Colors may be evolution’s most beautiful accident. Mutations that perturbed the arrangement of structural components, such as cellulose, collagen, chitin, and keratin, inadvertently created nanoscale landscapes that catch light in the most vibrantly diverse ways—producing iridescent greens, fiery reds, brilliant blues, opalescent whites, glossy silvers, and ebony blacks. Structural colors, in contrast to those produced by pigments or dyes, arise from the physical interaction of light with biological nanostructures. These color-creating structures likely developed as an important phenotype during the Cambrian explosion more than 500 million years ago, when organisms developed the first eyes and the ability to detect light, color, shade, and contrast. Ever since, structural coloration has evolved multiple times across the tree of life, as a wide range of organisms developed ways to fine-tune the geometry of some of the most abundant (and often colorless) biomaterials on Earth, engineering grooves, pockets, and films that scatter light waves and cause them to interfere with each other in ways we humans happen to find aesthetically pleasing. For centuries, scientists have studied the minute structures that give peacock feathers, butterfly wings, and beetle carapaces their striking iridescence, but “nothing really compares to the interest we’ve seen in these species over the last 10 or 12 years”, says physicist Peter Vukusic at the University of Exeter in the UK. The recent interest stems from the birth of synthetic photonics—a field that aims to create materials that precisely control the flow of light and color through structure. Begun in the late 1980s and early 1990s, synthetic photonics has given rise to ubiquitous technologies such as Blu-ray and to major technological advances in telecommunications. “But if you look at a colorful butterfly, or beetle, or fish, or bird, you see these structures that have been doing a similar job for such a long time”, Vukusic says. At London’s Royal Botanic Gardens, Kew, the berrylike fruits of the African perennial herb Pollia condensata, still shine an intensely bright metallic blue despite having been collected more than 4 decades ago. When the specimens came to the attention of Silvia Vignolini at the University of Cambridge, she set out to understand how exactly the fruit produced its intense, iridescent color. “It was weird that the fruit would produce a color that wouldn’t fade in many, many years”, Vignolini says. “Some of the plants we studied were collected almost 100 years ago.” She and her colleagues were unable to extract any blue pigment from the Pollia fruits. But using transmission and scanning electron microscopes, the researchers found that the fruits’ long-lasting blue iridescence was generated by the outermost layers of thick-walled cells. Specifically, within the walls of those cells are layers of cellulose microfibrils, stacked in a manner resembling a helix—each layer slightly rotated with respect to the next. Within each cell wall are many such helicoids stacked one on top of another. The distance between two similar points in a given helix—a value known as the helical pitch—is about the same as the wavelength of blue light. As a result, blue light is selectively reflected while other color wavelengths pass through or get canceled out through a property called constructive interference. However, the spacing of the cellulose layers varies slightly from cell to cell, with a minority of cells giving off red, purple, and green hues, thus giving the fruit a pixelated appearance under a certain type of illumination. The brightest living tissues on the planet are found in fish. Under ideal conditions, for example, the silvery scales of the European sardine and the Atlantic herring can act like near-perfect mirrors—reflecting up to 90% of incoming light. The European sardine and the Atlantic herring come close to mimicking silvery mirrors thanks to intracellular stacks of thin, flat, highly reflective crystalline plates of guanine crystals, separated by cytoplasm. These reflective cells are found on the layer of skin called the stratum argenteum, which lies immediately beneath the scales. By randomly varying the spacing between the cytoplasm and guanine-crystal layers, a broad band of wavelengths is reflected, affording the fish a silvery, mirror-like appearance. To create structural colors, organisms must master architecture at the nanoscale—the size of visible-light wavelengths. “But it turns out that biology doesn’t do a good job of creating nanostructures”, Yale University evolutionary ornithologist Richard Prum says. Instead, organisms create the initial conditions that allow those nanostructures to grow using self-organizing physical processes. Thus, organisms exploit what’s known as soft condensed matter physics, or “the physics of squishy stuff” as Prum likes to call it. This relatively new field of physics deals with materials that are right at the boundaries of hard solids, liquids, and gases. “There’ve been huge advances in this field in the last 30 years which have created rich theories of how structure can arise at the nanoscale,” Prum says. “It has been very applicable to the understanding of how structural colors grow.” It is a lament heard universally throughout the field of structural colors: there is an overwhelmingly poor understanding of the biological mechanisms that control and fine-tune the formation of these color-producing nanostructures. “We don’t know anything about the genetics behind structural colors”, says Nicholas Roberts, a physicist at the University of Bristol.
Posted on: Thu, 20 Jun 2013 17:37:05 +0000
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