Structural color not just for animals and plants

Researchers from the Adolphe Merkle Institute and colleagues at the University of Fribourg’s botanical garden and the University of Salzburg have uncovered that structural color is far more prevalent in slime molds than previously thought.

Slime molds, also know as myxomycetes, are simple, single-celled organisms that often found as blobs on decaying wood or leaves. They move slowly to feed on bacteria and other tiny life forms, and when conditions are right, they form small, highly colorful, spore-producing structures that help them spread. Although they look somewhat like fungi, myxomycetes belong to a different group of organisms called protists.

Single species had been previously investigated. This time, the researchers took different mold species into consideration and found that all of them exhibited thin film interference, a phenomenon responsible for their iridescent appearance. Thin film interference happens when light reflects off the top and bottom surfaces of a very thin layer, like a soap bubble or an oil slick, in this case, the peridium, the membrane surrounding the spore mass. The two reflected light waves combine in ways that can either strengthen or cancel each other out. This interference creates bright, colorful patterns because some colors of light are amplified while others are reduced, depending on the thickness of the film and the angle of the light. It is also found in some insects and flowers.

All 22 studied species showed this interference in their outer layers, producing a range of colors across the visible spectrum. According to the researchers, the colors result from a combination of structural effects and pigment absorption. Some species, like Metatrichia vesparium, have complex, multi-layered structures including calcium-rich deposits.  However, the structural color likely doesn't serve a biological purpose but may be a by-product of the organisms' life cycle.

AMI Soft Matter Physics PhD student Viola Bauernfeind explained, "We were surprised to find that this structural coloration is so widespread across different slime mold species. It's an unexpected feature in organisms traditionally classified as simple amoebae."

The research team used a combination of advanced imaging techniques, including light microscopy, scanning electron microscopy, and optical simulations, to analyze the slime molds' structures in detail. Prof. Gregor Kozlowski, head of the University of Fribourg’s botanical garden, coordinated contacts with the Polish who made the samples available, and suggested the analysis.

While the colorful display doesn't appear to have a specific biological function, unlike in plants or animals where it might attract pollinators or mates, it provides valuable insights into the organisms' development and structure. The findings were published in the journal Optics Express.

Reference: Bauernfeind, V.; Ronikier, A.; Ronikier, M.; Kozlowski, G.; Steiner, U.; Wilts, B. D. Thin Film Structural Color Is Widespread in Slime Molds (Myxomycetes, Amoebozoa). Opt. Express, OE 2024, 32 (4), 5429–5443.