Cyanobacteria and the Oxygen Revolution

The Bacteria That Changed Everything

About 2.4 billion years ago, Earth experienced one of the most dramatic transformations in its history — and it was caused by bacteria. These tiny organisms, the cyanobacteria, invented a new way to harvest sunlight that produced a highly reactive waste product: oxygen.

This event, called the Great Oxygenation Event (GOE), would eventually make complex life possible — including you. But first, it caused a mass extinction unlike any before or since.

Who Are the Cyanobacteria?

Cyanobacteria (sometimes called blue-green algae, though they're bacteria, not algae) are among the oldest organisms on Earth. Fossil stromatolites built by cyanobacteria date back at least 3.5 billion years.

They're remarkable for one key innovation: oxygenic photosynthesis. Unlike earlier photosynthetic organisms that used hydrogen sulfide or other compounds, cyanobacteria learned to split water molecules using sunlight:

6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂

This reaction releases oxygen (O₂) as a byproduct. Today we take this for granted — plants do it constantly. But when cyanobacteria first evolved this ability, oxygen was a deadly poison to most life on Earth.

Before the Revolution: An Alien World

Early Earth would be unrecognizable to us:

• The sky was likely orange or pink, not blue
• The ocean was green due to dissolved iron
• There was no free oxygen in the atmosphere
• Life was entirely anaerobic (didn't use oxygen)
• Iron dissolved in the oceans like salt does today

This was the world where cyanobacteria began releasing oxygen.

The Great Oxygenation Event

For hundreds of millions of years, cyanobacteria pumped out oxygen, but it didn't accumulate in the atmosphere. Why? Because the Earth had massive "oxygen sinks" — dissolved iron and other reduced minerals that would react with oxygen immediately.

The evidence is written in stone: banded iron formations (BIFs) are striking striped rocks found around the world. They formed as oxygen reacted with dissolved iron, precipitating it out of the ocean in layers. These rocks are where we get most of our iron ore today.

Around 2.4 billion years ago, the oxygen sinks became saturated. For the first time, free oxygen began accumulating in the atmosphere. The Great Oxygenation Event had begun.

The Oxygen Catastrophe

For the anaerobic life that dominated Earth, oxygen was toxic. Oxygen is highly reactive — it damages DNA, proteins, and lipids through oxidation. Anaerobic organisms had no defenses.

The result was likely the largest extinction event in Earth's history — far worse than the asteroid that killed the dinosaurs. Most life on Earth perished or was driven into oxygen-free refuges (where anaerobes still survive today, like your gut or swamps).

Oxygen also reacted with methane in the atmosphere, removing a powerful greenhouse gas. This may have triggered "Snowball Earth" events — periods when the planet froze over almost entirely.

The Rise of Aerobic Life

But life adapted. Some organisms evolved ways to tolerate oxygen, then to use it.Aerobic respiration — using oxygen to extract energy from food — is far more efficient than anaerobic alternatives, yielding about 18 times more energy per glucose molecule.

This energy bonanza would eventually enable:

• Complex multicellular life
• Large, active animals
• Brains that consume enormous amounts of energy
• Human civilization

The Ozone Shield

Atmospheric oxygen had another crucial effect: it created the ozone layer. When UV radiation hits O₂, it forms O₃ (ozone), which absorbs harmful UV radiation. This allowed life to eventually colonize land — before the ozone layer, the surface was sterilized by UV light.

Modern Cyanobacteria

Cyanobacteria are still everywhere — in oceans, lakes, soil, and even on rocks in deserts. They're estimated to produce 20-30% of Earth's oxygen today.

They also formed one of evolution's greatest partnerships. When a larger cell engulfed a cyanobacterium but didn't digest it, the symbiosis that resulted became the chloroplast — the organelle that powers all plant photosynthesis. Every leaf on Earth contains the descendants of ancient cyanobacteria.

Lessons from the Oxygenation Event

The Great Oxygenation Event teaches us several things:

• Life can transform planets: Tiny organisms fundamentally changed Earth's atmosphere, oceans, and climate
• One organism's waste is another's opportunity: Oxygen was pollution that became the basis for complex life
• Mass extinctions can enable new evolution: The catastrophe cleared the way for entirely new kinds of life
• Planetary change can be slow: It took hundreds of millions of years for oxygen to accumulate

Breathing History

Every breath you take connects you to those ancient cyanobacteria. The oxygen molecules entering your lungs right now were created by photosynthesis — a process invented by bacteria billions of years ago and still carried out by their descendants in every plant, algae, and cyanobacterium on Earth.

We live in the world that cyanobacteria made.