Humans go to a lot of effort to create portable artificial light sources to carry around with them. Like so many other problems overcome by man, nature has long had its own solution. Many bioluminescent lifeforms make they’re own light and carry it around with them, either to disguise themselves when hunting (like the bobtail squid), as a warning to predators (fireflies) or to attract other species (some mushrooms).
Some species of single celled plankton even glow when disturbed by tides, storms or passing marine life. These dinoflagellates are responsible for the phenomenon known as ‘milky-sea’ where whole ocean regions start to glow, this can even interfere with marine navigation! Scientists aren’t sure exactly why the plankton have developed like this. The most popular explanation is the ‘burglar-alarm theory’; if a small fish begins to feed on a plankton, its neighbours start to glow. This light attracts larger fish nearby which are likely to be the plankton-eaters predators. However, this system doest seem to be as foolproof as some of the better-understood uses of bioluminescence, which has left scientists questioning.
In general, bioluminescence involves the combination of two substances in a light-producing reaction. One is a luciferin, or a light producing substance, and the other is luciferase, which catalyses the reaction. In some cases the luciferin is a protein known as photoprotein, and the light-making process requires a charged ion to activate the reaction. Neurological, mechanical or chemical triggers can start these reactions.
The terms luciferin and luciferase both come from a Latin term lucifer, which means “light-bringer.” They are generic terms rather than the names of particular chemicals. Lots of different substances can act like luciferins and luciferases, depending on the species of the bioluminescent life form. For example, the luciferin coelenterazine is common in marine bioluminescence. Dinoflagellates that obtain food through photosynthesis use a luciferin that resembles chlorophyll. Their luminescence is brighter after very sunny days. Some shrimp and fish appear to manufacture their luciferin from the food they eat.
Animals can either house these substances in their own bodies or develop a symbiotic relationship with light-producing bacteria. These bacteria live in a light organ in the host organism’s body. The bacteria produce light all the time, so in order to turn their lights on and off, some animals can pull their light organs into their bodies. Others cover them with pieces of skin similar to eyelids. Some organisms also use a fluorescent substance, like green fluorescent protein (GFP), to adjust the colour of the light they create. The fluorescent substance absorbs the blue-green light and emits it as a different colour. Because of all these variations in luciferins, luciferases and how animals use them, many researchers believe that the ability to make light simultaneously and independently evolved in multiple forms of life.
Whatever the evolutionary origin of bioluminescence, these organisms and their luciferase systems are widely used in the fields of genetic engineering as reporter genes. Luciferase systems have also been harnessed for biomedical research using bioluminescence imaging and various bioluminescent organisms are in use as model systems. Additionally, the structures of bacterial photophores, the light producing organs in bioluminescent organisms, are being investigated by industrial designers. Engineered bioluminescence could perhaps one day be used to reduce the need for street lighting, or for decorative purposes. In June 2013 the Glowing Plant project successfully raised nearly $500,000 on the crowd funding site Kickstarter to create a bioluminescent plant. The project’s long term goal is the creation of trees that can be used for street lighting. Whatever the future, there remains a lot we can learn about bioluminescent systems.