AsianScientist (Apr. 09, 2025) – When Voyager 1 spacecraft captured a photo of the Earth from six billion kilometres away, astronomer Carl Sagan famously described our planet as a “pale blue dot.” That blue hue was sunlight scattering and reflecting off of the oceans. But now, researchers in Japan have found evidence that Earth’s oceans may not have always been blue — they were green.
A group led by Taro Matsuo from Nagoya University in Japan, has found evidence that cyanobacteria, often called blue-green algae, an important bacteria in the evolutionary process, flourished in green seas. “When I first had the idea that the oceans used to be green, back in 2021, I was more skeptical than anything else,” Matsuo said. “But now, after years of research, as geological and biological insights gradually came together like pieces of a puzzle, my skepticism has turned into conviction.”
“For me personally, a major turning point was our field survey on Iwo Island in the Satsunan archipelago in 2023,” Matsuo added. “From the boat, we could see that the surrounding waters had a distinct green shimmer due to iron hydroxides, exactly like how I imagined the Earth used to look.” This new discovery, published in Nature Ecology & Evolution, suggests how different the world was 2.4 billion years ago during a period called the Great Oxidation Event.
This event was triggered when cyanobacteria started using sunlight to make their own food — a process known as oxygenic photosynthesis. While doing this, they also released oxygen as a byproduct. Over millions of years, this oxygen built up in Earth’s atmosphere and made it possible for new types of life — like animals and humans who need oxygen to breathe — to eventually evolve.
In the modern world, most oxidation is performed by plants, using chlorophylls. However, the ancient cyanobacteria also used additional pigments called phycobilins, which were an integral part of their light-harvesting antennas. The researchers became interested in the question of why cyanobacteria required phycobilins in addition to chlorophylls.
Using advanced simulations, they found that the underwater light spectrum during the Archaean era, 4 to 2.5 billion years ago, changed to green due to iron precipitation. At the time, the Earth’s oceans contained high levels of ferrous iron, which was released by hydrothermal vent systems. Later, during the Great Oxidation Event, when oxygen started building up in the atmosphere, this oxygen reacted with the ferrous iron and turned it into ferric iron. Ferric iron doesn’t dissolve well in water, so it settles out as tiny, rust-like particles. These particles affected how light traveled through the ocean — absorbing most of the blue and red light and letting green light pass through. As a result, the oceans looked green underwater during that time.
“Genetic analysis revealed that cyanobacteria had a specialized phycobilin protein called phycoerythrin that efficiently absorbed green light,” Matsuo said. “We believe that this adaptation allowed them to thrive in the iron-rich, green oceans.”
Matsuo also believes his research may help in the search for life in outer space. On Earth, the ocean appears blue because water absorbs red light and scatters blue; however, the green oceans in the Archean era might have efficiently reflected green light due to iron precipitation. Therefore, looking for green oceans could be used as a sign of primitive life on distant planets.
Matsuo is excited about the possibility of green oceans improving the search for alien life.
This research shows how photosynthetic life and Earth’s environment have continuously influenced each other, evolving together over time.
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Source: Nagoya University ; Image:
The article can be found at Archaean green-light environments drove the evolution of cyanobacteria’s light-harvesting system
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