Our planet has hosted oceans for approximately 3.8 billion years, but their current blue appearance is relatively recent. Research indicates that it hasn’t always been this way.

In the ocean’s depths today, the water appears blue because it absorbs longer wavelengths of sunlight, particularly those at the red end of the spectrum.

This absorption allows shorter, bluer wavelengths to penetrate further and scatter back into our eyes. Billions of years ago, various colors may have masked the blue waters.

During that era, the earliest life forms emerged in the oceans, particularly unicellular cyanobacteria. These organisms were crucial in shaping our planet’s habitability by capturing sunlight energy through photosynthesis, resulting in Earth’s first oxygen availability.

Researchers in Japan have recently developed a computer model demonstrating that the initial oxygen released by cyanobacteria reacted with dissolved iron in the seawater, leading to the formation of oxidized iron that turned the ocean’s surface green.

Moreover, early cyanobacteria likely adapted to thrive in the greenish water.

In their study, scientists engineered cyanobacteria that possess a specific type of photosynthetic pigment responsive to green light, known as phycoerythrobilin.

In contrast, most current plants utilize red and blue light through chlorophyll pigments.

In laboratory settings, these modified cyanobacteria were cultivated in tanks filled with green water, revealing a phenomenon that also occurs naturally.

The waters surrounding Iwo Jima in Japan are naturally high in iron oxide, imparting a unique green hue. The cyanobacteria prevalent along its coastlines possess pigments that make use of elevated green light levels.

This study suggests that exobiologists searching for extraterrestrial life should not only consider blue liquid water but also various shades of green that may hint at primitive life forms.

This article addresses the inquiry (by Philip Burke of Somerset): “Has the sea always been blue?”

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