Earth’s Oxygen Production

The Amazon is often called “the lungs of the Earth.” It isn’t. Roughly half the oxygen in the air you’re breathing right now came from something far smaller and far less famous: ocean plankton.

A NASA satellite view of a phytoplankton bloom in the Southern Ocean, January 2016. Marine phytoplankton, not rainforests, produce roughly half of Earth’s atmospheric oxygen. Photo: NASA (public domain), via Wikimedia Commons.

mcqquestion.com

Who Really Makes Our Oxygen?

Not rainforests — the ocean

RainforestsNet oxygen output close to zero

Ocean Phytoplankton~50% of world’s oxygen

Dissolved O2 in Water~8-11 mg/L — far less than air

Air~21% oxygen by volume

Q40 answer: Only Statement 2 (ocean phytoplankton ~50% of oxygen) is correct.

⬇ Download as image

📑 Contents

1Must Know
2Good to Know
3Great to Know
4Test Yourself
5Previous Year Question

🏛️ Must Know

The Rainforest Oxygen Myth

Not “The Lungs” Rainforests do not produce more oxygen than oceans — a mature rainforest’s animals, microbes, and decomposition consume roughly as much oxygen as its plants produce, so its net oxygen output is close to zero.
Ocean’s Share Marine phytoplankton and photosynthetic bacteria produce roughly 50% of the world’s atmospheric oxygen — more than all the world’s tropical rainforests combined.
Prochlorococcus A single genus of tiny marine cyanobacteria, Prochlorococcus, is thought to produce up to a fifth of all oxygen in the biosphere by itself.

🏘️ Good to Know

Oxygen in Water vs Air

Water Holds Far Less Well-oxygenated surface water holds far less oxygen than atmospheric air, not several folds more — saturated water carries only about 8–11 mg/L of dissolved oxygen, while air is about 21% oxygen by volume.
Solubility Limit Oxygen’s low solubility in water is the reason: even fully saturated water can dissolve only a small fraction of the oxygen present in the same volume of air.
Ocean, Not Permanent Storage Oxygen phytoplankton produce doesn’t just accumulate in the ocean — most of it escapes into the atmosphere, while respiration and decomposition in the ocean use up roughly as much oxygen as photosynthesis creates.

⚡ Great to Know

How Earth Got Its Oxygen

The Great Oxidation Event Earth’s atmosphere wasn’t always oxygen-rich. Around 2.4 billion years ago, oxygen produced by early cyanobacteria began accumulating in the atmosphere in what’s called the Great Oxidation Event, after first being absorbed by iron and other elements in the oceans for hundreds of millions of years.
Today’s Balance Oxygen has stabilised at roughly 21% of the atmosphere by volume, held steady by a global oxygen cycle in which production and consumption are nearly balanced.
Photosynthesis vs Respiration Each year, photosynthesis on land and in the ocean releases roughly 100,000 megatons of oxygen — and respiration by animals, plants, and microbes consumes almost exactly the same amount, which is why atmospheric oxygen levels stay roughly constant year to year.
Oxygen Sinks Besides respiration, weathering and oxidation of exposed rock and organic carbon also consume atmospheric oxygen over long timescales, balancing the oxygen released when organic carbon gets buried in sediment.

Test Yourself

📝 Previous Year Question

UPSC CSP 2025 — Earth’s Oxygen Production Statements

UPSC 2025 Only Statement 2 is correct: marine phytoplankton and photosynthetic bacteria produce about 50% of world’s oxygen. Statement 1 (rainforests produce more oxygen than oceans) is false, and Statement 3 (surface water contains several folds higher oxygen than air) is false — water actually holds far less oxygen than air. See UPSC CSP 2025 GS Paper I, Q40.

Take an Environment and Ecology QuizRandom practice question from this subject →