Could alien oceans be green: Earth’s past may hold the key to finding life elsewhere
Billions of years ago, the Blue Planet wasn’t exactly blue.
Researchers have found that Earth’s first oceans may not have been blue at all like today.
Instead, they may have gleamed in shades of green for billions of years.
Although Earth formed about 4.5 billion years ago, life didn’t emerge on the planet for at least another 800 million years.
Yet, even in its lifeless stage, the planet was already covered in vast oceans, where hydrothermal vents pumped out ferrous iron.
As some of the earliest organisms to perform oxygenic photosynthesis, cyanobacteria—commonly known today as algae—emerged around 4 billion years ago.
The Blue Planet’s green past
Unlike modern plants that rely solely on chlorophylls, these ancient microbes also harnessed the sun’s energy using specialized pigments called phycobilins.
With the rise of cyanobacteria—key players in Earth’s evolutionary history—around 2.4 billion years ago, a transformative period known as the Great Oxidation Event began.
During this time, oxygen began accumulating in Earth’s atmosphere, setting the stage for life as we know it. However, one mystery puzzled researchers for years—why did cyanobacteria need phycobilins in the first place?
A group of researchers at Japan’s Nagoya University recently explored one potential explanation.
Led by Taro Matsuo, the researchers employed advanced computational chemical simulations to estimate how light filtered through Earth’s oceans during the Archean era, 4–2.5 billion years ago.
Their findings suggest that as cyanobacteria and other organisms began producing oxygen, it reacted with the ocean’s iron, shifting it from ferrous to ferric form, altering the water’s chemistry forever.
Ferric iron is insoluble. This means it precipitates out of water in the form of rust-like particles.
Researchers suggest that Earth’s ancient oceans, which were heavy on iron, interacted differently with light wavelengths.
These ferric iron particles absorbed blue and red light, mostly refracting the green light into the water, giving the oceans a distinctly green hue—at least to the human eye, had mankind existed at the time.
Green: A sign of life
Consequently, cyanobacteria adapted by evolving phycobilins to efficiently absorb light through the green-tinted waters, according to the Matsuo-led research team.
“Genetic analysis revealed that cyanobacteria had a specialized phycobilin protein called phycoerythrin that efficiently absorbed green light,” Matsuo said in a press release.
“We believe that this adaptation allowed them to thrive in the iron-rich, green oceans.”
Matsuo stated that the study could broaden the search for life in other galaxies. While blue-tinted planets often signal the presence of water, he suggests that astronomers should widen their color spectrum in search of potentially habitable worlds.
“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 said.
“This leads us to think that green oceans might be observable from a longer distance, making them easier to detect.”
The study titled ‘Archaean green-light environments drove the evolution of cyanobacteria’s light-harvesting system’ was published in the journal Nature Ecology & Evolution.
Source: Interesting Engineering
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Could alien oceans be green: Earth’s past may hold the key to finding life elsewhere