Imagine looking at Earth from space and seeing not a deep blue planet, but a glowing green world. That’s not science fiction — it might be ancient history. A new study published in Nature suggests that billions of years ago, Earth’s oceans weren’t blue at all — they were green.
Back in the Archean eon, more than 3 billion years ago, Earth looked very different. There were no trees or animals, just simple, single-celled lifeforms floating in vast iron-rich oceans. These early organisms were already performing a basic form of photosynthesis — but without the help of oxygen.
Instead of using water to create energy from sunlight, these microbes used iron. This chemical reaction turned the seas green and started releasing oxygen, slowly transforming the planet’s atmosphere over millions of years. Eventually, this built up to what scientists call the “Great Oxidation Event,” a dramatic shift that allowed more complex life to evolve.
The green-tinted waters near Iwo Jima, a volcanic island in Japan, inspired the study’s authors. Here, unusual iron-rich waters support thriving colonies of blue-green algae — bacteria that carry two special light-absorbing pigments: chlorophyll and phycoerythrobilin (PEB). While chlorophyll loves sunlight, PEB thrives in green light. These algae gave scientists a peek into how early life might have evolved in green seas.
Using computer models, the researchers showed that early oceans were filled with tiny particles of oxidized iron, turning the surface waters green. This wasn’t just a beautiful twist of nature — it played a big role in helping photosynthetic organisms thrive, eventually leading to more oxygen in the oceans and air.
But here’s the wild part: the oceans could change color again in the future.
As the Sun ages and its heat increases, Earth’s climate and ocean chemistry could change dramatically. Scientists predict that more sulphur in the water, from increased volcanic activity or pollution, might give rise to purple-colored oceans filled with sulphur-loving bacteria. Red oceans might bloom near polluted coasts, where red algae thrive on runoff filled with fertilizers. Even brown or orange waters could appear in areas with certain microbial life and unique water chemistry.
The oceans we know and love today might just be a temporary phase in Earth’s long, colorful history. So the next time you look at the ocean’s sparkling blue surface, remember — it hasn’t always looked this way. And in the far future, it might change yet again.
For more details, refer to this article from Nature.
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