Color in living organisms can be formed two ways: pigmentation or anatomical structure. Structural colors arise from the physical interaction of light with biological nanostructures. A wide range of organisms possess this ability, but the biological mechanisms underlying the process have been poorly understood. …
Two years ago, an interdisciplinary team from UC Santa Barbara discovered the mechanism by which a neurotransmitter dramatically changes color in the common market squid, Doryteuthis opalescens. That neurotransmitter, acetylcholine, sets in motion a cascade of events that culminate in the addition of phosphate groups to a family of unique proteins called reflectins. This process allows the proteins to condense, driving the animal’s color-changing process.
Now the researchers have delved deeper to uncover the mechanism responsible for the dramatic changes in color used by such creatures as squids and octopuses. The findings – published in the Proceedings of the National Academy of Science, in a paper by molecular biology graduate student and lead author Daniel DeMartini and co-authors Daniel V. Krogstad and Daniel E. Morse – are featured in the current issue of The Scientist.
Structural colors rely exclusively on the density and shape of the material rather than its chemical properties. The latest research from the UCSB team shows that specialized cells in the squid skin called iridocytes contain deep pleats or invaginations of the cell membrane extending deep into the body of the cell. This creates layers or lamellae that operate as a tunable Bragg reflector. Bragg reflectors are named after the British father and son team who more than a century ago discovered how periodic structures reflect light in a very regular and predicable manner.
“We know cephalopods use their tunable iridescence for camouflage so that they can control their transparency or in some cases match the background,” said co-author Daniel E. Morse, Wilcox Professor of Biotechnology in the Department of Molecular, Cellular and Developmental Biology and director of the Marine Biotechnology Center/Marine Science Institute at UCSB.
“They also use it to create confusing patterns that disrupt visual recognition by a predator and to coordinate interactions, especially mating, where they change from one appearance to another,” he added. “Some of the cuttlefish, for example, can go from bright red, which means stay away, to zebra-striped, which is an invitation for mating.”
The researchers created antibodies to bind specifically to the reflectin proteins, which revealed that the reflectins are located exclusively inside the lamellae formed by the folds in the cell membrane. They showed that the cascade of events culminating in the condensation of the reflectins causes the osmotic pressure inside the lamellae to change drastically due to the expulsion of water, which shrinks and dehydrates the lamellae and reduces their thickness and spacing. The movement of water was demonstrated directly using deuterium-labeled heavy water.
When the acetylcholine neurotransmitter is washed away and the cell can recover, the lamellae imbibe water, rehydrating and allowing them to swell to their original thickness. This reversible dehydration and rehydration, shrinking and swelling, changes the thickness and spacing, which, in turn, changes the wavelength of the light that’s reflected, thus “tuning” the color change over the entire visible spectrum.
“This effect of the condensation on the reflectins simultaneously increases the refractive index inside the lamellae,” explained Morse. “Initially, before the proteins are consolidated, the refractive index – you can think of it as the density – inside the lamellae and outside, which is really the outside water environment, is the same. There’s no optical difference so there’s no reflection. But when the proteins consolidate, this increases the refractive index so the contrast between the inside and outside suddenly increases, causing the stack of lamellae to become reflective, while at the same time they dehydrate and shrink, which causes color changes. The animal can control the extent to which this happens – it can pick the color – and it’s also reversible. The precision of this tuning by regulating the nanoscale dimensions of the lamellae is amazing.” …
Could these findings one day have practical applications? “In telecommunications we’re moving to more rapid communication carried by light,” said Morse. “We already use optical cables and photonic switches in some of our telecommunications devices. The question is – and it’s a question at this point – can we learn from these novel biophotonic mechanisms that have evolved over millions of years of natural selection new approaches to making tunable and switchable photonic materials to more efficiently encode, transmit, and decode information via light?”
Clothes that use this concept to blend in with whatever colors they are around will be a big seller one day.