Bioluminescence

©Paul o’Dowd, 2013

Bioluminescence

 In many environments, various organisms emit a ghostly green, blue, yellow or even red radiance.

 On the reef and in the rainforest, bioluminescent displays are an important feature of the nocturnal landscape.

 The spooky emanations may appear as points of light, blinking or otherwise, dotting the rainforest floor.

 It may look like inexplicable patches of moonlight under the rainforest canopy on a moonless night.

 It may appear as flashing pin-points of brilliance, carving punctuated tracks through the gaps in the vegetation.

 Some generators of this living light will flock into large groups that shimmer with random scintillations which occasionally become synchronized, causing pulses of light to course through the collective.

 Sometimes, simply moving through certain environments can result in vivid blue emissions which leave persistent glowing records of your passage.

 In each of these cases, the light is being emitted by an amazing chemical reaction which appears to have evolved independently in a number of very different organisms.

 Some important ideas in chemistry are very useful at this point.

 For a chemical reaction to occur there needs to be enough energy available to drive the process.

 Some reactions happen easily, like acid eating limestone, just drop it in and it’ll be white froth in seconds. Other reactions need a push, like scrubbing poisons from your car exhaust, which requires heat and a controlled environment and a catalyst.

 Catalysts help reactions to occur by holding the reactants in exactly the state they need to be in, in order to react.

 Enzymes are biological catalysts.

 Organic pigments called “luciferins” can, under certain conditions, react with oxygen to produce light.

 These conditions are not easily arranged for inside living tissues because this reaction requires lots of energy to kick it off.

 Enzymes, collectively referred to as “luciferase”, catalyze the reaction, lowering the energy required for the process and bringing it within the energy budget available to cells.

 Various approaches to bioluminescence include the addition of other chemicals like ATP to help energize the reaction or modify the colour of the light it produces, but luciferin, luciferase and oxygen are the basis of the reaction.

 Bioluminescence serves many functions in nature.

 Fireflies flash their lights as signals to mates and rivals.

 They can also form large tree-bound swarms that become semi-synchronized, and in such a state the light may serve as a means of coordinating the group for social or reproductive purposes.

 Flashlight fish do something very similar. They swim in dense schools which display a bright scintillating flicker across the school.

 As the fish approach other animals they begin to coordinate their flashes and gradually fall into perfect pulsing synchrony. As the school passes, the pattern falls apart and the random scintillations once again come to dominate the display.

 This behaviour probably serves to keep the school together on their nocturnal meanderings and probably has a defensive function by either dazzling, confusing or intimidating would-be predators.

 It might also help to illuminate prey or even light the way.

 Various deep sea fish use their light production skills to directly illuminate their world for finding food and for hazard detection and avoidance.

 Recently, red bioluminescence was discovered in a number of deep sea fish including anglerfish and dragonfish.

 Most fish do not see the colour red. Their eyes are not equipped with the hardware to detect light of that wavelength.

 The fish with red lanterns are also equipped with eyes sensitive to red light.

 The ability to produce and perceive red light provides these fish with a band of illumination that none of their prey, or their own predators, can perceive. This represents a stealth vision system on par with any radar or night goggles the military might devise.

 Just as stealthy is the emission of light as a means of camouflage.

 When viewed from below, objects in the ocean appear as clear silhouettes against the even illumination of the surface.

 A carefully tuned emission of light from the lower surface of many fish enables them to blend into the luminous background of the surface, making them nearly invisible when viewed from below.

 The sudden appearance of bright light can perturb many small predators. This may be due to the fear of higher order predators noticing the action and popping by.

 Many small marine creatures and a good number of forest invertebrates, appear to produce light for this reason, highlighting to anyone watching, a spot in the landscape where something is going on.

 Glow worms in the rainforest are in fact larval fireflies which are themselves, not flies but beetles. They produce a steady, or slowly pulsing, bead of light in the leaf litter.

 Many predators of insect larvae seek their prey amongst the dark environment of the forest floor. They seek out dark places and have little love of the light.

 An animal with an aversion to light is described as “negatively phototactic”.

 The glow worm may be employing a repulsive defense against the dark loving predators of the rainforest leaf litter habitat.

 On a similar note, and still within the rainforest, fungi are amongst the best known bioluminescent organisms.

 Different parts of a fungus may glow.

 In some cases the mushroom produces sharp pinpoints of relatively bright light.

 In others, the microscopic threads, or mycelium, of the fungus, produce dim and diffuse patches of light that look like moonlight striking the rotting humus through which the mycelium grows.

 In the case of the mushroom, it is a reproductive organ and it has spores to distribute.

 Conscripting the use of flying creatures that are generally positively phototactic, is a great way to spread these spores.

 In the case of the mycelium, it has nothing to distribute, but it is edible.

 Creatures who eat fungus must seek it where it grows; in dark, wet places. They are usually repelled by light.

 When a negatively phototactic, fungus feeding, leaf litter critter finds itself in a place full of edible fungus but lit like a stadium, they may think twice about sticking around for long enough to do any real damage.  

 Bioluminescence attracts, repels, illuminates and obscures. It also inspires art, with fantasy landscapes glowing, and science with powerful lighting technologies already on the market.

 The next time you’re on a night dive, spare a thought for the pedigree of the light stick you’ve tied to your tank and its direct relationship to the light you see in the creatures all around you.