The ancient ocean tinted the Earth green for 2.5 billion years.

Cyanobacteria are a group of photosynthetic bacteria. Like plants, they have the ability to convert solar energy into chemical energy and release oxygen as a byproduct of photosynthesis. These organisms are considered some of the oldest forms of life on Earth: their traces have been found in the form of stromatolites (sedimentary deposits created by cyanobacteria, fossilized bacterial mats from ancient times) that are over 3.5 billion years old, indicating their existence during the early stages of our planet's evolution.

In plants, the central element of photosynthesis is chlorophyll—a pigment found in the chloroplasts of cells. It absorbs light in the blue and red regions of the spectrum and reflects green. The reflection of green light is what makes plant leaves green.

Cyanobacteria also contain chlorophyll, and some species have an additional group of water-soluble pigments known as phycobilins. This group includes phycoerythrobilin, which absorbs light in the green and yellow regions of the spectrum, with wavelengths ranging from approximately 495 to 570 nanometers.

A team of Japanese scientists led by Taro Matsuo from Nagoya University posed two questions: why do cyanobacteria need additional pigments (phycoerythrobilin) if they already have chlorophyll? What might the presence of such pigments indicate about the environment in which the first photosynthetic organisms evolved? The researchers conducted a series of experiments and simulations to understand how the ancient ocean, which nearly entirely covered the Earth's surface billions of years ago, influenced the evolution of cyanobacteria.

According to Matsuo, during the Archean Eon—one of the four eons in Earth's history, spanning from four to 2.5 billion years ago—the ocean was saturated with iron, which dissolved in water as iron hydroxide (Fe(OH)₃). This substance resembles rust—loose and brownish-red.

Matsuo and his colleagues performed modeling to determine how much iron hydroxide could have been present in the ocean at that time and to identify the spectrum area necessary for photosynthetic organisms. The scientists used computer models to "recreate" conditions similar to those in the Archean.

The modeling showed that the Archean ocean contained so much iron hydroxide that it acted as a giant filter: absorbing light in the blue region of the spectrum. The water, in turn, absorbed light in the red region of the spectrum, just as it does today. Green light was likely to remain "unscathed" and could penetrate the depths. The researchers concluded that green light in the wavelength range that is today absorbed by the additional water-soluble pigments of cyanobacteria—phycobilins—most likely penetrated the depths.

However, modeling is only part of the work. To validate their findings, the researchers conducted an experiment. They attempted to grow several species of cyanobacteria (with and without phycoerythrobilin) in the laboratory under different light conditions. The cyanobacteria containing phycoerythrobilin grew significantly faster than the "normal" ones when exposed to green light (in the wavelength range that presumably penetrated the depths of the ancient ocean). Previous genetic analysis had shown that the phycoerythrobilin pigment was present in the common ancestor of modern cyanobacteria. Thus, the green "skill" is not a random mutation, but an ancient evolutionary invention.

The researchers went even further and traveled to the Japanese volcanic island of Io, part of the Kazan archipelago. There, at a depth of 5.5 meters, hot springs saturate the water with iron, causing the water around the island to appear green, much like in the Archean ocean.

It turned out that at this depth, cyanobacteria with additional "green pigments" literally dominated the area, while at the surface, where more blue light was present, other species without phycoerythrobilin prevailed. In other words, nature itself replicated the laboratory experiment, confirming that life always adapts to environmental conditions.

The scientists concluded that three billion years ago, the ocean resembled a giant "iron soup." It was saturated with iron hydroxide, which absorbed blue light, while the water, as it does now, "took" red light. A significant portion of green light penetrated the ocean depths, while some was reflected off the surface. This light had the greatest impact on the overall hue of the Earth.

In such conditions, cyanobacteria evolved. They had to adapt to the available light. They developed additional pigments that captured green rays for photosynthesis. These pigments helped the bacteria survive in a unique environment. Over time, cyanobacteria began to release oxygen, which reacted with dissolved iron in the ocean, forming iron oxides, the heavy particles of which settled to the bottom.

The green period of Earth ended about 600 million years ago when the oxygen produced by the bacteria completely oxidized the iron in the ocean. Traces of this event remain in the form of banded iron formations—layers of oxides in ancient rocks.

Over time, the water became clear, the green tint disappeared, and the Earth gradually "dressed" in a pale blue color visible from space. The modern color of the Earth is due not only to the oceans but also to the atmosphere. Rayleigh scattering (phenomenon where blue light is scattered more strongly in the air) paints the sky blue, blending with the dark blue of the oceans.

Even today, in coastal waters rich in organic matter (leaves, algae, silt), the water often appears greenish. Organic matter absorbs blue light, leaving green for cyanobacteria, which still use it for photosynthesis. Additionally, the green tint of the oceans serves as a clue for astronomers: if a distant exoplanet appears green, it may have its own "cyanobacteria" oxidizing metals in the water.

The scientific work of Matsuo's team has been published in the journal Nature Ecology & Evolution.