In the waters off Iwo Jima, a volcanic island in Japan’s Satsuma archipelago, the sea has a distinct green tint. It’s not just a trick of the light — it’s caused by microscopic particles of oxidized iron, Fe(III), suspended in the water. And in this greenish glow, blue-green algae flourish.

Despite their name, blue-green algae aren’t algae at all — they’re cyanobacteria: ancient, light-harvesting microbes that helped shape Earth’s atmosphere. These microbial pioneers were among the first organisms to photosynthesize using sunlight and water, releasing oxygen in the process. Their ancestors evolved more than 2.5 billion years ago in a radically different ocean — one rich in dissolved iron.

Back then, the oceans were riddled with ferrous iron (Fe(II)), and early photosynthetic microbes likely relied on it as a source of electrons for metabolism. These iron-laden waters set the stage for a fundamental planetary shift. Now, a new study led by Taro Matsuo and colleagues suggests that ancient Earth’s seas didn’t just contain iron — they glowed green because of it.

The Pale Green Dot Theory

Carl Sagan famously called the Earth a “pale blue dot“, but before that, it may have been a pale green dot. Long before forests or fish, Earth’s oceans teemed with microbes. Chief among them were cyanobacteria — tiny architects of photosynthesis.

Using numerical simulations based on iron chemistry, particle physics, and light penetration models, Matsuo’s team reconstructed the underwater “light window” of that time. The results were surprising. The light that filtered through to cyanobacterial habitats was mostly green. It peaked between 500 and 600 nanometers — wavelengths that chlorophyll a, the main pigment used in photosynthesis, barely absorbs.

So how did cyanobacteria survive in a world where chlorophyll a was poorly suited to the available light?

They hacked the system — with pigments.

Unlike plants, cyanobacteria don’t rely solely on chlorophyll. They also build huge protein structures called phycobilisomes. These proteins are studded with accessory pigments that act like energy funnels. One pigment in particular (phycoerythrobilin or PEB) soaks up green light in particular.

Implications for Earth — and beyond

To understand why cyanobacteria evolved this peculiar system, the researchers genetically engineered modern strains to mimic their ancient ancestors. They added PEB-producing genes to Synechococcus elongatus, a cyanobacterium that doesn’t naturally make PEB. Under green light, the modified cells grew faster than their wild-type peers. In other words, evolution picked the right pigment for the job.

The implications stretch far beyond Earth.

If green seas once nurtured the rise of life on Earth, then similar spectral environments on distant planets might do the same. In the absence of atmospheric oxygen, planets with iron-rich oceans could develop the same kind of light-filtering chemistry. These green seas would subtly select for microbes evolving pigments tuned to that spectrum, just like early cyanobacteria did. Phycobilins, particularly PEB, are not only well-suited for harvesting green light but also support long-distance energy transfer, so they have evolutionary advantages.

In other words, a planet needn’t be a “pale blue dot” to host life. A pale green dot — bathed in iron-filtered light — might be just as biologically promising. Spectral fingerprints of green oceans, detectable by future telescopes, could serve as subtle biosignatures.

Oceans on other planets could have other colors, too. Seas rich in sulfur compounds might take on a yellow or orange hue, while methane-dominated oceans — like those on Saturn’s moon Titan — could appear dark and oily. High concentrations of certain minerals or pigments produced by microbial communities could also tint the water red, purple, or even black. In fact, climate change is already changing the color of Earth’s oceans.

Ultimately, the color of an ocean isn’t just about the pigments inside it — it’s a reflection of the chemistry, biology, and light filtering through a planet’s atmosphere, making it a potential clue in the search for alien life.

The study was published in Nature Ecology & Evolution.