Color is abundant in the natural world, as are the mechanisms used to create it. Two recent studies explore the basis for the iridescent sheens in fish and plants. Gur et al. focused on Cyprinus carpio, or Japanese koi. Though it is known that the coloration of many fish is due to the presence of anhydrous guanine crystal arrays and that other organisms contain stable layers of amorphous guanine, there was no prior demonstration within biological systems that an organic molecule could pass through an amorphous phase en route to a crystalline phase. The authors used cryo-SEM imaging, microspot X-ray diffraction and X-ray fluorescence to identify a pool of guanine distinct from crystalline guanine and suggest that this material arises from vesicles whose elongation matches that expected of a growing crystal. Vignolini et al. determined that the blue coloration of Pollia condensata fruit is created by the combined action of three cell layers with different functions: the first reflects circular polarized light using a multilayered cell wall, the second layer absorbs the transmitted light using tannins, and the last layer back-scatters the remaining light to the tannin layer. The study also showed the surprising presence of both right- and left-handed helicoidal microfibrils in the same tissue.
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Goodman, C. In living color. Nat Chem Biol 8, 873 (2012). https://doi.org/10.1038/nchembio.1104
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DOI: https://doi.org/10.1038/nchembio.1104
