Intracellular opal-like photonic crystals, formed from lipids within photosynthetic cells, have been observed to produce vivid structural color in the algae Cystoseira tamariscifolia. The bright blue and green iridescent sheen from the brown algae arises when the plant self-assembles a nanostructure of tiny oil droplets, made by the seaweed, to control how light reflects from the seaweed cells containing chloroplasts. The seaweed, known commonly as “rainbow wrack,” can switch this self-assembly on and off over the course of hours, creating changing opal-like photonic crystals that react to the changing sunlight in tidal rock pools. Morphology and structural color of the brown algae seaweed known as Cystoseira tamariscifolia. C. tamariscifolia at collection site showing structural color (a). Low-magnification (scale bar, 500 μm) image of a specimen with two different colors (b). Close-up of tips of blue (c) and green (d) specimens (scale bar, 50 μm). Single vesicles in the epidermal cells of two different specimens under a high-magnification optical microscope (e and f) (scale bar, 2 μm). Courtesy of University of Bristol. The nanostructures are composed of tightly, regularly packed spheres of the oil droplets, and the spheres are dynamically responsive to environmental illumination. Researchers at the University of Bristol found that the structural color was present in samples adapted to low light levels, whereas higher light levels produced a slow disappearance of the structural color such that it eventually vanished completely. Once the seaweed was returned to low-light conditions, the color re-emerged. By imaging the seaweed at a subnanoscale level, the team discovered that the seaweed was switching on and off the self-assembly, going from a disordered, unreflective state to an ordered, opalescent one. Previous examples of photonic structures in plants have been of one-dimensional structures. The presence of such a complex photonic structure in algae, such as the ordered 3D opal structure observed by the Bristol team, was surprising in itself, but even more so was that this structure was dynamic. How the algae is able to control the level of order or disorder, reversibly, remains unknown, but researchers surmise that because the opal-like photonic structures sit in the same part of the cell as the chloroplasts, they are likely to be controlling the light levels, scattering the light evenly to all the available chloroplasts inside the cell, possibly to help the seaweed adjust to different light levels during high and low tides. According to researchers, the seaweed’s ability to form opal-like structures surpasses the capabilities of current artificial opal synthesis. “The formation of opals from oil droplets is a completely new discovery," said researcher Martin Lopez-Garcia. "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.” The research was published in Science Advances (doi:10.1126/sciadv.aan8917).