An OXYGEN CRISIS That Poisoned Cyanobacteria but Sparked Life

Oxygen Crisis Cyanobacteria

Last Updated: Dec 29, 2018

The oxygen crisis that changed Earth’s atmosphere

Great Oxygenation Event

There wasn’t always a breath of fresh air on Earth. Methane and nitrogen choked any potential life in Earth’s early atmosphere. But this doesn’t mean that life didn’t exist.

Long ago, tiny, microscopic cyanobacteria flourished in ocean environments. They evolved to carry out photosynthesis anaerobically.

The key to their existence was that they didn’t need oxygen to survive. As a byproduct, they released oxygen which was toxic for them.

As oxygen filled the oceans then the atmosphere, this event was the great oxygenation event. This paved the way for aerobic organisms to carry the torch.

An oxygen crisis that almost ended cyanobacteria


For millions of years, cyanobacteria used heat from volcanoes or mid-ocean ridges, then released oxygen into the oceans and air. Eventually, they filled the oceans with oxygen.

The irony of cyanobacteria is that the oxygen they released was toxic to them. As a whole, the number of anaerobic organisms dropped in the Proterozoic Eon.

The reason why this event is called an “oxygen crisis” is because they threatened their own existence through their own waste oxygen.

Cyanobacteria didn’t completely vanish in the Proterozoic Eon. By hiding in low oxygen environments, cyanobacteria avoided their mass extinction.

A struggle for constant oxygen

Banded Iron Formations

Banded iron formations (BIF) consist of alternating layers of red (oxidized) and (unoxidized) black iron.

Because iron rusts when it reacts with oxygen, these layers were red. So the red (oxidized) layers mark a time when there was oxygen.

The alternating layers of oxidized and unoxidized minerals gives a sneak peak of how oxygenation was just starting to get a foothold.

We can find banded iron formation all over the world such as Australia, Canada and South Africa. Most of the world’s iron ore deposits were produced in the Archean Eon.

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