Most of Our Oxygen Doesn’t Come From Where You Think

Forget rainforests. Here’s the real source of our air — and why it’s in danger

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This “Rainforest” stock photo is beautiful, luscious, and probably full of invisible oxygen — but it’s not the main source of our breathable air. Photo by Hidayat Abisena.

a child, when I learned about how plants produce the oxygen that we breathe, I imagined that I could see that oxygen in the air. In my mind’s eye, I saw vast waves of oxygen rising from a lush tropical rainforest.

The image makes sense — if plants produce oxygen, it stands to reason that bigger plants produce more, and areas of dense vegetation produce more oxygen than areas with only sparse plants. A rainforest, with its multiple levels of vegetation, surely produces the majority of our oxygen, right?

Not quite.

Consider the Amazon, the world’s largest rainforest. By one estimate, this entire area is only responsible for a piddling 6 percent of the world’s oxygen production.

So if lush rainforests, filled with many different plant species all living in somewhat tenuous and competitive harmony, aren’t responsible for the creation of most of the air we need to breathe… where’s it coming from?

And are we in danger of accidentally destroying our own air supply in a climate catastrophe?

The Earth is a Blue Marble

It’s important to remember that sunlight shines on all areas on the surface of the planet, including both land and water. In fact, since about 71 percent of the Earth is covered by water, it’s reasonable to say that the Sun is shining more on the water than on the land.

And even though the surface of the ocean looks dull, lifeless, and uninteresting compared to the business of a lush tropical forest, there’s a lot going on beneath the surface.

The top 200 meters (or approximately 650 feet) of the ocean is known as the epipelagic zone, from epi-, meaning “above”, and pelagic, meaning “surface of the sea”. This top zone of the ocean absorbs most of the sunlight that hits it — and is filled with algae, single-celled plants that flourish from that light.

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Algae, clinging to rocks. Looks like green scum, but we’d all gasp if it vanished (and would keep gasping, haha, air jokes). Photo by chuttersnap.

This algae produces a huge amount of oxygen, simply because there’s so much of it growing throughout much of the ocean. It also provides the primary food source to a large percentage of the life in the ocean — and has been serving in this role for hundreds of millions of years.

By various estimates, the total percentage of oxygen that’s produced from these tiny, modest single-celled critters ranges from 50 to 85 percent! And there’s one breakout star, a single-celled algal organism called Prochlorococcus, which produces roughly 20% of the world’s oxygen just on its own.

Why is this estimate of the percentage of oxygen from the ocean vary so widely? Part of this is due to the changing nature of the ocean — the amount of sun, the seasons, and the tides will all shift the level of oxygen that any one region of open ocean produces.

Another challenge is that a significant amount of this oxygen that’s produced doesn’t go directly into our atmosphere, but stays absorbed in the oceans. This is how fish are able to breathe down there, without needing very long snorkels.

But finally, that oxygen can sometimes get used up in a dramatic reversal of its production, when something called a harmful algal bloom, or HAB, occurs.

The Paradox — Too Much Algae, No Oxygen

It sounds crazy. Too much algae grows — and this somehow depletes the oxygen, instead of creating more? Shouldn’t more algae equal more oxygen being made?

The problem isn’t the living algae, but what happens when they die.

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Water clouded with algae, likely in a bloom. This is often caused by human intervention (such as fertilizer runoff) or as an effect of climate change. Photo by Ivan Bandura.

There are a few stages to an algal bloom:

  1. The bloom is triggered by a change in conditions. This is sometimes natural, but often is caused by human-associated effects such as unseasonal warmth from climate change, or fertilizer runoff from farms.
  2. Huge numbers of algae grow, clogging the water, in a “bloom.” The algae may produce toxins that can kill sea life, and can also kill fish and wildlife by covering them and suffocating them.
  3. As the fuel for the bloom runs out, the algae die in huge quantities. The dead algae lead to huge growths of bacteria, which consume oxygen from the water as they feed on the rotting algae.

If an algal bloom is large enough, it leaves behind an area of the ocean, sometimes many square kilometers, that no longer contains enough oxygen in the water to support life. These regions are known as “dead zones.”

Dead zones have a whole host of problems — they can’t support fish, they may spread if there’s still more fertilizer, sewage, or other pollutant runoff contributing to them, and they can wipe out important species that we need to maintain the ocean food chains.

But in the case of this article, dead zones also reduce the amount of oxygen produced. And while it hasn’t had an impact on our atmosphere yet, scientists are warning that, unless we reduce pollution flowing into the ocean and contributing to algal blooms, we may render much of our oceans uninhabitable for fish.


To wrap up: trees and forests look verdant and brimming with life, but they contribute only a small fraction of the total oxygen in the atmosphere. Most of this oxygen comes from modest, single-celled algae that grows in the ocean. The sheer surface area of the ocean means that these little blobs of green fuzz are the biggest overall producers of oxygen on the planet.

However, while they contribute more than half the oxygen to the atmosphere, they’re also susceptible to blooms, where the algae grows out-of-control — and then dies. That death leads to rotting algae that’s consumed by bacteria, using up far more oxygen than the algae produced in the first place.

The ocean can eventually recover from these harmful algal blooms, but it takes time, and the removal of the cause — which means cutting back on pollution, such as fertilizer runoff, that flows into rivers, lakes, and oceans.

It’s hard to see the life teeming below the surface of the ocean, but we can’t be complacent — if it dies, the entire world above the sea level will suffer.


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PhD in genetics, bioinformatician, scientist at a Silicon Valley startup. Microbiome is the secret of biology that we’ve overlooked.

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PhD in genetics, bioinformatician, scientist at a Silicon Valley startup. Microbiome is the secret of biology that we’ve overlooked.

Observations and analysis from scientists on bacteria, biotech, health, and how we live and interact with our natural world.

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