Bioluminescence - The Prometheus Within
Bioluminescence, the ability of an organism to produce its own light, is widespread in nature from mushrooms to deep sea fish. Yet, we often overlook the richness of the bioluminescent world because we bring our own lights wherever we go. In this article, join us on a journey into the what, where, and how-it-all began of bioluminescence.
Many organisms produce light without a Greek God of Fire, and unraveling the mystery of bioluminescence has fascinated humans for thousands of years, from Aristotle to modern scientists. In our modern world, where the glare of artificial lights dominates the nights, we often miss the natural lights around us. In temperate or tropical climates, the iconic, bioluminescent fireflies1 are familiar sights as they conspicuously signal to one another on warm summer nights. Yet they are only the tip of the bioluminescent iceberg. Thousands of organisms are known to emit light, such as the Jack O’Lantern mushroom, bioluminescent click beetles (Deilelater physoderus), and most marine organisms, including the tiny dinoflagellates that illuminate waves breaking on tropical beaches at night.
I grew up in rural Delaware and was enthralled with the fireflies who came out at dusk during the summer. Their lights broadcast coded messages, blinking in the darkness, “I’m a male Photinus pyralis. Are there any females out there?” But communication isn’t so simple. He is lucky if a female of the same species responds. Woe to him if a female Photuris firefly intercepts his love letter. She can mimic the Photinus females’ flashes to lure and eat the unsuspecting Photinus male. Not to be outdone, however, a Photuris male will often mimic the flash pattern of a Photinus male to attract the female of his own Photuris species – she thinks she’s getting a meal, and he mimics the intended meal in order to mate. Sir Walter Scott could have been talking about firefly bioluminescence when he said, “Oh, what a tangled web we weave, when first we practice to deceive!2”
So, why does bioluminescence even exist, and why is it so widespread among certain organisms3? Bioluminescence can have adaptive value for the organism, as we’ve seen for the different firefly species. It is a communication strategy analogous to acoustic (e.g. bat, insect, or bird calls) or chemical (e.g. moth pheromones) signals that other species use. But, unlike the ambient light most animals like us rely upon, it is a visual signal that can penetrate the darkness of the night or the ocean depths. To a bioluminescent organism, the world of darkness is just another environment to be exploited, and exploit it they have. Examples of bioluminescence conferring adaptive value are legion and tend to fall into a few broad categories:
Attraction of mates: the charismatic lightning bugs are the poster children for using bioluminescence to attract mates.
Attraction of prey: deep sea anglerfish employ a luminous, bacteria-laden lure that lights up to attract prey (and mates, as well).
Avoidance of predators: the tiny dinoflagellates that light tropical waves flash when disturbed and are less likely to be eaten by predators. Certain fish emit anti-predator flashes to distract predatory elephant seals.
Generally cool but not completely understood: Humboldt squid swim in formation in the darkness of the Pacific Ocean, aided by luminous organs throughout their bodies called photophores.
Fungal bioluminescence, also called foxfire, can be so dim that some scientists have considered it a mere metabolic byproduct rather than confering an obvious advantage. In Maine where I live, bioluminescent mushrooms appear in late summer, and I often sit for hours at night trying to photograph them. I may need to dark-adapt for 15 minutes in total darkness before I can detect their faint glow.
In contrast, the Brazilian fungus Neonothopanus gardneri can emit very bright light. Experiments suggest it attracts insects to the mushroom, aiding in the dispersal of the mushroom’s spores. The difference in the brightness of fungal bioluminescence may well determine whether the light can attract spore dispersers, so the jury is still out on the adaptive significance of different types of foxfire.
Evolution of Bioluminescence
The pervasiveness of bioluminescence among living organisms is a mystery. Was there just one evolutionary Prometheus in the distant, dimly-lit past that gave rise to this widespread ability, or were there many? That is, how did bioluminescence evolve? First, we need to know more about the mechanisms of bioluminescence.
Generally, a light-emitting molecule, called luciferin4, and an enzyme called luciferase are needed to produce light. Don’t be fooled by the names of the molecules. These are generic terms that don’t imply that the actual molecules are the same between different organisms. Nor do they even have a common evolutionary history (nor that they are Satanic molecules). For example, the firefly and the Jack O’Lantern mushroom have luciferases, but the actual molecules are quite different. Similarly, the luciferins are diverse as well. To avoid confusion, one can say firefly luciferase or bacteria luciferase to help distinguish them.
An alternate, less common bioluminescence found in marine organisms like jellyfish and comb-jellies uses the light-emitting luciferin but substitutes a non-enzymatic photoprotein for luciferase. The chemistry of bioluminescence is as varied as the organisms themselves.
Yet, amid this diversity, stands a feature common to all bioluminescence in bacteria, jellyfish, or lightning bugs: oxygen is required for light to be emitted from luciferin. Photoproteins need oxygen to be manufactured and are then destroyed in the light production reaction, while luciferases, being enzymes, are constantly recycled as they accelerate the reactions between oxygen and luciferin that produce light.
This fundamental requirement for oxygen has led some researchers to hypothesize that the origins of bioluminescent machinery lie in the protection of early life from toxic oxygen5 and not in the adaptive value of producing light per se. This potentially places the origins of bioluminescence over a billion years ago.
The multiple bioluminescent systems that have since appeared represent fascinating examples of convergent evolution, that is the independent evolution of similar traits in distantly-related organisms. Despite superficial similarities, bioluminescence has evolved in very different living things more than 40 times in response to a similar conundrum – how to survive when the lights are out. In fishes alone it has been estimated to have evolved independently at least 27 times, with some fishes evolving intrinsic6 bioluminescence and others developing symbiotic relationships with bioluminescent bacteria.
I often wonder at the beauty of bioluminescence. There is beauty in the fireflies at night and in the faint mushroom glow. There is beauty in the complexity of those marvelous mechanisms that have evolved over such a long time. I look forward to more nights and dark oceans filled with these light-bearing organisms.
1 They are also called lightning bugs but are in fact beetles instead of flies or bugs.
2 From “Marmion: A Tale of Flodden Field” by Sir Walter Scott.
3 Bioluminescence is widespread among the life domains Eukaryota and Bacteria. Among the vertebrates, only fish are capable of bioluminescence; humans and all other vertebrates are not.
4 Luciferin comes from the Latin word lucifer, meaning “light-bearing.”
5 Earth’s early atmosphere lacked oxygen which appeared when plants evolved. The reactive oxygen produced by plants was toxic to many organisms until protective mechanisms evolved.