The ocean produces roughly half of Earth’s oxygen — not the rainforest, which uses up most of what it makes — through photosynthesis by phytoplankton, microscopic marine organisms so abundant that a single teaspoon of seawater can contain as many as a million of them.

About half of the oxygen on Earth comes from the ocean. Not from the rainforests, which consume close to as much oxygen as they make, but from phytoplankton, the microscopic algae and bacteria that drift through the sunlit surface of the sea and photosynthesise on a scale that is hard to picture.

The figure is real, and widely cited by agencies including the United States National Oceanic and Atmospheric Administration. It also comes with a footnote that is more interesting than the headline.

The invisible forest

Phytoplankton are tiny, individually invisible, and almost unimaginably numerous. A single teaspoon of seawater can hold more than a million microscopic organisms, of which phytoplankton are a large part, although the count rises and falls enormously with season, sunlight and the nutrients in the water.

One group stands out. Prochlorococcus, the smallest known photosynthetic organism, is so abundant that, by NOAA’s account, it alone produces up to a fifth of the oxygen in the entire biosphere. That is a larger share than all the world’s tropical rainforests combined.

These organisms do much the same thing a tree does. They use sunlight to turn carbon dioxide and water into food, releasing oxygen as a by-product. What differs is the scale and the speed. There are astronomical numbers of them, they live fast, and they are grazed, infected and replaced within days.

The trouble with “the lungs of the Earth”

The claim usually attached to the Amazon, that it produces 20 per cent of the world’s oxygen, does not hold up. The number appears to trace back to the rainforest’s share of land-based photosynthesis, and once the ocean is counted the figure is closer to 6 to 9 per cent.

More to the point, a mature forest consumes almost all the oxygen it makes. Yadvinder Malhi, an ecosystem scientist at the University of Oxford, has put the Amazon’s net contribution to atmospheric oxygen at close to zero. The trees release oxygen by day, but respiration by the plants and animals, and especially by the microbes breaking down dead matter, uses up about the same amount. A forest in balance is not adding to the air.

Why “half our oxygen” needs its footnote

The ocean works the same way, and this is the part the popular version leaves out. NOAA is explicit that while the ocean produces at least half of Earth’s oxygen, marine life consumes roughly the same amount, through respiration and the decay of dead organisms.

So the oxygen in any single breath is not freshly made by plankton and piped ashore. The breathable atmosphere is a reservoir built up over hundreds of millions of years, as a small fraction of the carbon fixed by photosynthesis was buried in sediment instead of being respired back. That slow leak into storage, rather than the daily output of the plankton, is what filled the air with oxygen.

The distinction changes how the claim should be read. If phytoplankton vanished tomorrow, the immediate disaster would be the collapse of the marine food web and the carbon cycle, not people gasping for air. The atmosphere would hold its oxygen for a very long time.

Counting it from orbit

The reason the ocean’s share is an estimate, rather than a measured certainty, is that nobody can sample the whole sea. The global picture comes from space.

Satellites read the colour of the ocean, because water rich in chlorophyll looks greener, and from that colour researchers work out how much phytoplankton is present and how much photosynthesis is going on. Older sensors such as SeaWiFS and MODIS built the first global maps. In February 2024, NASA added PACE, short for Plankton, Aerosol, Cloud, ocean Ecosystem, a satellite whose hyperspectral instrument can tell different communities of phytoplankton apart rather than just measuring how much is there.

That detail matters, because which species are blooming changes how much carbon is drawn down and how the food web behaves. PACE is built to track those shifts as the ocean warms.

The same reasoning, pointed outward

There is a longer reach to all of this. Oxygen is the biosignature astronomers most hope to find in the atmosphere of another planet, on the logic that life is the obvious way to keep an atmosphere stocked with so reactive a gas.

Earth is the cautionary case. Its oxygen is the product of a particular balance, built over geological time and held up by the burial of carbon, not a simple readout of how much life is present at any one moment. Reading oxygen on a distant world will need the same care, and the same wariness of the tidy headline.

For now, the nearer task is the one PACE was built for: watching how the ocean’s invisible forest changes, season by season, from orbit.