Earth’s oceans may not have always been as brilliantly blue as they are today. In fact, there may have been a time when they weren’t blue at all. According to researchers at Japan’s Nagoya University, Earth’s earliest oceans glimmered with green hues for over 2 billion years. The ramifications may not only better our understanding of Earth’s distant past—it may help expand our search for life elsewhere in the galaxy.
Although Earth itself cohered into a planet around 4.5 billion years ago, most estimates suggest it took at least another 800 million years before the earliest lifeforms developed. But while lifeless during that time, the planet was already covered by vast oceans dotted with hydrothermal vent systems that released large amounts of ferrous iron into the water.
The earliest cyanobacteria, commonly known today as algae, emerged around 4 billion years ago as some of the first organisms to perform oxygenic photosynthesis. While modern plants use chlorophylls for this process, ancient cyanobacteria also collected the sun’s energy through phycobilins in their antennae. Cyanobacteria’s rise ultimately helped kick off a period roughly 2.4 billion years ago known as the Great Oxidation Event. During this time, oxygen accumulated in Earth’s atmosphere, which in turn had major ramifications for the evolution of life. But the reason why cyanobacteria needed those phycobilins remained a mystery to researchers for years.
A group led by Taro Matsuo at Nagoya University recently explored one potential explanation in a study published in Nature Ecology & Evolution. Using advanced computational chemical simulations, Matsuo’s team was able to approximate how the light spectrum diffused underwater during the Archean era 4–2.5 billion years ago. They determined the increasing amounts of oxygen produced by organisms like cyanobacteria eventually interacted with an ocean’s iron content, changing it from ferrous to ferric iron.
Unlike ferrous iron, ferric iron is insoluble, which means it precipitates out of water in the form of rust-like particles. These ancient iron-heavy oceans interacted differently with light wavelengths, with ferric iron particles absorbing mostly blue and red light and leaving the green to refract into the water. According to Matsuo’s team, this would have given oceans a much greener tint to the human eye (had humans been alive at the time). Consequently, cyanobacteria continued to evolve phycobilins in order to absorb light through all that green.
“Genetic analysis revealed that cyanobacteria had a specialized phycobilin protein called phycoerythrin that efficiently absorbed green light,” Matsuo said in a statement. “We believe that this adaptation allowed them to thrive in the iron-rich, green oceans.”
Matsuo wasn’t always a believer in his green ocean hypothesis. When he first began pondering the idea in 2021, he was “more skeptical than anything else. “
“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.”
A major moment in this journey occurred in 2023 during a field study on the Satsunan archipelago’s Iwo Island. Located southwest of Kyushu, the area’s ocean water is known for its unique coloration.
“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,” said Matsuo.
The implications also go beyond Earth itself. While blue-tinted planets may hint at potential water elsewhere in the galaxy, Matsuo believes astronomers could consider expanding their color palettes.
“Remote-sensing data show that waters rich in iron hydroxide, such as those around Iwo Island in the Satsunan archipelago, appear noticeably brighter than typical blue oceans,” he explained. “This leads us to think that green oceans might be observable from a longer distance, making them easier to detect.”
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