These images show rainbow wrack (Cystoseira tamariscifolia) reflecting light. Credit: University of Bristol (Click to enlarge).

Rainbow wrack (Cystoseira tamariscifolia) is a type of brown alga found in the Mediterranean Sea and off the Atlantic coast of Europe. In the water, these algae glow. And although there are many glimmering organisms that live in the water, such as bioluminescent jellyfish and lantern fish, most produce their own light. The iridescent algae rainbow wrack, on the other hand, uses a crystal structure to reflect sunlight, according to a new study published April 11 in the journal Science Advances.

To study the shimmering seaweed, a group of researchers gathered the plant from a typical tourist-inhabited beach in southwest England during low tide. Using a variety of microscopy techniques, they discovered that the alga’s cells contained baggies of “opals.” Physicists use the term “opal” to describe any material with a very specific, tightly packed lattice structure, said senior study author Ruth Oulton, a physicist at the University of Bristol.

Whereas gemstone opals are made from spheres of silicon dioxide, this algal opal is made from oil droplets, or lipids. But all “opals” reflect light in very similar ways.

It’s very rare for plants to have opal-like structures, but if they do, they’re usually found in a hard exterior, like cellulose in cell walls, Dr. Oulton told Live Science. In the case of rainbow wrack, “it’s the first time that an opal’s been found that’s not made of hard material inside a living thing.”

What’s more, the researchers found that rainbow wrack reacted to the light, changing its structure to dim and brighten itself, depending on the conditions. When there was ample light, the alga took apart its tightly packed opal structure to dim its glow. But when surrounded by near darkness, within a few hours it re-ordered all of the spheres back together into a lattice. Soon, it was glowing again.

The researchers don’t know exactly why rainbow wrack adopted this mechanism. But because this species lives in an area where changes in the tides sometimes leave it exposed on the beach and other times buried under 9 feet of water, they think it could have evolved to regulate the amount of light that reaches its chloroplasts for photosynthesis. It’s more than likely not a coincidence that the baggies of opals are surrounded by chloroplasts, Dr. Oulton said.

“What we know is seaweed itself can change its opal… when it gets lighter, the opal structure disappears,” she said. “When a beetle dies, the opal is still there, but if the seaweed were to die, all of it would disappear,” she added.

Scientists can’t yet replicate the process of turning the opals on and off in the lab, but they’d like to be able to. After talking to some chemists, the team figured out that this new finding could open up new possibilities, such as biodegradable displays. For example, if they can mimic rainbow wrack’s process of packing and unpacking opal structures based on light, researchers may be able to create biodegradable packaging and labels from something as commonplace as coconut oil.

This could take the form of labels on food packaging that turn a different color, based on expiration dates; or plastic in packaging that totally disintegrates after a while, the researchers said.