The alga Calyptrolithophora papillifera is encased in a shell of calcite crystals with a two-layer structure (visible on oblique face). Calculations show that this protective covering reflects ultraviolet light. Image Credit: J. Young/Natural History Museum, London
Many of the oceans' algae have evolved natural "sunscreens" as protection from the sun's ultraviolet rays. But researchers recently showed that geometrical arrangements of tiny calcite crystals surrounding certain algae achieve the same effect as chemical sunscreens, reflecting ultraviolet radiation while allowing longer wavelengths through for photosynthesis. This natural "photonic" device parallels similarly ordered structures that generate the colored patterns of butterfly wings and may lead to new designs for technological devices that manipulate light.