Iridescence is a structural color phenomenon where varying-colored hues arise from the interaction of light with physical surfaces, rather than from chemical pigments. Iridescence was thought to be unique to specific organisms such as butterflies, peacocks and birds, but it has been recently reported in bacterial species. Here we isolated a previously uncharacterized iridescent marine bacterial species from the ocean in Woods Hole, MA. Under epi-illumination the iridescence changes from vibrant red/orange/yellow to green. How living bacterial cells produces iridescence is an intriguing question, given that the architecture that gives rise to iridescence in terrestrial organisms are typically made up of non-living biological material in fish scales, bird feathers, or the arthropod exoskeleton. Notably, when the bacteria are grown in liquid culture, they do not display iridescence, suggesting that the cells' ability to self-organize into a community on solid surface determines their iridescence. Recent studies have shown gliding motility and physical sub-structures and arrangements are associated with iridescent properties in bacteria. Iridescent bacteria have also been investigated for their potential use as iridescent "bioink". In this study, we report a surprising phenomenon, where a natural, spontaneous change in iridescence occurred during biofilm growth, giving rise to different iridescent color hues than normal. This is the first study describing the natural formation of iridescence mutants, not only in bacteria, but in any living system.
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