The ocean's invisible inhabitants are the true lungs of our planet

Space · Published 24 October 2026 Swipe more like this →

In short: You likely believe that forests are the primary source of the oxygen you breathe. While trees are vital, the vast majority of our atmosphere's oxygen is actually produced by a microscopic world hidden beneath the waves. These tiny organisms have been engineering our air for hundreds of millions of years.

While forests are often called the 'lungs of the planet,' they are not the primary source of breathable air. Most of the oxygen in our atmosphere—which currently makes up about 21 percent of the air—is generated by the world's oceans. Specifically, microscopic organisms like phytoplankton, cyanobacteria, and diatoms are responsible for producing between 50 percent and 80 percent of the oxygen we breathe. These tiny life forms are much smaller than anything visible to the human eye. They are also significantly older than the first trees. This invisible world remains essential for every single breath humans take today.

This process is driven by photosynthesis, where these organisms use sunlight to convert carbon dioxide and water into sugar for food, releasing oxygen as a byproduct. However, not all oxygen produced is released into the atmosphere. Most living cells consume oxygen through cellular respiration to create energy, and decomposition also uses it up. The oxygen atoms are not destroyed during these cycles. They simply transform into different molecules. This means the atoms remain present even when they are being used up by various biological processes.

The oxygen that eventually reaches our atmosphere is the small remainder left over when these organisms die and sink to the ocean floor without fully decomposing. This shift in atmospheric chemistry was a monumental geological event. Until roughly 600 million years ago, Earth's atmosphere contained less than 5 percent oxygen, consisting mostly of nitrogen and carbon dioxide. Land plants did not even exist until 470 million years ago. The 'Great Oxygenation Event' was driven by ocean-based life, not trees. Today, this life thrives primarily in the photic zone—the top 200 meters (about 656 feet) of the ocean—where sunlight can penetrate deep enough to power the photosynthetic process. The ocean's role was to simultaneously decrease carbon dioxide levels while building up oxygen. This occurred over millions of years as these organisms pumped out gas. The process continues today in the specific area where light reaches the water.

General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.

Intellect Plus is a visual educational discovery platform for exploring fascinating facts and ideas. Brain Food, Not Brain Rot. What is Intellect Plus?

Related

• Two-thirds of our oxygen comes from a single drop
• The vast majority of Earth's oxygen is locked away beneath your very feet
• Every breath you take owes something to blue-green bacteria
• Oxygen is being produced in total darkness without any sunlight
• Photosynthesis captures about eight times the power human civilisation uses
• NASA initially rejected the Landsat program, now its most valuable investment in space technology.
• The impossible task of peering into another mind
• Why the most advanced civilizations might skip the Dyson Sphere
• The International Space Station is set to retire by 2030, but what comes next?
• Could antimatter propulsion move from the realm of science fiction to reality?
• Why molecular biology needs philosophy to understand the human experience
• Why modern spacecraft rely on two distinct types of electric propulsion engines

More in Space · Latest · A–Z · All topics