Researchers from Nagoya University suggest Earth's earliest oceans were green for over 2 billion years, providing insights into both Earth's past and the search for life beyond.

Earth’s oceans may not have always been the vibrant blue we associate with them today. Researchers from Nagoya University in Japan have proposed that Earth’s earliest oceans, for over 2 billion years, glimmered with green hues instead. This discovery, published in Nature Ecology & Evolution, could significantly enhance our understanding of the planet's distant past and potentially aid in the search for life elsewhere in the galaxy.

Although Earth formed approximately 4.5 billion years ago, it took around 800 million years before life began to emerge. During this lifeless period, vast oceans covered the planet, interspersed with hydrothermal vent systems that released large amounts of ferrous iron into the water.

The earliest lifeforms, cyanobacteria, appeared around 4 billion years ago. These organisms, now known as algae, were among the first to engage in oxygenic photosynthesis.

Unlike modern plants that use chlorophylls, ancient cyanobacteria used phycobilins in their antennae to capture sunlight.

This adaptation played a crucial role in the Great Oxidation Event, which occurred around 2.4 billion years ago, leading to an oxygen-rich atmosphere.

However, the exact reason cyanobacteria relied on phycobilins remained a mystery for many years.

Taro Matsuo, a researcher at Nagoya University, led a team that explored one potential explanation. In their study, Matsuo's group used advanced computational chemical simulations to reconstruct the light spectrum underwater during the Archean era, between 4 and 2.5 billion years ago.

Their findings suggested that increasing oxygen levels, produced by organisms like cyanobacteria, interacted with the ocean’s iron content, converting ferrous iron to ferric iron.

Unlike ferrous iron, ferric iron is insoluble, precipitating out of water as rust-like particles. This transformation altered how the oceans interacted with light.

Ferric iron absorbed mostly blue and red light, allowing green wavelengths to refract into the water.

As a result, ancient oceans would have appeared much greener to the human eye, had humans existed at the time.

To adapt to this environment, cyanobacteria evolved phycobilins to absorb green light more efficiently.

“Genetic analysis revealed that cyanobacteria had a specialized phycobilin protein called phycoerythrin that efficiently absorbed green light,” Matsuo explained.

“We believe that this adaptation allowed them to thrive in the iron-rich, green oceans,” Matsuo added.

Initially, Matsuo had been sceptical about the green ocean hypothesis when he first considered it in 2021. “I was more skeptical than anything else,” he said.

“But after years of research, as geological and biological insights gradually came together like pieces of a puzzle, my scepticism has turned into conviction.”

A pivotal moment in Matsuo’s research occurred in 2023 during a field study on the Iwo Island in the Satsunan archipelago, southwest of Kyushu.

The surrounding waters there exhibited a distinct green shimmer due to iron hydroxides, matching Matsuo's hypothesis of how ancient Earth’s oceans may have appeared.

“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,” Matsuo recalled.

The implications of this discovery extend beyond Earth. Matsuo believes that astronomers searching for life on distant planets might need to broaden their search criteria.

While blue oceans are often associated with water on other planets, he suggests that green oceans could also be indicative of life.

“Remote-sensing data show that waters rich in iron hydroxide, such as those around Iwo Island, appear noticeably brighter than typical blue oceans,” Matsuo stated.

“This leads us to think that green oceans might be observable from a longer distance, making them easier to detect.”