LIFELIGHT: The Chemical Secrets of Flickering Fireflies

Fireflies’ summertime glow is really a fascinating nature’s show to watch. And we might have wondered - how and why do these little creatures produce light? The chemical reaction within fireflies’ bodies allows them to make and emit light. 

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This kind of light production is called bioluminescence, the conversion of chemical energy into light energy, and is quite characterized by many other organisms, mostly marine organisms. Perchance an epitome of bioluminescence is the manner how fireflies produce light. To be able to do this, they contain photic organs on their lower abdomen that make light. Generally, luciferin chemical reacts with ATP, and when exposed to oxygen, light is produced and emitted.

CREDIT: www.firefly.org

Fireflies contain an organic compound called luciferin in their bodies. Luc represents firefly luciferase, adenosine triphosphate (ATP), the universal biochemical energy source, and PPi is inorganic pyrophosphate. Firefly luciferase has extraordinary specificity for this nucleotide triphosphate. The adenylate is the true substrate of the subsequent oxidative chemistry. Luciferase converts firefly D-luciferin (LH2) into the corresponding enzyme-bound luciferyl adenylate. In fact, D-LH2-AMP produced synthetically reacts with oxygen in the presence of luciferase to produce light emission identical to that obtained with the natural substrates D-luciferin and Mg-ATP. (Bruce R. Branchini, CHEMISTRY OF FIREFLY BIOLUMINESCENCE)

The luciferase enzyme functions as a mono-oxygenase, although it does so in a very unusual manner without the apparent involvement of a metal or cofactor. In some way that has not been yet determined, luciferase amino acid residues are recruited to promote the addition of molecular oxygen to the D-luciferin adenylate, which is then transformed to an electronically excited state oxyluciferin molecule and carbon dioxide, each containing one oxygen atom from molecular oxygen. Visible light emission results from the rapid loss of energy of the excited state oxyluciferin molecule via a fluorescence pathway. The symbol (*) denotes an electronic excited state.

The light wavelength is between 510 and 670 nanometers (pale yellow to reddish green color). To reflect the light away from their abdomen, fireflies’ cells that make the light have uric acid crystals.

credit: www.x-weibo.net

The pulsating pattern is due to the fireflies’ regulation of airflow into their abdomen. When oxygen is available, the firefly’s abdomen lights up and when there is no oxygen, there is no glow. The oxygen transport from outside to the interior cells takes place through abdominal trachea because these organisms do not have lungs. It is really amazing how some firefly species produce a high flash rate, pointing out the relatively slow speed of the muscles for oxygen transport.

Experts believed that in flash control, nitric oxide gas is very important. Without this gas, oxygen that enters is bound to mitochondria’s surface, and is thereby not available for transport further within the light organ. The presence of the gas, which binds to the mitochondria, allows oxygen to flow into the light organ where it combines with the other chemicals needed to create an awesome bioluminescent reaction. Because nitric oxide breaks down very fast, as soon as the chemical is no longer being produced, the oxygen molecules are again trapped by the mitochondria and are not available for the production of light.

A firefly's light is “cold light”, without a lot of energy being lost as heat, contrary to a light bulb which produces a lot of heat in addition to light. This is necessary because if a firefly's light-producing organ got as hot as a light bulb, the firefly would not survive the experience. 

By recreating the firefly’s glow in the lab, scientists continue to tease out the secrets behind how these little guys light up, the American Chemical Society (ACS) announced in a new video. Scientists had known that a compound called luciferase produced the firefly’s glow. A recent article published in the Journal of the American Chemical Society, Dr. Bruce Branchini of Connecticut College, reveals a molecule toxic to most animals, called a superoxide ion, plays a key role in the reactions that cause luciferase to produce light. Superoxide ion is a form of molecular oxygen which contains an extra electron, a very reactive specie. It can cause inflammation and cell damage in humans and other animals but doesn’t appear to harm the bug because the reactions are contained and happen quickly, the scientists say. 

Reasons of Flashing and Flickering

Fireflies’ flashing and flickering acts have a variety of reasons. Other experts think that the firefly’s flashy style may warn predators of their bitter taste and to stay away. Their bodies are equipped with the chemical called lucibufagens, and after a predator gets a mouthful, it quickly learns to associate the firefly's light with a bad taste. 

credit: cleantechnica.com

As adults, many fireflies have flash patterns unique to their species and use them to identify other members of their species as well as to discriminate between members of the opposite sex. Several studies have shown that female fireflies choose mates depending upon specific male flash pattern characteristics. Higher male flash rates, as well as increased flash intensity, have been shown to be more attractive to females in two different firefly species.

The adult fireflies of some species are not luminous at all, instead, they use pheromones to locate mates. The use of pheromones as sexual signals appears to be the ancestral condition in fireflies with the use of luminous sexual signals as being a more recent development. There are species that employ both pheromonal and luminous components in their mating systems. These species appear to be evolutionarily intermediate between the pheromone-only fireflies and flash-only fireflies.

While each firefly species has its own pattern of flashing, some females imitate the patterns of other species. Males land next to them – only to be eaten alive by tricky fireflies. Their light is quite a deadly weapon, not just motivation of romance.

References:

https://en.wikipedia.org/wiki/Firefly (Retrieved: April 28, 2016) 

http://animals.howstuffworks.com/insects/question554.htm (Retrieved: April 28, 2016) 

http://earthsky.org/earth/bugs-firefly-light (Retrieved: April 28, 2016) 

http://www.scientificamerican.com/article/how-and-why-do-fireflies/ (Retrieved: April 28, 2016) 

http://learn.genetics.utah.edu/content/molecules/firefly/ (Retrieved: April 28, 2016) 

https://www.sciencemag.org/news/sifter/watch-chemistry-behind-how-fireflies-glow (Retrieved: April 28, 2016) 

http://news.nationalgeographic.com/2015/07/150724-fireflies-glow-bugs-summer-nation-science/ (Retrieved: April 28, 2016)