It’s June. You’re in a North American temperate forest, strolling through damp Tennessee woodlands enjoying the warm summer evening. Away from all urban qualms and artificial lights. The surrounding is humid and the temperature is just right for one of nature’s most flamboyant wonders to be showcased. The night goes haywire with seemingly chaotic little light bulbs flashing as though short bursts of miniature fireworks tantalizing you.
It’s hard to believe that each and every species has its own distinctive flashing pattern. Some species have synchronized flashing patterns and it’s really hard to fathom the reason for this complex ordeal of pyrotechnic display and coordination. This entire snafu is just an old mating ritual. Often, flying firefly males flash to attract mates, some species doing so synchronously, a capacity called ‘entraining’ which means responding to an external rhythm. Flashing is done to attract stationary females who may flash the same signal back to the one they find someone interesting, after which the male waits for precisely 4 seconds and flashes again. If the female flashes back the same signal yet again, the male knows that he’s welcome to proceed. From a female firefly point of view, for an initial assessment, along with the precise timing of flashing and the pattern of flight whilst flashing, the brighter the flash and the longer it lasts, the better the quality of genes possessed by the male and the greater is his chance of mating with her.
Fuel For The Explosions
Also known as lightning bugs, fireflies are not actually flying but beetles belonging to the species Lampyridae. The secret behind their ‘cold light’, as it is called, is an enzyme called luciferase, which catalyzes the reaction between a protein called luciferin, and oxygen. Energy in the form of a photon is emitted when luciferin and oxygen react to form oxyluciferin. In some organisms instead of the enzyme luciferase, a photoprotein is used to aid this reaction. Photoproteins were first studied in bioluminescent crystal jellyfish found off the west coast of North America. The photoprotein in crystal jellyfish is called ‘Green Fluorescent Protein’ or GFP. GFP is an extremely important molecule in imaging studies. Photoproteins combine with luciferins and oxygen but need another agent, often an ion of the element calcium, to produce light.
In English, ‘Lucifer’ is a name for Satan before his fall from heaven; in Latin, it means ‘bringer of light.’ Luciferin produces 100% light with little or next to no heat production making it the most efficient source of light with nearly 100% efficiency and this is the very reason why the emitted light is called cold light. For pedantic folks, ‘cold light’ means less than 20% of the light generates thermal radiation or heat. By contrast, an incandescent light bulb is only 10% efficient — 90% of the energy is lost as heat. For another comparison, the heat produced by a candle is 80,000 times greater than the amount of heat given off by a firefly’s light of the same brightness!
The overarching term for such and other related phenomena is ‘Bioluminescence’. It is a form of chemiluminescence, which means that light is produced by means of a chemical reaction. Bioluminescence is at times autogenic meaning self-generated by the organism though most of the time it has something to do with the symbiotic relationship between the host organism and the bacteria sheltered within the host’s body. In some organisms, it is set according to the organism’s circadian rhythm which is a body’s natural 24-hour clock of sleep and wakefulness. Closely related terms are often used interchangeably by some folks and that can lead to a murky understanding of concepts. To shed light, bioluminescence is different from fluorescence in that it involves a chemical reaction. The fluorescing light is only visible in the presence of the stimulating light. This is exhibited by some coral reefs, swell sharks, sea turtles, algae, bird feathers, vitamin B and even your own fingernails!
The phenomenon may seem pretty rare at the first glance. The only thing we have to do is to delve a bit deeper. Here outside our comfort zones, in the deep seas and oceans, a whopping 76% of marine animals are bioluminescent. At least 1500 species of fish are known to be bioluminescent. On land too, more than 90% of fungi glow in the dark.
Reasons for Bioluminescence:
Now, what’s the reason for this wonder? Is all of this just for mating? Are the circling flashes of Atolla jellyfish or the rippling, sparkly waves of Bareo comb jellies just to put on a show?
Turns out an organism may emit light for a variety of reasons:
- To attract mates: just as fireflies and Brazilian click beetles do. But then you would ask, what are pheromones for? Pheromones are chemicals which have the ability to act as hormones outside the body secreted or excreted by organisms to attract potential mates of the same species. Pheromones may be used for long distance communication while bioluminescence is a sort of close-range personal invitation.
- To fend off predators: just like millipedes do. A millipede, typically Motyxiasequoiae glows in the dark to let mice and other predators know with whom they’re messing with. Any animal ignoring the warning gets laced with cyanide. The technical term for this phenomenon is ‘aposematism’.
- To lure prey: which is seen clearly in the caves located in Australia and New Zealand inhabited by glowworms.Actually the larvae of the small fly Arachnocampa Flava. These fungal gnat larvae, found in dark and damp caves, have a rather trailblazing way of fooling their prey. When night falls, these bioluminescent organisms cover the entire cave ceiling, tunnels and walkways with dazzling bright and rather sticky bioluminescent threads whichattract prey. The prey mistakes the cave ceiling for the sky as the lights overhead look like bright stars streaming across the milky way. This way the prey gets caught in the overhanging saliva beads’ dangling strings formed by the fungal gnat larvae, usually during the day. Also consider the very iconic anglerfish. Dubbed as the ugliest animal yet to be found, they compensate this by exhibiting deft shrewdness to lure their prey. Their females sport a lure of glowing flesh called ‘esca’ which is actually a part of their dorsal spine that protrudes over their mouth and acts as bait for any prey close enough to be snatched. The glowing is caused by bacteria living inside the esca. Angling is a method of fishing with an ‘angle’ or a fish hook and hence the reason for naming it ‘anglerfish’.
- To camouflage: As in the case of Hawaiian bobtail squids which light up with the help of bacteria which reside in one of their organs and help them to conceal their shadows and camouflage in moonlight thus evading predators. Or consider the shrimp Sergestussimilis who has its underbelly match with the color of the incoming light from above the water surface, a process known as ‘counter-illumination’, and as an added feather to its cap Sergestussimilis are even accompanied with switches to dim and brighten their light according to necessary requirements. Cookie-cutter or cigar sharks (talk about creative naming) and firefly squids also exhibit counter-illumination.
- As a decoy: As in the case of brittle stars who detach their arms from their body to throw off predators. Their arms just like those of sea stars can regrow. Another example highlights sea cucumbers who are interesting marine invertebrates in the shape of giant cucumbers. When threatened, some species can break off the luminescent parts of their bodies onto nearby fish. The predator will follow the glow on the other fish, while the sea cucumber crawls away. Nifty, right?
- To surprise or delay: As is the case with spew bioluminescence where sticky bioluminescent mucus is spewed out by vampire squids and shrimp to blind their predators momentarily which gives them enough time to escape.
- As deception in the form of mimicry: As in the case of female Photuris fireflies. They mimic the mating flashes of other firefly species to attract them, whereupon they are pounced on and eaten. This femme fatale can even snatch the male fireflies (who have a constant glow even after death) stuck in a spider’s web.
- To confuse the predator: As hatchet fish do by using their dazzling array of colors to confuse the predators about their true body shape.
- As per the adage “The enemy of my enemy is my friend”, bioluminescence may serve to summon the predator of the predator. Known as the “burglar alarm” effect, this may be especially important for tiny life-forms, such as dinoflagellates, that cannot swim fast. The chief defence for these creatures is not fight or flight — but light. Another example is of a sperm whale which may seek out a habitat with large communities of bioluminescent plankton, which are actually not a part of the whale’s diet. The plankton’s predators have a surprise waiting at their scintillating arrival. This may even benefit other creatures who wish to stay alive from the same predators.
Why Marine Bioluminescence Predominates:
Studies point out the fact that bioluminescence is mostly observed in marine biomes. Why is that so?
Well, most scientists point to two reasons for the dearth of freshwater organisms exhibiting bioluminescence:
First, freshwater habitats have not been around as long as marine habitats — evolution is a slow process and freshwater habitats do not yet have the biodiversity of oceans. According to studies, very few bioluminescent fish can tolerate low salinity, as of now. Evolution can indeed be a really long process.
Second, freshwater species wouldn’t really benefit from bioluminescence. Freshwater habitats are often murkier, and deep-water species use other adaptations (such as a catfish’s sensitive ‘whiskers’) to hunt and defend in the environment.
The only instance of bioluminescence in a freshwater animal is Latia neritoides, a limpet (marine mollusc often found clinging to rocks)-like snail native to New Zealand’s streams, where it emits a glowing slime when bothered.
Why Is The Ocean So Blue:
Now that we know the plausible causes for bioluminescence, here’s a general question. Ever wondered why the ocean looks so blue in all those marine photographs. The reason is as you go deeper and deeper, the longer wavelengths of light all get absorbed quicker than blue-green light (wavelength around 470 nanometers) which travels the furthest in water. By 700 feet (around 200 meters) the water becomes a perpetual twilight with only blue light remaining and at 2000 feet (around 600 meters) even that disappears and all that remains is a midnight zone. In these regions, most of the marine organisms are only sensitive to blue light and lack the necessary pigments to absorb either higher wavelengths (yellow or red) or lower wavelengths (indigo or ultraviolet.)
A notable exception is a proud member of the Malacosteid family, the loose jaw dragonfish. Loosejaws have adapted to emit red light; which most fish can’t see and so they have an enormous advantage to use it as a flashlight to search for unsuspecting prey.
Female railroad worms (Phengodeslaticollis) which are actually beetle larvae are like tiny trains with greenish-yellow window lights which warn predators of their toxicity. The only insects to see red colour, their red headlamp gives them an edge for capturing prey. As most of the marine bioluminescence is bluish-green, cameras have to be equipped with a yellow light filter (absorbs nearly all colours except yellow) which blocks out most blue light, to capture marine bioluminescence.
Applications Of Bioluminescence:
The applications of bioluminescence are as exciting as the plethora of creatures that possess it.
- GFP (Green Florescent Protein) found in the bioluminescent jelly fish Aequorea Victoria, is an excellent ‘reporter gene’. It is able to attach itself to another gene without interfering with the gene’s natural behaviour. This allows scientists to trace and monitor the activity of the studied gene ; its expression in a cell, or its interaction with other chemicals. GFP emits light by a process called as ‘resonant energy transfer’. This process basically converts the blue light released by aequorin(a type of photoprotein) or even ultraviolet light, into green light.
- Bioluminescent trees, for instance, could help light up city streets and highways. This would reduce the need for electricity and also give rise to a magical view. This can be used for decorative and ambient lighting purposes too. Bioluminescent crops and plants could luminesce when they need water or other nutrients, or when they were ready to be harvested. This would prove very beneficial for farmers and agribusiness.
- Luciferase-based systems are used in genetic engineering and biomedical research. It has been used as a marker to detect blood clots, to tag tuberculosis virus cells, and to monitor hydrogen peroxide levels in living organisms. Hydrogen peroxide is believed to play a role in the progression of some diseases, including cancer and diabetes. Scientists now can use a synthetic form of luciferase for most research, so the commercial harvest of fireflies has decreased.
- Bioluminescent activated destruction is a viable method to kill cancer cells which involves specific targeting of mutated cells by highlighting and eliminating them.
The Limitless Potential Of Bioluminescence:
Now that we have metaphorically and literally noticed the sheer usefulness of this unique phenomenon, future researchers have much to look forward to. Imagine a deep-sea squid squirting nebulous light in our direction before darting into the gloom. We have found this ethereal phenomenon and the direction before us is blurred. Now, imagine tiny ostracods releasing flash bombs before us, which yield one of the brightest of lights to be emitted by any living being. Hence, we are also blinded by the endless possibilities we can sculpt this to our advantage and to the advancement of research of the upcoming years.
Before concluding, we take two short trips.
Our first trip is to Vieques island in Puerto Rico.
Shimmering waves of electric blue are washing across the shore rendering a mystical vista. Each undulation of the waves and every movement in water bring about new patterns of mesmerizing blue. Now that we have entered the poet’s dream, we explore what actually causes this phenomenon. This region is known for bahíabioluminiscente or ‘bio bay’ which is home to countless speck-of-dust size beings called as dinoflagellates. At least 18 genera of dinoflagellates show luminosity.
Named for their two whip-like flagella and the whirling motion they make while swimming(‘dinos’ is a Greek word that means ‘whirling’) Dinoflagellates are a type of plankton and are the largest single-celled organisms (having a size 1000 times greater than bacteria). They possess the characteristics of both, plants and animals. Millions of them create a beautiful glistening effect, particularly when there’s little moonlight. The dinoflagellates emit blue light by using luciferin, on encountering any movement. If the movement is intense then the light may even appear white to human eyes. The luciferase found in dinoflagellates is related to the green chemical, chlorophyll found in plants.
Bioluminescent dinoflagellate ecosystems are rare, mostly forming in warm-water lagoons with narrow openings to the open sea. Bioluminescent dinoflagellates gather in these lagoons or bays, and the narrow opening prevents them from escaping. Sometimes dolphins and other fish are outlined by ethereal blue lines while moving in the water and so their movements can be tracked. The whole lagoon can sometimes be illuminated at night painting a picturesque scene which is indeed a treat for the eyes.
On our next trip, we travel on a ship to a part of the Northwestern Indian Ocean near Indonesia.
It feels as if the ship were travelling on clouds or snow and we’re being escorted along this seemingly endless milky white path. This is called as ‘milky seas effect’ or ‘mareel’ and is caused by the bioluminescence of the dinoflagellate noctiluca scintillans which is popularly known as ‘sea sparkle’. This phenomenon is so bright that it can even be observed from satellites orbiting Earth. This phenomenon was reported by early sailors along with their exhilarating experience with this phenomenon described in their logs.
Ranging from poignant Darwin snippets to the research for detecting and hiding submarines by the US navy during the cold war, bioluminescence has indeed come a long way, arising in more than 50 independent species while treading along the thorny path of evolution. There’s a whole new world to explore still. Pyrosomes, red scorpionfish, green bombers and many others are equally fascinating organisms and I do implore you to explore this coruscating cosmos. More than 95% of the ocean is yet to be discovered and species are already drastically on the decline as witnessed never before. Our actions affect this once teeming life in ways we can’t even begin to comprehend. What we know currently know might just be a tiny flash of light in the vast wilderness of myriad fireflies, heralding the beacon light for future generations to come.
By Amey Danole
- https://www.nationalgeographic.com/animals/reference/bioluminescence-animals-ocean- glowing/
- https://www.livescience.com/32688-fireflies-synchronous-flashes-are-booty-calls-study-reveals.html#: ~:text=Fireflies%2C%20which%20use%20bioluminescence%20for%20sexual%20selection%2C%20synchronize,to% 20help%20female%20fireflies%20recognize%20potential%20mates%20
Note: This blog is a part of Science Communication by iGEM IISER Bhopal 2020 Team. They are a bunch of undergrads from diverse academic backgrounds ranging from Chemistry, Biology, Engineering to Economic Sciences! United by their love for science especially synthetic biology, they aim to create an inexpensive and non-invasive therapy for diabetes! The idea is to have a probiotic pill that can transform the gut cells in the human body into insulin-producing cells!