Have you ever walked on a beach and noticed that the waves seem to glow as they roll onto shore? Perhaps you have seen fish or jellyfish that glow in the dark, or maybe you’ve chased fireflies in your backyard or on a camping trip. These are all forms of luminescence (the production of light without adding heat), but the manner that these organisms produce their light can be quite different.
Fluorescence, Phosphorescence and Bioluminescence: What’s the Difference?
The luminescent process in which organism uses a chemical reaction that results in the emission of light is called bioluminescence. Fireflies light up using a bioluminescent reaction in which the enzyme luciferase acts on a light emitting pigment, luciferin, in the presence of ATP, magnesium and oxygen. Some organisms absorb radiant energy and then reemit that energy as light. This process is referred to as fluorescence (or biofluorescence). Fluorescence occurs when part of an organism absorbs ultra violet rays, x-rays or another form of radiant energy (excitation source) and then almost immediately reemits light waves (usually at a longer wavelength than the absorbed rays). Because the reemission is almost immediate (within 10–8 seconds), the emission also ceases almost immediately when the exposure to the exciting rays ends. This immediate cessation of light when the excitation source is removed distinguishes fluorescence from phosphorescence, which is the luminescent process where the light-emitting substance absorbs radiant energy, but then continues to give off light (glow) for a period of time (i.e., glow-in-the dark watch dials).
Creatures that produce light using fluorescence range from corals, jellyfish and fish to parrots and butterflies. Most recently, both the South American spotted tree frog (Hypsiboas punctatus) and the hawksbill sea turtle have been added to the list of fluorescing organisms.
Under the Sea
Although fluorescent organisms are found in many different environments, fluorescence in living creatures appears most often in marine life. Because water filters both incident and reflected light, visible signaling based on pigmentation can be unreliable under water. Color added using fluorescence (typically green, orange or red) is more visible because the spectrum of the light emitted is at a higher contrast with the predominantly blue illumination of aquatic settings. Fluorescence, although widespread throughout fish species, is particularly common in coral reef dwelling marine species (1). Many of the species that exhibit florescence (i.e., sharks, lizardfishes and flatfishes also poses yellow long-pass intraocular filters, enabling them to better detect fluoresced light (1).
Corals also contain fluorescent pigments. These pigments may be brightly colored with low fluorescence, or they may be highly fluorescent. Because corals are dependent on the photosynthetic activity of algae, with whom they have a symbiotic relationship, they must survive in the photic zone of the ocean. The intensity of sunlight can vary within this zone from damaging levels to very shaded; so clearly the coral needs a method for adapting to the lighting conditions. This is done using fluorescent pigments. The pigments act to regulate the light environment for the coral. In low light conditions the pigments might act to enhance light availability. In intense sunlight, the pigments are photo protective (2), dissipating the energy from light waves that have low photosynthetic activity and reflecting visible and infrared light (2).
Other examples of fluorescent marine life include mantis shrimp, which have a complex system of color visualization. Mantis shrimp have fluorescent yellow patches on their antennal scales and carapace that they use for signaling (3). Lolignid squid have fluorescent yellow “eye spots” that are most likely used for visual signaling, possibly to indicate a change of direction in schools (4). Green fluorescent protein (GFP), widely used in experimental biology today, was first isolated from a jellyfish (Aequorea victoria; 5)
Both hawksbill and loggerhead sea turtles have recently been shown to fluoresce, marking the first occurrence of fluorescence in a marine vertebrate other than fish. You can see a fluorescent hawksbill turtle in the video below.
On the Land and in the Air
Although it is most common in aquatic settings, fluorescence also occurs in land-dwelling creatures. Swallowtail butterflies in the Princeps nireus group use the fluorescent blue or blue-green bands or spots on their dark dorsal wings for signaling. The fluorescence is directionally enhanced by the crystal structure of their wing scales (6). Budgerigars (a type of parrot) have yellow fluorescent feathers on their cheeks and crown, which is used in courtship displays (7).
Earlier this year, a report in PNAS described the first incidence of fluorescence in an amphibian (8). The South American tree frog (Hypsiboas punctatus) has translucent skin that fluoresces green under UV-blue light. This fluorescence falls into the expected visual range of amphibians and is important in low-light conditions where it dramatically enhancing the brightness of the individuals. The authors further identified several amphibian families with similar characteristics to the South American tree frog that should also be tested for fluorescence.
As fascinating as fluorescence in the natural world is, the discovery of fluorescent proteins truly revolutionized experimental biology. Scientists have harnessed the power of these fluorescent proteins to use as a reporter of gene expression. They are useful because cells expressing them can be visualized in a noninvasive way simply by illuminating the sample with blue light. These Fluorescence-based assays are sensitive and there are a large selection of fluorescent proteins to choose from. Researchers have found a wide variety of applications for fluorescent assay including investigating molecular dynamics and interactions, enzymatic activities, signal transduction and cell health. Fluorescent dyes are now used to quantitate DNA and RNA prior to use in downstream experiments. As new fluorescent proteins, like those recently identified in the South American tree frog, are identified, new uses in science could be just around the corner.
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