Lurking in the rocky depths off the Atlantic coast of Europe and in the Mediterranean Sea, a hidden gem of the waters exists in plain sight. Brown algae known as Cystoseira tamariscifolia may appear as average kelp but don’t be deceived. Although their color, a uniform sea-green, is rather boring to the naked eye, C. tamariscifolia are actually quite complex photosynthetic organisms and at closer inspection display strikingly vivid colors resembling the iridescence of opal gemstones. Similar to the skin of a chameleon or the wings of a butterfly, the cells of this algae contain tiny, reflective spheres, known as photonic crystals, which are organized to resemble nanosized honeycombs and are only visible under the microscope. These three-dimensional crystals are tightly ordered, multilayer structures which control how certain wavelengths of light are diffracted and in which direction.

C. tamariscifolia algae’s cells contain tiny, reflective spheres, known as photonic crystals.                                                                                                                                   Credit: University of Bristol

In April, scientists from Bristol, UK and Braga, Portugal have found that C. tamariscifolia, more affectionately known as Rainbow Wrack, contain opal-like photonic crystals which are able to control light within photosynthetic cells, producing their exceptional deep blue to green and even turquoise colors. They published their findings in the Journal Sciences Advances where they demonstrate the composition of the vesicles, unlike that of natural opals which are composed of the second most abundant material on earth after oxygen, silicon dioxide (SiO₂), are uniquely comprised of fat molecules known as lipids. Although the natural function of Rainbow Wrack’s structural color remains a mystery, the proximity of the crystals to chloroplasts, the organelles responsible for photosynthesis, and the fact the algae reside in environments with changing light availability, suggest the crystals play a role in regulating photosynthesis. Scientists further observed when replicating a high tide environment with little light, the iridescent colors are prominent and visible, yet under low tide with abundant light conditions, the color fades. However, most astonishingly, when the algae was once again in a low light environment, the color reappeared and was able to disappear once more indicating the algae can manipulate the crystals continuously. They attributed this effect to the ordering of the lipid spheres inside the vesicles. In darkness, the visible lipid spheres become ordered forming the opal-like photonic crystal and scattering the light to the chloroplasts, yet in the presence of light, the spheres are disordered and transmit the light through the vesicle.

It is remarkable not only are the algae capable of forming opal-like photonic crystals from lipids and dynamically controlling their packing structure’s order, but they are able to reverse it and back again. The scientists point out this ability even surpasses the capabilities of current artificial opal synthesis. For this reason, C. tamariscifolia are truly the hidden gems of the sea.

Although the algae’s structural color is surprising and astonishing, the potential applications which stem from understanding this process are numerous including drug delivery, biodetection, sensors, and tumor screening. Currently there are biosensors utilizing photonic crystals such as measuring glucose levels, however they produce single-shot measurements unable to monitor continuously. If scientists are able to understand the way in which photonic crystals in C. tamariscifolia dynamically respond to the environment, they could harness the photonic crystals to improve biomedical devises.

A visit to the Atlantic coast of Europe and the Mediterranean Sea not only provides exceptional experiences on land, but now also under the water’s surface. If you can find this ordinary brown algae, you will have found a hidden gem of the waters existing in plain sight, whose beauty is not just cell deep.

Original Research Article: Lopez-Garcia, Martin, et al. “Light-Induced Dynamic Structural Color by Intracellular 3D Photonic Crystals in Brown Algae.” Science Advances, vol. 4, no. 4, 2018, doi:10.1126/sciadv.aan8917.