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Planet Earth is blue. Most of its surface is covered in water, the vast majority of which is ocean, and water generally absorbs longer wavelengths of light, such as red and orange, and reflects back the shorter blue wavelengths. When astronauts travel into space and look back at their home, they see a blue sphere in an expanse of jet black.
But the blue color we associate with Earth’s oceans is a relatively recent development in the planet’s evolutionary history. During the Archaean Eon, which spanned 4.0 to 2.5 billion years ago, the oceans shimmered green, according to new research, and this green light may have helped to set the stage for the rise of an ancient blue-green algae that transformed the planet. Known as cyanobacteria, that algae precipitated the Great Oxygenation Event, when oxygen came to dominate the atmosphere and the oceans, which preceded the emergence of higher order creatures in water and on land.
“This research is a good example for the coevolution of Earth and life, because the surface environment and cyanobacteria affected each other,” says Taro Matsuo, a space physicist at Nagoya University in Japan who led the research. “Cyanobacteria could thrive under the green environment.”
The new findings were published in Nature Ecology & Evolution by Matsuo and a team of space physicists, chemists, and earth scientists. The researchers began their investigation by modeling the biogeochemistry of the oceans from the Archaean Eon, a time when high levels of iron bubbled up from hydrothermal vents. At first, the iron would have dissolved completely, but as cyanobacteria began to multiply and to produce oxygen, that iron would have changed into iron hydroxide, a form that is known to scatter green light.
Early light-absorbing bacteria had a hand in turning the water green.
The green light, the scientists hypothesized, could have in turn worked as a selection pressure on cyanobacteria. They tested this idea by cultivating and genetically engineering two distinct kinds of cyanobacteria under different light conditions, one dominated by white light and one by green light. Under green-light conditions, the bacteria with certain specialized light-harvesting pigments grew more quickly. Scientists had long puzzled over why some cyanobacteria made these extra pigments, known as phycobilins, alongside chlorophyll, which is used by all photosynthetic organisms to absorb energy from white light. Now an answer presented itself: Perhaps the phycobilins evolved to help these bacteria turn green light into energy.
Environments resembling those of the Archaean still exist today, according to Matsuo; around Iwo Island in the Satsuma archipelago, iron from thermal vents creates a similar green light window at 18 feet deep. And at a number of lakes around the world, iron hydroxide gathers in shallow surface layers: Lake La Cruz in Central-Eastern Spain, Lake Matano in Indonesia, Lake Pavin in France, and the Red Sea.
“I like this paper,” says Cameron Thrash, a marine ecologist who studies oceanic microbes at the University of Southern California. Biological oceanographers think a lot about how phyotplankton exploit different niches based on available wavelengths of light, he says, for example in open ocean versus coastal areas. “In this case, the authors are thinking about how the evolution of one of these spectral tunings may have happened based on the occurrence of predominantly green surface water in the Archaean.”
Thrash says he finds it particularly interesting that the researchers postulate that the early light-absorbing bacteria also had a hand in turning the water green—showing how Earth and life influence one another in intricate ways.
There are also glimmers that our oceans could change color—again. A 2023 study in Nature showed that the color of the global oceans has shifted slightly over the last two decades, likely as a consequence of climate change. In particular, tropical ocean regions near the equator have become greener—likely because of changes in phytoplankton at the surface.
Looking ahead, Matsuo says he is working with NASA Ames Research Center to study whether green oceans could be an indicator for life on distant planets. Green oceans also reflect more light than blue ones—which makes it more likely they could be spotted by life-searching probes.
Matsuo says he hopes green oceans will become an indicator of the early evolution of life elsewhere in the universe. 
Lead image: 100Y Design / Shutterstock
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