Fluorescence seems like technology, but we actually have a lot of fluorescence in nature too. Fluorescence in the animal world can be important signals vital to the species. In stones, fluorescence is due to impurities and inclusions, or, in fluorite, one of the main features defining the rock.
How did we get Fluorescence in Nature?
In nature, organisms use their naturally evolved fluorescence for purposes including communication, attraction of mates, luring of prey, camouflage from predators, and UV protection from sunlight.
Scientists also really like to alter genes to express fluorescent compounds for harvesting in fluorescence microscopy applications. Nearly any brightly stained micrograph you’ve seen is from fluorescent dyes targeting certain molecules to make them visible. For example, GFP, the first fluorescent protein used in fluorescence microscopy, was harvested from special jelly fish. You can also check out these genetically engineered silkworms to producing fluorescent silk.
1. Scorpions
Several types of scorpions, including the species Lychas scutilus, found all over Asia and Africa, have a fluorescent glow. They produce blue-green light under 320-400nm (blue/UV light) excitation. Specifically, they have compounds in their exoskeleton that act as fluorophore pigments, beta-carboline and 4-methyl, 7-hydroxycoumarin. Most insects, including bees, can see in this range of light, while humans can’t. Thus several arthropods have fluorescent properties to signal to each other. Coumarin compounds, also found in plants like cinnamon and tonka beans, have a sunblock property, so these developments may also help critters tolerate time in the sun.
2. Fluorite – the namesake of fluorescence in nature
Fluorite, first found as fluorspar, naturally occurs as a component in many rock formations. Fluorspar was first mentioned in a 1530 text about metal working by Georgius Agricola. When purified from the rock as calcium fluoride (CaF2), fluorite can appear in colors from sea-foam to emerald green, and lavender to magenta purple. The stone can also appear as yellow, red, or blue.
Fluorite has a lot of commercial and technological uses, in addition to simply being sold as a beautiful collector’s crystal. Fluoride used in industrial and pharmaceutical applications is often derived from purified fluorite rock. Fluorite cleaved sheets are also used in optics when researchers need to pass light in the very low or very high wavelength bands, since conventional glasses can actually be opaque in those ranges.
3. Lignum nephriticum
As discussed in The History of Fluorescence, the first formal study came in the 1560s when Franciscan explorer Bernardino de Sahagún observed people of Mexico using bark of the Eysenhardtia polystachya tree, known as “palo azul” (blue wood). A tonic for the kidneys is made from a tincture of the bark. Beyond the medical properties, the tinctures had a distinct glow to it.
1646 Athanasius Kircher performed his own experiments and found the liquid really turned many different colors dependent upon light source. Johann Bauhin in 1650, a Swiss botanist, described the color as being like opal, changing in the light. Newton and Robert Boyle also later mentioned Lignum Nephriticum in their own studies. Samples of the bark and fluid were used to study fluorescence, and later on connected with fluorite rock, giving birth to the name of the phenomenon we now call fluorescence.
4. Spiders and other arthopods
I’ve mentioned many insects and arthropods have evolved fluorescent compounds. Here you can read about the understudied butterflies too. Note there is a difference between this and bioluminescence. Bioluminescence is a form of phosphorescence, which is a chemical reaction given off by the exchange of compounds. Fluorescence is the re-emitting of light already absorbed. Rest assured, I’ll cover bioluminescence in other articles.
Spiders, unlike the scorpion, have fluorescent compounds in their blood, including their eyes. They have a wide variety of fluorescent compounds spread across many species and taxa. These ultraviolet cues, which other bugs can see,are important in their predator-prey relationships. Jumping spiders also need fluorescence to differentiate between males and females, so it plays a role in their courtship rituals.
Here is Nicky Bay’s album of fluorescent spiders and more scorpions. Note that the spiders have more complex fluorescent patterning.
5. Jellyfish – the fluorescence in nature that went to the laboratory
Most biologists remember learning about GFP – green fluorescent protein, in the context of the origins of fluorescence microscopy. The protein was first isolated from jellyfish species Aequorea victoria in the 1960s by Osamu Shimomura, a Japanese marine biologist and organic chemist. There is a bioluminescent protein that has fluorescence properties under calcium influx as a secondary feature. GFPs usefulness in microscopy was only discovered in the 1990s, and molecular biology has been able to put it to a lot of use since. Now there are many other synthesized dyes, such as mCherry, chosen for colors and properties, based on this doscovery.
6. Amber
Amber is a resin-like organic solid structure with pure formula C12H20O. Often collectors find amber that has preserved very old, prehistoric specimens like bugs inside. People in the Stone and Bronze age, three to four thousand years ago, already knew about amber.
Within the amber, there are chains of hydrocarbon with 20 carbons to 12 hydrogen, making a perylene compound. Perylene throughout the samples gives amber its fluorescent properties. In fact, perylene is used in new OLEDs for the blue emission. ITs known absorbtion is at 432 nm, UV.
Younger, unripened amber, called copal, does not fluoresce. So collectors can us UV illumination a a diagnostic tool for the quality and age of their specimens.
Studies on blue amber and its enhanced fluorescent properties: Here and here.
Bonus fluorescence in nature: perylenes are also found in at least two species of lichen: Laurera sanguinaria Malme and Graphis haematites Fée.
7. Emeralds
Emeralds have a bit more weaker fluorescence, but what’s interesting about them is that they fluoresce RED, taking in light around 430 nm and spitting it back out at 730 nm. And emerald specimens have varying levels of fluorescence since its due to certain mineral inclusions like Chromium ions. The most fluorescent emeralds can be found from Colombia and Peru.
Again the independent collectors at mindat.org have the best photo documentation of fluorescent emerald, which you can view some of here.
8. Coral
Coral is a marine species widely known to be very fluorescent across many varieties. First discovered in the 1990s, coral glows to attract their “prey”. Plankton like to swim towards the Green Fluorescent proteins.
Another proposed reason for fluorescent coral is the sunblock properties already discussed. The proteins can express up or down in concentration to ensure optimal light levels received to the coral. Coral has been studied widely by marine biologists to explain the diversity of their colorings. Indeed, like all of the animal kingdom, the colors always play an important role in the success of the species.
9. Chlorophyll
Chlorophyll is the pigment in leaves that give them a green color and captures light for photosynthesis. Photosynthesis is like the plants breathing, converting sunlight and carbon dioxide into energy for the plant and off-putting oxygen.
A dark-adapted leaf will rapidly emit light under illumination. Researchers use the degree of fluorescence in leaves to determine their efficiency in photosynthesis. The degree of fluorescence changes depending on how long the leaf has been in light as well. So the chlorophyll extractions in experiments can tell scientists a lot about the effects of certain variables on the efficiency of the plant, as well as its stress tolerance. There are even special chlorophyll fluorometers that let researchers measure this directly off the plant.
10. Rubies
Even artificial rubies fluoresce under blacklight. And while the emeralds glow red, rubies can glow blue or red. Artificial rubies have less impurities and striations, so they glow more evenly. Many natural rubies will glow much less brightly.
And, just like in the emeralds, the fluorescent properties of rubies is from the ionic inclusions. Not just chromium, but iron and others, present in the crystal matrix, will have the absorption and emission effect needed for fluorescence.
There’s a lot more Fluorescence in nature too
Many more insects, gems, and animals glow under fluorescence. This is all invisible to our human eyes unless we go out exploring with some blacklights. It’s very fun to imagine being a bug and seeing in a wider range of colors than we do no. Indeed the bug kingdom has its own secret world we rarely see.
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