Bioluminescence [46]

Light of Life

By Richard Birdsey

A few weeks ago, several Club members posted messages across the NSWSKC e-mail network discussing their experiences with bioluminescence. Paddlers observed bioluminescence around particular types of coastal formations and at different times of the year. Others talked about interesting experiences they have had with light-emitting plants and animals. Having spent a couple of years studying marine plankton on and off, and at Dave Winkworth’s suggestion, I thought Club members might be interested in knowing a bit more about this fascinating spectacle of nature.

What is Bioluminescence?

There are a number of natural and human-made processes that create light. Chemiluminescence is a broader set of light producing chemical reactions of which bioluminescence is just one. Certain types of chemicals when mixed together produce energy. This energy ‘excites’ other particles which vibrate and generate light. A chemiluminescent product known to many kayakers are Cylume light sticks. A thin glass vial containing hydrogen peroxide sits within a plastic tube containing a chemical called an ester and a fluorescent dye. When the vial is broken the hydrogen peroxide and ester release energy which excites the fluorescent dye and generates light. Bioluminescence is a form of chemiluminescence and, as the name implies, light produced within an organism.

Another type of light emitting process is fluorescence. This is where external energy (as opposed to internal chemical reactions) are absorbed by a fluorescent material and then immediately remitted. This is the central principle behind the operation of fluorescent light tubes. A form of fluorescence is phosphorescence which is where external energy is stored for longer periods of time and slowly reemitted. Watch faces and glow-in-the-dark toys use phosphorescent materials. Some animals and plants are phosphorescent as opposed to being bioluminescent.

Which organisms are Bioluminescent?

Bioluminescence is primarily a marine phenomenon. It has been observed in over 700 marine genera ranging from bacteria through to fish, molluscs (such as squid), sponges, jellyfish, echinoderms (starfish family) and crustaceans (crabs and the like). A common myth is that bioluminescence mostly comes from bacteria. In fact, a vast range of single-celled plankton, zoo and gelatinous plankton are bioluminescent. The ‘sparks’ flying off your paddle is most probably plankton such as copeopods or dinoflagellates. The glowing ‘trails’ in your wake are billions of scared, hungry or over-sexed plankton (more on this later). Interestingly there are almost no freshwater bioluminescent organisms. Land based bioluminescent organisms include some insects (such as fireflies), fungi, worms (‘glow worms’), and some terrestrial micro-organisms.

How is the light made?

As mentioned above, bioluminescence results from light-producing chemical reactions. The group of chemicals involved are broadly termed luciferins. Light is produced by a series of oxidation reactions set off by a catalyst called luciferase (a catalyst is chemical accelerant that speeds up reactions). Luciferin is oxidised by luciferase to produce energy, oxygen, oxyluciferin and light. How plants and animals create luciferins varies. Some secrete it from special glands. ‘Fresh’ luciferin must be captured in the diet or synthesised internally.

Larger animals such as squid and fish have specialised light organs called photophores (photo = ‘light’ and phore = ‘bearing’) which are wart or blister-like structures packed full of bioluminescent bacteria. When the animal wants to generate light they ‘stir’ or stimulate the bacteria in the photophore and hey presto – instant light!

Why make light at all?

At the end of the day no-one can definitively say why some organisms generate light. Taking a Darwinian perspective, if being bioluminescent was hugely advantageous and ensured one’s survival then all plants and animals would be bioluminescent. However, mariners and scientists have come up with some interesting ideas as to why you might want to glow in the dark:

  • Defence: Blasting your attacker with light will help you to escape. Some molluscs, worms and squid emit a cloud of luminous slime when attacked. Interestingly air forces use a similar tactic by dropping brilliant flares when flying low over enemy territory to dazzle anti-aircraft gunners and heat-seeking missiles.
  • Offence: Similar to defence, blinding potential prey will improve your chance of getting a meal. Some deep-sea fish have photophores which can be uncovered by flaps of skin so they can be used like a torch.
  • Camouflage: Deep-sea fish and squid may adjust the intensity of light they emit to prevent them being silhouetted against a darker or lighter background.
  • Communication: Different patterns, sequences and colours of photophores can identify what species you are, your gender, territoriality, mating status, etc. This ability would be vital in low light environments such as nocturnal or deep-sea species.

The real reason is probably a combination of the above – and a couple we haven’t dreamed up!

Where Bioluminescence is found

Bioluminescent plankton occur in all the world’s oceans. The prevalence of plankton in coastal waters and the ocean results from vastly complex interplays between temperature, currents, wind, location of food sources, nutrients, etc. As sea (and therefore living) conditions change so do the appearance of bioluminescence. For example, ‘milk seas’ (where large expanses of the sea ‘glows’) is a result of mass spawning of bioluminescent plankton triggered by a combination of ideal sea temperature and nutrients. ‘Red tides’ are another result of these types of ideal breeding conditions.  Distribution of bioluminescent plankton also varies vertically through the water column. Plankton will congregate or actively migrate between layers of water with different temperature or nutrient load. John Lipscombe’s Navy diving observations of bioluminescence at specific depths is an example of this type of distribution.

Bioluminescence and humans

There are many interesting examples of the interaction between humans and bioluminescent organisms. Cages of fireflies are used a lights in some traditional communities. Japanese troops rubbed a dried extract from bioluminescent starfish on their fingers to help them read maps at night. The ability of bioluminescent plankton to give away the position of submarines and warships is being studied by the US and Royal Navies. Studies have found that even very low concentrations of bioluminescent plankton can ‘paint’ a submarine with enough light to be detectable by satellites. Interestingly the last German U boat sunk during World War I was because bioluminescent plankton enabled the non-sonar equipped corvette to accurately track and depth charge it. Scientists are looking at using bioluminescent plankton to detect heavy metal pollution in the environment (generically called ‘bio-indicators’).

Despite these applications of the science of bioluminescence, the sight of sparks flying from your paddles or glowing trails across are something to be beholden. I hope you can now further observe and appreciate the complexity of these fascinating creatures.