In the broadest sense, “light” is electromagnetic radiation, and it’s one of the most fundamental pieces of data that organisms can collect about their environment. The ability to process light stimuli is extremely useful — 96% of animals have some type of visual system, and many microorganisms possess light-sensitive proteins (known  as “opsins”) that allow them to respond to light, even if they aren’t technically “seeing” it. ^[1]1. Oakley, T.H., and D.C. Plachetzki. “The Evolution of Opsins.” In Encyclopedia of the Eye, 82–88. Elsevier, 2010. https://doi.org/10.1016/B978-0-12-374203-2.00150-0. ^[2]2. Frentiu, Francesca D., and Adriana D. Briscoe. “A Butterfly Eye’s View of Birds.” BioEssays 30, no. 11–12 (November 2008): 1151–62. https://doi.org/10.1002/bies.20828. ^[3]3.Land, M F, and R D Fernald. “The Evolution of Eyes.” Annual Review of Neuroscience 15, no. 1 (March 1992): 1–29. https://doi.org/10.1146/annurev.ne.15.030192.000245.

It’s useful to be able to generate light, too. Humans, for example, have found a number of methods for producing light —  many of which involve (at the most basic level) burning something. But did you know that some organisms can produce light completely on their own?

Some of you might already know what I’m talking about — if you’ve been outside during the twilight hours of a summer night, you may have noticed little flashes of light made by fireflies; you may have even seen (or at least seen pictures of) marine microorganisms glowing bright blue in the surf along the coast of certain beaches. These are both examples of organisms generating their own light through a process known as bioluminescence.

It may come as a surprise that bioluminescence is actually fairly common; most major groups of organisms have some bioluminescent species (terrestrial vertebrates and flowering plants are notable exceptions). Nowhere is bioluminescence more common, though, than in the ocean. Some studies suggest that up to 77% of marine organisms may be bioluminescent, and there are certain taxa of marine organisms (such as Cnidaria) that are composed of almost entirely bioluminescent organisms. ^[4]4. Martini, Séverine, and Steven H. D. Haddock. “Quantification of Bioluminescence from the Surface to the Deep Sea Demonstrates Its Predominance as an Ecological Trait.” Scientific Reports 7 … Continue reading

Let’s explore what bioluminescence is, how it developed, and why it is so useful for marine organisms.

What is bioluminescence?

Simply put, bioluminescence is the production of light in an organism using chemical reactions. 

In general, bioluminescent reactions involve three key ingredients: a luciferin, a luciferase, and oxygen. Let’s go into some more detail about each ^[5]5. Hastings, J.Woodland. “Chemistries and Colors of Bioluminescent Reactions: A Review.” Gene 173, no. 1 (January 1996): 5–11. https://doi.org/10.1016/0378-1119(95)00676-1. :

Luciferin (from the Latin lux-, meaning “light,” and ferre, meaning “bearing” or “producing”) is a generic term for the molecule that actually produces light in a bioluminescent reaction; they are oxidized by luciferases and release light as a product of the reaction (“oxidized” can mean a couple different things — in this case, it just means that the molecule gained an oxygen atom). ^[6]6. Lee, John. “Bioluminescence: The First 3000 Years (Review),” September 2008. http://elib.sfu-kras.ru/handle/2311/935. ^[7]7.“Luciferin.” In Merriam-Webster, December 31, 1969. https://www.merriam-webster.com/dictionary/luciferin. Interestingly, despite the fact that organisms independently evolved bioluminescence at least 40 times, there isn’t much variation among luciferins themselves; only 4 different luciferins are responsible for the majority of bioluminescence in the ocean.

Luciferases are the enzymes that catalyze the oxidation of luciferin, allowing a reaction that would happen too slowly on it’s own to happen fast enough that a meaningful amount of light can be generated. Sometimes, a luciferin, a luciferase, and oxygen are all bound to a single, large protein. In this case, they are collectively called a photoprotein. In order to stop all of the components from reacting in an uncontrolled way, photoproteins are often controlled by calcium or magnesium ions — the ions can be released when needed, and then they will bind to the photoprotein, changing its shape and allowing the bioluminescent reaction to occur. Unlike luciferins, luciferases and photoproteins vary tremendously across organisms.

Bioluminescence, fluorescence, and phosphorescence

Bioluminescence isn’t the only way an organism can produce light — they can also fluoresce or phosphoresce. When a fluorescent or phosphorescent molecule absorbs this energy, its electrons become energized; as the electrons return to their resting state, they release that light again, typically with less energy (i.e., at a longer wavelength). ^[8]8. Johnson, Iain, ed. “Fluorescence Fundamentals.” In The Molecular Probes Handbook: A Guide to Fluorescent Probes and Labeling Technologies, 11. ed. Carlsbad, California: Life Technologies Corp, … Continue reading ^[9]9.LibreTexts. “Fluorescence and Phosphorescence.” In Physical & Theoretical Chemistry, 2020. There are some subtle differences between fluorescence and phosphorescence, but the most important distinction for our purposes is that fluorescent molecules release their light almost immediately, whereas phosphorescent molecules can release their light much more slowly.

As you may have noticed, these other ways of producing light require energy from photons; while those are easy to obtain from sunlight, they are a scarce resource in the deep ocean. Bioluminescence is the only way organisms can generate light completely on their own, so if a deep-sea organism is fluorescent, it is often also bioluminescent — the light emitted from the bioluminescent reaction is partially or completely absorbed by the fluorescent molecules before it is reemitted.

How might bioluminescence have developed?

As mentioned previously, bioluminescence has likely independently evolved at least 40 times, which suggests that it is relatively easy to develop. There are a few possible explanations for this:

1. It’s possible that luciferins already had a preestablished role ^[10]10. Devillers, Ingrid, Bertrand de Wergifosse, Marie-Pierre Bruneau, Bernard Tinant, Jean-Paul Declercq, Roland Touillaux, Jean-François Rees, and Jacqueline Marchand-Brynaert. “Synthesis, … Continue reading — Luciferins have strong antioxidant properties, so they may have originally been used for (the much more ordinary role of) scavenging up free radicals (these highly reactive molecules are an ordinary part of biological activity but can damage cells if they are uncontrolled). Deep water is colder and darker than shallow water, and both factors reduce the amount of free radicals generated, so it’s possible that these luciferins became available for other purposes as organisms migrated into deeper water.
2. Predators don’t need to make their own luciferin — For some predators, luciferin could be considered a kind of nutrient; something that can be acquired through their diet. If a predator can eat organisms that produce luciferin, all they need is the genes to produce luciferases or photoproteins.

Of course, bioluminescence is also extremely useful, which is what we will dive into now.

What are the functions of bioluminescence?

Fundamentally, bioluminescence can be considered a form of communication — organisms can provide a visual stimulus that influences other organisms around them. Researchers have found a variety of ways that organisms leverage their bioluminescent ability ^[11]11. Haddock, Steven H.D., Mark A. Moline, and James F. Case. “Bioluminescence in the Sea.” Annual Review of Marine Science 2, no. 1 (January 1, 2010): 443–93. … Continue reading :

Defense

Bioluminescence can aid in defense in the following ways:

• If a predator is approaching, an organism can flash to startle them
• Similarly, they can flash to alert other organisms of nearby predators
• Organisms can illuminate their undersides so that they have the same brightness as the light above, making them harder to see.
• Organisms can use glowing body parts as distractions or sacrifices so that they can escape (even if it’s at the expense of a body part)
• Much like other forms of bright coloring, bioluminescence can be used to signal that an organism is potentially poisonous and shouldn’t be eaten

Offense

• Predators and parasites can use bioluminescent body parts to lure prey or attract hosts
• Predators can also use bright lights to stun and confuse prey, or simply make them easier to see.

Other

• Bioluminescence can also be used as a tool for mate attraction and recognition.

Conclusion

Bioluminescence is a remarkable quality that allows organisms to use visual information to communicate in and interact with low-light environments. Due to its usefulness and the fact that it likely took advantage of pre-existing molecules, it has independently developed many times and can be seen across almost every major group of organisms. Next time you are in a dark environment, see if you can spot an example of bioluminescence, and try to imagine what that organism might be using it for!

Nicholas Milazzo

Writer, Copy editor 

Learn More About NIck

References[+]

References

↑1	1. Oakley, T.H., and D.C. Plachetzki. “The Evolution of Opsins.” In Encyclopedia of the Eye, 82–88. Elsevier, 2010. https://doi.org/10.1016/B978-0-12-374203-2.00150-0.
↑2	2. Frentiu, Francesca D., and Adriana D. Briscoe. “A Butterfly Eye’s View of Birds.” BioEssays 30, no. 11–12 (November 2008): 1151–62. https://doi.org/10.1002/bies.20828.
↑3	3.Land, M F, and R D Fernald. “The Evolution of Eyes.” Annual Review of Neuroscience 15, no. 1 (March 1992): 1–29. https://doi.org/10.1146/annurev.ne.15.030192.000245.
↑4	4. Martini, Séverine, and Steven H. D. Haddock. “Quantification of Bioluminescence from the Surface to the Deep Sea Demonstrates Its Predominance as an Ecological Trait.” Scientific Reports 7 (April 4, 2017): 45750. https://doi.org/10.1038/srep45750.
↑5	5. Hastings, J.Woodland. “Chemistries and Colors of Bioluminescent Reactions: A Review.” Gene 173, no. 1 (January 1996): 5–11. https://doi.org/10.1016/0378-1119(95)00676-1.
↑6	6. Lee, John. “Bioluminescence: The First 3000 Years (Review),” September 2008. http://elib.sfu-kras.ru/handle/2311/935.
↑7	7.“Luciferin.” In Merriam-Webster, December 31, 1969. https://www.merriam-webster.com/dictionary/luciferin.
↑8	8. Johnson, Iain, ed. “Fluorescence Fundamentals.” In The Molecular Probes Handbook: A Guide to Fluorescent Probes and Labeling Technologies, 11. ed. Carlsbad, California: Life Technologies Corp, 2010.
↑9	9.LibreTexts. “Fluorescence and Phosphorescence.” In Physical & Theoretical Chemistry, 2020.
↑10	10. Devillers, Ingrid, Bertrand de Wergifosse, Marie-Pierre Bruneau, Bernard Tinant, Jean-Paul Declercq, Roland Touillaux, Jean-François Rees, and Jacqueline Marchand-Brynaert. “Synthesis, Structural Characterization and Antioxidative Properties of Aminopyrazine and Imidazolopyrazine Derivatives.” Journal of the Chemical Society, Perkin Transactions 2, no. 7 (1999): 1481–88. https://doi.org/10.1039/a900289h.
↑11	11. Haddock, Steven H.D., Mark A. Moline, and James F. Case. “Bioluminescence in the Sea.” Annual Review of Marine Science 2, no. 1 (January 1, 2010): 443–93. https://doi.org/10.1146/annurev-marine-120308-081028.