Earth's oceans were once green not blue and they may change colour again — but when?

Earth’s Oceans Were Once Green, Say Japanese Scientists (Image: Canva)

It’s hard to picture now, but Earth’s deep blue oceans were once green. According to a new study by Japanese researchers, early photosynthesis helped shift the colour of our seas. The discovery offers fresh insight into how simple life changed the planet over billions of years.

The Earth's Waters in the Green Age

There existed only single-celled creatures in the seas of Earth long before the plants and animals. Due to their chemistry, the ancient seas looked green, not blue. The land was desolate, covered in black, brown, and grey rocks. Banded iron formations are characteristic patterns that occurred below the surface between 3.8 and 1.8 billion years ago. The shifting chemistry of Earth's atmosphere and seas was recorded by these strata.

When the world contained little or no oxygen during the Archean aeon, the transformation began. Early life forms utilised a type of photosynthesis that used iron instead of water. The "Great Oxidation Event"—a turning point that paved the way for the advent of more complex life—was the eventual product of this process. This shift from oxygen-rich to oxygen-poor conditions is evidenced by the red and grey layers found in ancient iron deposits.

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Clues from a Modern-Day Island

Scientists studied the sea surrounding Japan's volcanic island, Iwo Jima, in order to add weight to their hypothesis. The greenish color of this sea is caused by oxidised iron, or Fe(III). The environment is one that is favorable to blue-green algae, meaning that early seas might have favored similar life. These algae evolved along with bacteria that generated photosynthesis through iron instead of water.

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Ocean and Life Colours

Chlorophyll and other pigments are utilized by photosynthetic organisms to trap sunlight. They use carbon dioxide and sunlight to produce sugars for energy. Contemporary blue-green algae with the pigment phycoerythrobilin (PEB) grew more efficiently in green water under laboratory conditions, indicating that early life was adapted to the marine environment that prevailed on Earth.

The color of our oceans directly affects the chemistry of the water and the presence of life.
Oceans might have become purple in the ancient past because of high sulphur content.
They could have been red in tropical climates.

The "red tides" are also caused by an algae that alters the color of water. Earth's oceans can again become a different colour as the Sun grows old, reminding us that everything changes with time.