Our planet has had an ocean for around 3.8 billion years, but new research suggests it hasn't always been blue.
Away from coastal areas, the sea today is blue because of the way water absorbs longer wavelengths of sunlight towards the red end of the spectrum.
This leaves the shorter, bluer wavelengths to penetrate deeper, scattering and reflecting back into our eyes. Several billion years ago, the blue water may have been masked by other colours.
Back then, the earliest forms of life were evolving in the ocean, including single-celled cyanobacteria. They played a key role in making the world habitable by harnessing energy from sunlight through photosynthesis and turning it into the first freely available oxygen on Earth.
Researchers from Japan recently built a computer model that shows that the first oxygen released by cyanobacteria combined with iron dissolved in the water, producing an oxidised form of iron that turned the ocean’s surface green.
What’s more, early cyanobacteria may have evolved to thrive in green-tinted waters.
In the same study, scientists produced genetically engineered cyanobacteria with a form of photosynthetic pigment that harnesses green light, called phycoerythrobilin.
In contrast today, most plants use chlorophyll pigments to harness red and blue light.
Under lab conditions, these engineered cyanobacteria flourished in tanks of green water. The team also noticed a similar phenomenon in nature.
Waters around the Japanese island of Iwo Jima are naturally rich in oxidised iron and are distinctly green. The cyanobacteria growing along the coastline here have higher levels of the green-harnessing pigment.
The study suggests that exobiologists searching other planets for signs of life should keep an eye out not just for blue, liquid water, but also greener tints that could hint at the earliest signs of life.
This article is an answer to the question (asked by Philip Burke, Somerset) 'Was the sea always blue?'
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