospheres have modeled how this mechanism could build up detectable levels of oxygen and ozone on temperate rocky planets even without any life present.8The Astrophysical Journal. Buildup of Abiotic Oxygen and Ozone in Moist Atmospheres of Temperate Terrestrial Exoplanets and Its Impact on the Spectral Fingerprint in Transit Observations That finding complicates the search for extraterrestrial life, because oxygen in a planet’s atmosphere does not automatically mean something is alive there.
Cryogenic Air Separation
When industry needs large quantities of high-purity oxygen, the workhorse technology is cryogenic distillation. The idea is deceptively simple: cool air down until it becomes liquid, then exploit the fact that nitrogen and oxygen have different boiling points. Nitrogen boils at about −196°C while oxygen boils at about −183°C, so if you carefully warm liquid air in a distillation column, nitrogen evaporates first and oxygen concentrates at the bottom.
In practice, this requires enormous, heavily insulated towers operating at very low temperatures, with multiple stages of separation to achieve the desired purity. Industrial cryogenic plants use distillation columns with anywhere from a couple dozen to nearly 200 separation stages, carefully calibrated reflux ratios, and precisely managed temperature profiles to ensure clean separation of oxygen, nitrogen, and argon.9PubMed Central. Novel Study on Cryogenic Distillation Process and Application by Using CHEMCAD Simulation These facilities are energy-intensive, but they can produce oxygen at purities above 99.5%, which is necessary for applications like steelmaking, chemical manufacturing, and rocket propulsion.
Cryogenic plants operate around the clock and can produce hundreds or thousands of tons of oxygen per day. The oxygen is typically stored and transported as a cryogenic liquid in insulated tanker trucks or piped directly to nearby industrial consumers. If you have ever seen a large, windowless industrial facility near a steel mill or hospital complex with thick white pipes running out of it, there is a good chance it is an air separation unit.
Smaller-Scale Industrial Methods
Not every application needs the volume or purity that cryogenic distillation provides. Two other technologies fill the gap for moderate quantities or portable use.
Pressure swing adsorption, commonly called PSA, is the technology inside the oxygen concentrators that many people with lung conditions use at home. It works by pushing compressed air through beds of a zeolite mineral that preferentially grabs nitrogen molecules. Nitrogen has a stronger electrical interaction with the zeolite than oxygen does, so it sticks to the mineral while oxygen-enriched air passes through.10PubMed Central. Quadrupolar Interaction with Zeolite and Pressure Swing Adsorption in Portable Medical Oxygen Concentrators for Breathing of Covid-19 and COPD Patients Then the pressure drops, the nitrogen releases from the zeolite, and the cycle repeats. Modern PSA systems using lithium-exchanged zeolites can achieve oxygen purities above 90%.11Applied Mechanics and Materials. Simulation of Pressure Swing Adsorption for Oxygen Concentrator Using LiX Zeolite
PSA technology scaled up dramatically during the COVID-19 pandemic, when hospitals worldwide needed supplemental oxygen faster than cryogenic plants and supply chains could deliver it. Portable concentrators became critical in regions where medical-grade bottled oxygen was scarce.
Membrane separation is a newer approach that pushes air through thin polymer membranes that allow oxygen to pass through more readily than nitrogen. These systems are mechanically simple, with no moving parts in the separation stage itself, and they scale well for moderate volumes. Economic analyses have found that a well-designed two-stage membrane process can produce 90% pure oxygen at a cost of roughly five cents per kilogram, competitive with PSA for certain applications.12Elsevier (Separation and Purification Technology). Technoeconomic analysis of oxygen-nitrogen separation for oxygen enrichment using membranes Membrane units are particularly attractive where portability matters or where electricity is cheap and available.
Electrolysis and Chemical Generators
Water electrolysis is the most direct way to make oxygen from scratch rather than extracting it from air. Pass an electric current through water and it splits into hydrogen and oxygen, one gas at each electrode. This has been understood since the early 1800s, but recent interest has surged because of the hydrogen economy: if you want green hydrogen for fuel cells or industrial chemistry, you get pure oxygen as a free byproduct.
Proton exchange membrane water electrolyzers are one of the leading technologies for this. They split water using electricity and, when powered by renewable sources, produce both green hydrogen and oxygen with no carbon emissions. These systems are expected to make up a substantial share of the green hydrogen market alongside alkaline and solid oxide electrolyzers.13ACS Publications. Proton Exchange Membrane (PEM) Water Electrolysis: Cell-Level Considerations for Gigawatt-Scale Deployment High-temperature solid oxide electrolysis offers even higher conversion efficiency, with some system designs achieving energy efficiencies above 85%.14Elsevier. Modeling and performance optimization of a solid oxide electrolysis system for hydrogen production
For situations where reliability matters more than efficiency, chemical oxygen generators provide a no-electricity option. The classic example is the chlorate candle, a solid block of sodium chlorate mixed with iron powder. When ignited, it undergoes a chemical reaction that releases oxygen gas at a steady rate. Submarines have used these for decades, and the International Space Station carries a backup oxygen delivery system consisting of 33 chlorate candles for emergencies.15SAE International. Development of a Solid Chlorate Backup Oxygen Delivery System for the International Space Station Commercial aircraft also carry chemical oxygen generators above the passenger seats, which is what activates when the masks drop during a cabin depressurization. The generators run hot, which is why flight safety briefings emphasize not smoking when masks deploy.
Making Oxygen on Other Worlds
If humans are going to live and work on the Moon or Mars, shipping oxygen from Earth becomes impractical very quickly. A single crewed Mars mission would need many tons of oxygen for breathing and as rocket propellant for the return trip. The solution being developed is to make oxygen from local materials, an approach called in-situ resource utilization.
On Mars, NASA’s MOXIE experiment, which flew on the Perseverance rover, demonstrated the first production of oxygen on another planet. MOXIE works by solid oxide electrolysis of carbon dioxide, which makes up about 96% of the thin Martian atmosphere. It heats CO₂ to high temperatures and electrochemically strips an oxygen atom from each molecule, producing breathable oxygen and carbon monoxide as a waste product.16PubMed Central. Mars Oxygen ISRU Experiment (MOXIE)-Preparing for human Mars exploration MOXIE was small, roughly the size of a toaster and designed as a proof of concept. A full-scale version for a crewed mission would need to be about 200 times larger.
The Moon presents a different challenge. There is no substantial atmosphere to work with, but lunar soil, called regolith, is rich in metal oxides. One promising approach is molten salt electrolysis, where regolith is dissolved in a bath of molten salt and electricity is passed through to separate oxygen from the metals. The FFC process, one variant of this technique, has demonstrated the ability to extract up to 96% of the oxygen present in simulated lunar soil without needing to melt the regolith itself.17Acta Astronautica. Characterization of metal products from the molten salt electrolysis of lunar highland regolith simulants As a bonus, the process also yields useful metals like iron, aluminum, and titanium. Molten salt electrolysis is being studied for both Moon and Mars applications as a way to reduce the mass that needs to be launched from Earth.18Journal of The Electrochemical Society. Electrochemical Processing of Planetary Regolith in Molten Salts – A Review
The Global Oxygen Cycle
With so much oxygen being produced and consumed constantly, it is reasonable to wonder why the atmosphere does not run out or overflow. The answer is that production and consumption are closely balanced. Photosynthesis releases oxygen while respiration and decomposition consume it. Every living thing that breathes, including the plants themselves at night, pulls oxygen back out of the air. Combustion of fossil fuels and wood does the same. The result is that atmospheric oxygen maintains a nearly constant concentration, currently about 20.9% of the atmosphere by volume.19Reference Module in Earth Systems and Environmental Sciences. Global Biogeochemical Cycling: Oxygen Cycle
This balance is not just a surface phenomenon. A deep Earth oxygen cycle also operates over geological timescales, with oxidized material cycling between the planet’s mantle and its surface environment through volcanic outgassing and subduction. About 82% of Earth’s total oxidizing capacity resides in the mantle, with the remaining 18% in the surface environment. Recent mantle outfluxes and subduction influxes appear roughly balanced, though outfluxes likely exceeded influxes earlier in the planet’s history.20Earth and Planetary Science Letters. The deep Earth oxygen cycle: Mass balance considerations on the origin and evolution of mantle and surface oxidative reservoirs
Where things get worrying is in the ocean. Global ocean dissolved oxygen has declined roughly 2% since 1960. Oxygen minimum zones, the deep layers where oxygen is naturally low, are expanding, and coastal hypoxic areas, sometimes called dead zones, have increased tenfold. The main culprits are warming waters, which hold less dissolved gas and mix less efficiently, and nutrient runoff that feeds microbial blooms whose decomposition consumes oxygen.21International Journal of Pure Science Research Studies (IJPSRS). Ocean Deoxygenation: Causes, Consequences, and Conservation Strategies This does not threaten the atmosphere directly, since the ocean holds only a fraction of Earth’s free oxygen, but it has serious consequences for marine ecosystems and fisheries.
Who Figured All This Out
The discovery of oxygen has a famously tangled history. The Swedish apothecary Carl Wilhelm Scheele was technically the first person to isolate the gas, sometime around 1771 to 1772, but his written account was delayed in publication. Meanwhile, the English clergyman and scientist Joseph Priestley independently prepared oxygen in 1774 by focusing sunlight on mercuric oxide and collecting the gas that came off. Priestley recognized that the gas had unusual properties and that a candle burned more vigorously in it, but he interpreted his findings through the lens of phlogiston theory, the prevailing (and wrong) explanation of combustion at the time. He called his discovery “dephlogisticated air” and stuck with that name for the rest of his life.22PubMed. Fire-air and dephlogistication. Revisionisms of oxygen’s discovery
Priestley shared his findings with the French chemist Antoine Lavoisier, who repeated the experiments and arrived at a fundamentally different interpretation. Lavoisier recognized that the gas was a distinct element, named it “oxygen,” and used it as the cornerstone of an entirely new theory of chemistry that replaced phlogiston theory. The debate over who “really” discovered oxygen has gone on ever since. Priestley got there first experimentally. Scheele may have gotten there even earlier. But Lavoisier understood what the gas actually was, and that understanding overturned an entire field.23PubMed. Joseph Priestley, oxygen, and the enlightenment A nineteenth-century commentary in Nature put it bluntly: the bare discovery of the gas was “a very minor matter” compared to the revolution in chemistry that Lavoisier built from it.24Nature. Lavoisier, Priestley, and the Discovery of Oxygen
Lavoisier’s name for the element came from the Greek words for “acid-former,” because he believed, incorrectly, that oxygen was a necessary component of all acids. That turned out to be wrong (hydrochloric acid contains no oxygen, for example), but the name stuck. Lavoisier himself did not live to see how far his ideas would go. He was executed during the French Revolution in 1794, reportedly prompting the mathematician Lagrange to remark that it took only a moment to cut off that head, and a hundred years might not produce another like it.