Cystoseira tamariscifolia, or rainbow wrack can be quite beautiful, but it's colors come from an unexpected source. Dr Martin-Lopez Garcia/University of Bristol

A common seaweed has been found to make opals out of oil droplets, even controlling the production so they only appear when needed. Botanists and physicists are still struggling to work out how this is happening, but a solution may help us build better solar cells.

Although it is classified as brown algae, residents of Europe's coastlines have long known it can be far more colorful than that, inspiring its nickname “rainbow wrack”. The blue and green iridescence Cystoseira tamariscifolia produces comes from tiny spheres packed tightly within its cells that scatters the light of shorter wavelengths in many directions, while longer wavelengths are absorbed. The process is similar to that in opal gemstones, and related to the way particles make the sky blue in daytime and red at sunset.

However, when Dr Martin Lopez-Garcia of the University of Bristol investigated the process he found the seaweed is using very different material from the stones.

Opals are formed from silicon dioxide glass with some water included. Tiny spheres of silica glass diffract light of specific wavelengths, depending on size and packing, to induce their famous rich mix of colors. Despite the ubiquity of their ingredients, they're only common at a few places on Earth, notably South Australia and Ethiopia. However, the discovery of deposits on Mars demonstrates biological processes are not required to make them.

Cystoseira Tamariscifolia as normally seen (A), and the opalescence that gives it its name (B), rainbow wrack. The oil droplets that produce this under microscopes (C-F) University of Bristol

Rather than using glass, the seaweed is producing the same effect from spherical oil droplets. “The formation of opals from oil droplets is a completely new discovery," said Lopez-Garcia said in a statement. "If nanotechnologists were able to understand and mimic the dynamic properties of this seaweed opal, we may in the future have biodegradable, switchable display technology that may be used in packaging or very efficient, low-cost solar cells.”

PhD student Nathan Masters shone a light on C. tamariscifolia and found the colors disappeared. Yet when the seaweed was kept in low-light conditions the colors came back. In Science Advances Lopez-Garcia, Masters and colleagues report the algae switches between assembling oil droplets in ordered and unordered ways, depending on the light conditions, with the opal effect only present when the droplets are ordered.

Not only does the team not understand how the ordered assembly is occurring, no one is sure why. It's probably not a coincidence that the opals neighbor the photosynthesizing chloroplasts inside the seaweed's cells. The authors think the droplets may help distribute what little light there is at high tide evenly among chloroplasts to capture as much of the Sun's energy as possible.

Since distributing light where it can be absorbed is a key challenge of solar cell design, any tricks we can learn from the humble rainbow wrack could be game-changing.