I’m very excited to say that this article features beautiful illustrations by the wonderful and hugely talented illustrator Holly Astle! Thank you for reading and be sure to check out her links at the end of this (quite long) article.
Walking along some famous beaches or swimming in certain oceans, you will see your footsteps glow in the sand, or the ocean light up with flashes of light as you swim through. This natural phenomenon is called bioluminescence and has captured the imagination of humans for many years. I have been lucky enough to swim with bioluminescent plankton myself, and it got me thinking about how this wonderful quirk of nature has evolved. So, what is bioluminescence, what causes it, and how do different organisms use it?
Bioluminescence is the emission of visible light by an organism as a result of a natural chemical reaction. While we humans use chemical energy gained from our food in various processes to power and heat our bodies, these organisms can use some of this chemical energy to emit light. It is important to note that bioluminescence is different to fluorescence – fluorescent pigments only glow in the presence of an external light source, while bioluminescent organisms’ glow in the complete absence of light.
Bioluminescence has evolved around 50 times completely independently and is primarily a marine phenomenon. 30 of these cases belonging to different species of fish, but there are a huge number of marine animals, microbes and land organisms that produce their own light including jellyfish, fish, algae, fungi and dinoflagellates (phytoplankton). It’s widespread distribution and ubiquity in the deep-sea environment hints at its importance to these organisms and its various roles including but not limited to: a defence mechanism, to attract mates, or luring prey.
So how do these organisms do it? They can either produce it themselves or through a symbiotic relationship with bacteria. In a symbiotic relationship, the light-producing bacteria live in an organ in the hosts body. To control the light and keep themselves hidden from larger predators, these organisms have adapted a few ways to turn the lights on and off. Some can retract the organs into their body muffling the light, while other cover the organs with pieces of skin a bit like closing your eyes.
They use a group of chemicals called Luciferins. The name comes from the Latin term lucifer, meaning light-bringer, and there are several forms, e.g. some use the luciferin coelenterazine, while photosynthesising plankton have a luciferin that resembles chlorophyll.
The Luciferase uses ATP (a source of energy) to allow the oxygen to bind to the Luciferin. This oxidises Luciferin to Oxyluciferin and light is released. In some cases, the luciferin, luciferase and oxygen are all bound into a single unit called a photoprotein. In this case it is the binding of calcium that releases light.
Although the chemistry is known, it is still a bit of a mystery how some species trigger bioluminescence. In multi-cellular species luminescence is neurologically controlled: in fireflies the transmitter is glutamate, while in some species of fish this is noradrenaline. In some species it is thought to be triggered by a significant event, such as environmental changes, but this is still unclear.
One such environmental change could be water disturbance, as it is for Dinoflagellates. These are single-celled plankton responsible for the “milky sea” fabled by sailors that can be seen from space. The water movement causes an action potential (a sweep of electrical energy) through the cell and is thought to admit protons from the acidic vacuole into contact with the cells components that control the light emission chemistry.
These organisms produce a flash of light when the water around this is disturbed, which scientists have hypothesised is their “burglar-alarm”. They theorised that this flash is a way to fight back – they light up to show larger animals that the smaller animals feeding on them are nearby and could make a good snack for them instead of them continuing to eat the dinoflagellates.
Swimming with these phytoplankton in Thailand is an absolutely magical experience, it’s like swimming in stars. If you get a chance to visit Thailand, I highly recommend making this one of your top priorities, so wonderful to see this phenomenon in person.
Deeper down in the ocean there is a sweet spot for bioluminescence called the twilight zone, also known as the disphotic zone, where very little light penetrates. Seawater absorbs longer red and yellow wavelengths giving the twilight zone a blue-green colour. The longer wavelength of blue light means the light has more energy to penetrate further, and organisms at this depth are incredibly clever and produce their light at the same wavelength, so it can blend from the light above and travel a longer way than the red light. The Pony fish uses bioluminescence on its underside to mimic the small rays of light coming down from the water surface, and this counter-illumination means their silhouette cannot be seen from below, rendering them invisible to predators.
A drawback to focussing on blue light is they tend to not be able to see the shorter red wavelengths. This is a prime weakness for others to exploit, like the Stoplight Loosejaw fish. These nightmarish looking fish (seen on the left) produced a red luminescence from the patch under their eyes in order to sneak up on its prey.
Bioluminescence can also be used as a final act of revenge for Ctenophores. Cannibalistic in nature, the tiny translucent comb jellies move around using tiny hairs called cilia and feed on each other, but as they break apart in the others stomach, they begin to glow. This is where being translucent has its downside – the body parts in their stomach light up their killer making them attractive to larger predators. Mutual destruction.
One of my favourite examples is the Hawaiian bobtail squid and its harmonious relationship with a species of bacteria called Vibrio fischeri. The bacteria do not express light when floating around freely in the ocean, but when housed in the squid’s light organ (on its underside), it emits light to mimic the moonlight or sunlight above.
There is a clever daily routine between the bobtail squid and its bacteria. The bacteria only produce the visible light when there is a certain population density in a colony, this is known as quorum sensing, and is also why they do not produce light in the ocean as the population density is too low. To use this as a method of control, the squid expels up to 95% of the bacteria in the morning, allowing it to stop emitting light while it
sleeps. When night falls the bacteria have multiplied enough to begin emitting light again.
Another amazing way the squid can control the luminescence is by withholding oxygen. As mentioned earlier the reaction requires oxygen to be present, so withholding oxygen limits the luminescence the bacteria emit. They can also cover the light as mentioned earlier. Reflective tissue lines the tube leading from the light-producing organ, ensuring light is directed outwards, and this tissue can also act like an iris by expanding or contracting to modify the amount of light released. There is also another muscle lens that diffuses the bacterial light, clever eh.
Even cleverer, it has been found by researchers at the University of Wisconsin that the bacteria themselves entrained the squid’s circadian rhythm (the 24hour cycle most organisms daily lives revolve around). The found escry1, one of the genes that entrains circadian rhythms, is dominant in their light organ and was not cycling with environmental light. They found that the bacteria luminescence to be controlling their hosts circadian rhythms, forming a feedback system to allow the pair to survive!
Although most of the bioluminescent organisms are in the sea, there are some great examples of organisms that glow on land, such as millipedes, fire flies, and a species of fungus.
Saprobe Panellus stripticus is one of 70 species of bioluminescent fungi, and is found in Asia, Australia, Europe and North America. It grows in clusters on wood, particularly beech, oak, and birch. Some species feeding on the rotting wood, and the glow is known as “foxfire”.
The bioluminescent glow is also entrained by their circadian clock, however this one revolves around temperature. As night falls the temperature follows suit, and they begin to emit a green light. Observations have dated back to Aristotle, but we are still not too sure why they have this strange ability. Scientists theorise that the glow attracts insects to increase seed dispersal, but other theories include it being a warning to those that eat them, or just a quirky by-product of another chemical reaction.
I could go on and on about bioluminescence, how awesome and interesting it is, many more different organisms that use it, and how we can and are using bioluminescence, but for now I think I will end it here.