Fluorescence, Bioluminescence, and Phosphorescence

Personal taste notwithstanding, let us be honestthe best corals are glowing corals! We all know how to intensify onlooker reactions to our reef tanks. Simply turn off the white daylight fixture, and flip on those actinic bulbs. People tend to stop talking and just stare. Dont kid yourself, I know you do it too! Suddenly the tank looks like one of those toys we had as kids, the kind where you put colored pegs in a board, and they would light up in colorful designs. The psychedelic shades of glowing greens, reds, oranges, purples, and blues set against an ethereal azure background remind me of Fourth-of-July fireworks, fired off at twilight. Creating this lighting effect is actually pretty easy. Answering questions as to why it happens is anything but.

As marine aquarists, we already inhabit an esoteric world of complexities that those not in the hobby find mystifying. Though this perception is not entirely justifiable, fluorescence, bioluminescence, and phosphorescence are three truly amazing coral qualities that enhance the mystery. That so many marine organisms possess one or more of these attributes is as astonishing as it is confusing. Though no definitive answer exists to explain the why question, we do know how these phenomena occur. There are also a few interesting theories about the functions of the essences, as I like to call them.

The following is a brief article which is intended to translate some of the intense scientific work on this subject into a more digestible explanation. Essentially, the discussion centers on some of the highlights of the topic, without getting too technical.

Bioluminescence

Bioluminescence is a term used to describe light created by living organisms. It is what gives a firefly its shine, makes waves glow at night, and lights up a deep-sea anglerfish lure. The animal itself is the light source; or rather, chemicals within the animals body excite electrons to give off light. Bioluminescence is truly special because it requires input from no other energy source than the food the animal consumes. Certain types of proteins, called luciferins, store energy in the form of excited electrons. These electrons are freed from their bonds by enzymes called luciferases. Once free they become less energetic, and in doing so, release energy in the form of photons, or light. Virtually all bioluminescent reactions occur in this manner, whether created by bacteria, vertebrates, or anything in between.

Though the chemical constituents are vastly different, bioluminescent reactions are similar in effect to that of a glow stick (chemiluminescence). That is, both sets of reactions rely on two separate molecules, which combine in an oxidation-reduction (redox) reaction that yields light and a third molecule of lower energy.

Fluorescence

On the atomic level, fluorescence and bioluminescence are similar. In fact, light in general is simply a byproduct of the excitement and subsequent calming of electrons. The difference is in the trigger. Whereas the bioluminescent trigger is the luciferin/luciferase complex, fluorescence is triggered when a pigment absorbs light from an outside source. In the case of anthozoan fluorescence, the external source is sunlight (or artificial light in an aquarium). As the light passes through the anthozoans tissues, some of it is absorbed by fluorescing pigments. These pigments make electrons available for excitation, which in turn give off light as they return to their normal energy levels.

Fluorescence accounts for more color varieties than either bioluminescence or phosphorescence, because the emitted color is dependant upon the fluorescent pigment which absorbs the incoming light. The resulting hue is determined by the excitement level the electron achieves. Originally, fluorescence was believed to come from a single pigment, the Green Fluorescent Pigment (GFP). Since that time, a number of similar molecules have been discovered. This group is referred to as the GFP-like Chromo-Proteins (CP).

Marine aquarists know that blue (actinic) light typically yields the most intense fluorescence. Ultraviolet light will produce an even stronger effect. This intensity occurs because blue and indigo are the most energetic colors of visible light, and therefore provide the most energy to excite electrons. White light, which carries all colors of the visible spectrum, contains more than enough blue light to induce fluorescence. Unfortunately, the other colors often drown out the glow, making it harder to see. Aquarium lighting can be adjusted to enhance the fluorescent qualities of the livestock, but in the ocean you must be at least 33 feet underwater to see the same effect with the naked eye. Beyond that depth, the water has filtered out most of the colors other than blue, allowing divers to witness naturally occurring fluorescence.

Phosphorescence

Phosphorescence is very similar to fluorescence in its chemistry, however the manifestation of the results is different. Rather than create bright light which ends as soon as the external light source is removed, phosphorescence is less intense but remains for a time after the external light has ended. Phosphorescence is what you see when a child has glow-in-the-dark stars glued to the ceiling. Instead of absorbing and releasing energy instantly, the electrons re-release the energy more slowly than it was absorbed. Though some cnidarians exhibit phosphorescent tendencies, often the light is too weak to be seen with the naked eye. It may also be overwhelmed by bioluminescent or fluorescent light produced by the same animal. Interestingly, bleached out coral skeletons sometimes visibly phosphoresce.

So Now We Ask Why!

Why do corals and other anthozoans make their own light? The question is more than appropriate. Unfortunately, the answer is more than unclear. It might be sensible, then, to explore how other animals use these abilities. For example, male fireflies use bioluminescent flashing to attract females with which to mate. However, cnidarians are broadcast spawners. They reproduce sexually by releasing their gametes into the water column, relying on chance to bring them together. Thus, reproduction is not a likely explanation for the cnidarian light show.

The two other major factors that dominate life in the wild are defense and feeding. Could the essences be related to one of them? Both factors play significant roles in the deep sea, where bioluminescence is the only source of light. In fact, it estimated that up to 90% of deep sea species utilize bioluminescence in some way. The quintessential example is the anglerfish, an animal which dangles a glowing lure from its head to attract potential pray. Another fish, the cookie cutter shark, has a small glowing spot on its belly to draw attention from would-be predators. Only the predators become the prey once they move in for the kill. On the other hand, some smaller animals create lighting effects to distract potential predators, or to attract even larger predators to consume the original threat.

The majority of anthozoans which concern us in the marine livestock trade are photosynthetic. Or more correctly, they live in relatively shallow water, where they can form symbiotic relationships with zoochlorellate and zooxanthellate algae. One theory on bioluminescence suggests that the positioning of these algal cells may be the key to understanding the usefulness of luminescence. Anthozoans from extremely shallow (read well-lit) waters have fluorescing pigments positioned above their algal symbionts; this arrangement may help to redirect some of the most harmful radiation away from the algae. Anthozoans from deeper waters typically have their fluorescing pigments between or below their algae. The assumption is that these pigments augment the photosynthetically available radiation by reflecting back light the algae missed on the first pass. In fact, it may also be true that the pigments make previously unusable wavelengths of light into more useful colors. There are even anthozoans known to use their own bioluminsence to trigger their own fluorescence, in order to add supplemental light for their algal symbionts.

There is also evidence suggesting that fluorescence may be a signal for other animals. The idea is not unique to cnidarians. For instance, it is known that bees and other insects can see patterns on flower petals that fluoresce in the UV spectrum. Some fish also can see in the UV spectrum. It is likely that anthozoan fluorescence is visible to a number of other animals. Though no definitive evidence exists, it has been postulated that fluorescence may be used to attract larger predators to clean the corals surface of pests. This idea explains why so much fluorescence occurs in the green part of the spectrum. Green is a dominant color in very shallow waters, and many animals from this zone see green particularly well.

Translation To The Aquarium

To achieve a stronger fluorescent effect in your reef tank, you must first be sure to choose fluorescing species. You may also want to mix in as many different colors as possible because green can quickly become the dominant tint. Be sure to regularly provide high-quality particulate or dissolved foods with amino acids, and maintain excellent water conditions. I usually run actinic lighting all day long, along with full spectrum daylight. Using 12,000 K or 14,000 K daylight, instead of 10,000 K, will also enhance the fluorescent effect. I have also tried using blue LEDs during the daylight cycle, to no ill effect. They add a slight sparkle to the fluorescence.

Conclusion

Coral luminescence is like natural fireworks. The phenomenon is both amazing and mysterious. If you are yet to see it firsthand, seek it out. There is little that can compare to this fantastic light show. In fact, the ability to luminesce is so unusual, it has given corals special standing in the scientific and medicals communities. Bioluminescent genes have been sliced into many varieties of plant and animals, while fluorescing proteins are being used as markers to aid in cellular biology. And all this can be reproduced at home, in a glass box. Simply astounding!

Works Cited:

Castel, Rob. Green Fluorescent Protein, from Background to Applications. Department of Structural Environment; Vrije Universiteit Amsterdam. 2003.

Castro, Peter and Michael E. Huber. Marine Biology, Fifth Edition. McGraw Hill: Boston. 2005.

Kerkenberg, Erwin. The Fluorescent Aquarium, A New Trend in the Marine Aquarium Hobby? Coral: The Marine Aquarium Magazine. Vol 5:3, June/July 2008.

Lloyd, JE. Firefly Mating Ecology, Selection and Evolution. Mating Systems in Insects and Arachnids. pp. 184-192. 1997.

Siebeck, E.U., A.N. Parker, D. Sprenger, L.M. Mathger, and G. Wallis. A Species of Reef Fish That Uses Ultraviolet Patterns for Covert Face Recognition. Current Biology. Vol 20:5. pp. 407-410. 2010.

Siebeck, U.E.. Communication in Coral Reef Fish: the Role of Ultraviolet Colour Patterns in Damselfish Territorial Behaviour. Elsevier Science Direct, The Association for the Study of Animal Behaviour. 68. pp. 273-282. 2004.

Wiedenmann, Jorg. Fluorescence and Bioluminescence. Coral: The Marine Aquarium Magazine. Vol 5:3. 2008.

Wiedenmann, Jorg. Fluorescent Coloration in Invertebrates. Coral: The Marine Aquarium Magazine. Vol 5:3. 2008.