The Amazon Rainforest—with its dense, green canopy—and the world’s vast, deep oceans are often considered the lifelines of Earth’s atmosphere. Both play a significant role in regulating gases crucial to life. But a persistent myth continues to circulate: that the Amazon Rainforest produces 20% of the world’s oxygen. While it makes for a compelling headline, this statement is misleading. The real powerhouse of oxygen production lies not on land, but beneath the waves—in the form of microscopic marine life.

This article takes a closer look at the oxygen-generating capacity of the Amazon Rainforest compared to that of the oceans, particularly through the activity of phytoplankton, the unsung heroes of Earth’s biosphere.

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Oxygen Production: The Basics

Oxygen is essential for the survival of most life forms. It is produced through photosynthesis, a natural process in which green plants, algae, and certain bacteria convert sunlight, carbon dioxide (CO₂), and water into energy (glucose). A byproduct of this process is oxygen, which is released into the atmosphere.

Photosynthesis occurs both on land and in water. However, there’s a key difference: how much oxygen is actually added to the atmosphere by these processes.

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Photosynthesis on Land: A Balanced Cycle

Forests, including the Amazon, are incredibly productive ecosystems. Trees and plants photosynthesize during the day, releasing oxygen as they grow. But they also consume oxygen—both through their own cellular respiration and through the decomposition of dead organic material by microbes and fungi.

This creates a closed oxygen loop: the Amazon Rainforest produces a large amount of oxygen, but also uses nearly all of it. The result? Its net contribution to the oxygen we breathe is close to zero.

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Marine Photosynthesis: Earth's True Oxygen Engine

In contrast, marine ecosystems operate differently. Tiny aquatic organisms called phytoplankton float near the surface of the ocean and photosynthesize in much the same way as plants. However, because much of the organic matter in the ocean sinks to the seafloor before it decays, the oxygen used in respiration is not always recycled back at the surface. This gives oceans a net positive oxygen output.

In fact, it’s estimated that over 70% of the oxygen in Earth’s atmosphere is generated by phytoplankton and other marine plants. These microscopic organisms may be small, but their collective impact is massive.

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Myth-Busting: The 20% Amazon Oxygen Claim

The idea that the Amazon produces 20% of Earth’s oxygen likely originates from the fact that it accounts for about 20% of photosynthesis on land—not total global oxygen production. However, when considering oxygen used by the forest itself, its net contribution drops dramatically, likely hovering around zero.

Why doesn’t the Amazon add more oxygen to the atmosphere?

It comes down to respiration. Both plants and microorganisms in the rainforest consume oxygen continuously, especially when organic matter decomposes. The same trees that photosynthesize by day also breathe around the clock, as do countless soil organisms breaking down dead plant material. In this way, the Amazon functions more like a giant recycling system for oxygen, rather than a net producer.

Additionally, the limiting factor is carbon dioxide concentration. The atmosphere contains only about 0.04% CO₂, compared to 21% oxygen, meaning there’s a natural ceiling to how much new oxygen any photosynthetic system can generate.

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Scientific Estimates: Just How Much Oxygen Does the Amazon Produce?

Dr. Yadvinder Malhi, an ecologist at the University of Oxford, estimates that the Amazon generates approximately 16% of the oxygen produced by terrestrial plants. However, when global photosynthesis is taken into account—including the oceans—this contribution drops to around 9% of total photosynthetic oxygen output. Again, this is gross production—not accounting for the oxygen consumed within the ecosystem.

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Oceans: The Silent Oxygen Giants

While forests play a valuable role in regulating global carbon and water cycles, the oceans dominate oxygen production. Marine photosynthesis, particularly by phytoplankton, is the main driver of this process.

What Are Phytoplankton?

Phytoplankton are microscopic, plant-like organisms that float near the surface of oceans, lakes, and rivers. Like land plants, they harness sunlight to photosynthesize, producing oxygen and removing carbon dioxide from the water and atmosphere.

One of the most prolific oxygen producers is Prochlorococcus, a tiny cyanobacterium found in the open ocean. According to marine biologist Dr. Sylvia Earle, this single species is responsible for generating the oxygen in one out of every five breaths we take.

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The Health of Oceans Matters

Healthy oceans are critical to maintaining this oxygen supply. Phytoplankton thrive in clean, nutrient-rich water with access to sunlight. But pollution, climate change, rising ocean temperatures, and acidification all threaten their survival. A decline in phytoplankton populations could have profound consequences—not just for marine ecosystems, but for atmospheric oxygen levels and climate regulation as well.

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Comparing the Amazon and Oceans in Oxygen Output

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Why We Must Protect Both Ecosystems

Though the Amazon is not the “lungs of the Earth” in the way it's often portrayed, its ecological importance cannot be overstated. It regulates rainfall patterns, stores vast amounts of carbon, and harbors unparalleled biodiversity. Meanwhile, oceans and their phytoplankton are vital for oxygen production, climate regulation, and carbon sequestration.

The health of both ecosystems is deeply intertwined with the health of the planet. One cannot thrive without the other.

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In Conclusion: Rethinking Oxygen Origins

The Amazon Rainforest may be awe-inspiring, but the real oxygen champions are the oceans. Understanding the true roles of these ecosystems in Earth's oxygen cycle helps us move past myths and focus on what truly matters: preservation and sustainability.

As we face the twin crises of climate change and biodiversity loss, protecting Earth’s critical ecosystems—whether rooted in soil or floating in the sea—has never been more urgent. Every breath we take is a reminder of the delicate balance that keeps life on this planet possible.