These researchers reconstructed how oxygen came to Earth

13 June 2025 Research news item

What caused oxygen to appear in our oceans and atmosphere? You can't ask much bigger questions than that, but thanks to research in the ‘ocean archive’, researchers may have found part of the answer. “Nowadays, we associate blue-green algae with polluted water, but these bacteria were the first organisms to produce oxygen on Earth.”

For us humans, it is unimaginable: a living environment without oxygen. Yet scientists know that 2.5 billion years ago, there was no oxygen in our atmosphere or oceans. “Planet Earth is about 4.6 billion years old,” says Wytze Lenstra, assistant professor of biogeochemistry and geobiology at Radboud University. Around 2.5 billion years ago, something happened in our oceans that brought oxygen to Earth. “We call this event the Great Oxidation Event.”

Together with colleagues from Utrecht University, Lenstra investigated what might have happened billions of years ago that has made our planet habitable today. And despite the enormous amount of time that has passed since then, there are still locations where rocks from this period can be found. 'My colleagues from Utrecht went to Australia to take measurements of so-called Banded Iron Formations (BIFs). These are red iron-rich rocks that were once formed in the ocean before there was permanent oxygen on Earth. These rocks are like an archive of the ocean. The deeper you look, the further back in time you go.'

Blue-green algae

By analysing the chemical composition of the rocks, the researchers discovered an interesting connection. “We saw that changes in the chemistry of the BIFs coincided with so-called Milankovitch cycles. These cycles describe, among other things, the variation in the Earth's climate as a result of variations in the Earth's axis. “The Earth's axis is tilted relative to the sun, but the direction of the axis relative to the sun varies on timescales of approximately 11,000 years.” These changes cause periods of more intense and less intense solar radiation on Earth. The chemical composition of the BIFs showed that the nutrients needed for bacterial growth varied according to these Milankovitch cycles.

Whereas in ‘normal’ periods, iron is particularly abundant in the BIFs, periods of more intense solar radiation show peaks in phosphorus, an important nutrient for algae in the ocean. These higher concentrations of phosphorus in the ocean allowed cyanobacteria (blue-green algae) to grow and produce oxygen. When the periods of more intense solar radiation decreased again, the researchers also saw that the phosphorus peak in the BIFs disappeared and that a lot of iron could once again be found in the rock layers.

Spectacular excursion

It is quite possible that one of these periods of increased solar radiation was at the root of the Great Oxidation Event and the growth of oxygen-producing blue-green algae. The question is whether our Earth would have become an oxygen-rich planet without this astronomical influence.

Lenstra normally studies the dynamics of iron and phosphorus in the modern ocean. The research into the Great Oxidation Event, for which his colleagues in Utrecht approached him, was a spectacular detour. A detour that has left him wanting more. “It has made me more aware of the impact that research into modern systems has on answering questions that go back much further in time. This research teaches us more about the ways in which our planet eventually became oxygen-rich and how this enabled complex life as we know it to emerge. With insights like these, we could potentially learn more about the habitability of other planets.”

Would you like to learn more about this research by Lenstra and his colleagues? Read their article

Photo via Wikimedia Commons

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Organizational unit: Faculty of Science, Radboud Institute for Biological and Environmental Sciences, Microbiology
Theme: Molecules and materials, Laws of nature