The Chemistry behind Bioluminescence

Introduction

The emission of light generated in a process named bioluminescence is a type of luminescence used by living organisms to communicate, defend themselves and attract prey. Bioluminescence is also called a type of “cold light”. Cold light is a type of illumination that only 20% or less of the light radiates heat.

Many of the bioluminescent creatures tend to thrive in the oceans. These include fish, plankton, jellyfish, bacteria, and much more. Not many bioluminescent organisms are found in freshwater habitats.

What is Bioluminescence and how is it produced?

Bioluminescence is a chemiluminescence type, a chemical reaction where light is emitted as one of the by-products. Bioluminescence is produced through a chemical reaction between a substrate (luciferin) and an enzyme (luciferase) that results in the molecules of luciferin being converted into a product in an excited state and emitting a photon on returning to the ground state, with visible light emitted along the products.

The bioluminescent colour depends on the specific arrangement and type of luciferin. The colours of bioluminescence are different based on the luciferases and corresponding luciferins found in nature. The most common colours are blue and green. Some type of bioluminescence is emitted through a reaction between luciferins, photoproteins, and oxygen, along with an agent like calcium.

Many luciferins are only formed by living organisms that express the corresponding luciferase, except the bacterial luciferin FMNH2, concentrated flavin mononucleotide (FMN), which is abundant in all cells.

How does bioluminescence work and how is it any different from chemiluminescence?

As explained before, bioluminescence is a certain type of chemiluminescence, although there are many differences between them. Bioluminescence is the type of luminescence only living organisms exhibit through chemical reactions in their bodies. Sometimes phosphorescence and fluorescence often get confused as bioluminescence, though no chemical reactions are happening in those two phenomena.

The bacterial bioluminescence reaction is catalyzed by an αβ-heterodimeric luciferase coded by the genes luxA and luxB. In addition to FMNH2, the luciferase binds molecular oxygen and a long-chain fatty aldehyde. The fatty aldehyde is oxidized to the corresponding fatty acid, and FMNH2 is oxidized to FMN, thereby emitting a blue photon with a wavelength around the spectral emission maximum λmax of roughly 490 nm:

FMNH2+RCHO+O2→FMN+RCOOH+H2O+hν.FMNH2+RCHO+O2→FMN+RCOOH+H2O+hν

In chemiluminescence, the enthalpy that leads to electronic excitation sources from a chemical reaction. In other types of luminescence such as fluorescence or phosphorescence, the energy comes from external sources, like an active light source. Chemiluminescence has been proven to be so useful because it is delicate and, in most scenarios, extremely selective, but it is not universally applicable because of the prerequisite that the enthalpy released in a chemical reaction must be manifested as the emission of photons.

An example of chemiluminescence is the luminol experiment, which is commonly known as a demonstration of chemiluminescence. In the following reaction, luminol (C8H7N3O2) reacts with hydrogen peroxide (H2O2) to release blue light (note that there is minimal heat/energy released in the process as most of the energy released is in the form of illumination or light; which is a form of energy). The amount of light released by the reaction is low unless a small amount of suitable catalyst is added. Classically, the catalyst is a small amount of iron (Fe) or copper (Cu).

The reaction is:

C8H7N3O2 (luminol) + H2O2 (hydrogen peroxide) → 3-APA (vibronic excited state) → 3-APA (decayed to a lower energy level) + light

Where 3-Aminopthalalate is given as 3-APA.

Note: There is no difference in the chemical formula of the transition state, only the energy level of the electrons.

Chemiluminescence is also sometimes seen in the solid composition of some elements. For example, the green glow of white phosphorous in damp air is a gas-phase reaction between vaporized phosphorus and oxygen. Chemiluminescence can also be affected by the same factors that affect other chemical reactions. For example: increasing the temperature of the reaction speeds it up, causing it to release more energy in the form of light. However, the light doesn't last as long compared to cooler or lower temperatures and gleams radiantly only for a brief moment.

A more detailed study of Bioluminescence

Now that we have seen what bioluminescence is, and how it is a form of chemiluminescence along with the differences in both, it is time to take a closer look at our topic at hand. That is the study of the chemistry behind the bioluminescence that living organisms use to guide them through the dark.

The most common example of bioluminescence we see almost every day is the common firefly. The process by which fireflies produce light is perhaps one of the most well-known examples of bioluminescence.

When oxygen is combined with calcium (Ca), adenosine triphosphate (ATP), and the chemical luciferin in the presence of luciferase, a bioluminescent enzyme, light is produced. The firefly provides the light organ with oxygen through an external supply. It was confusion among scientists about how fireflies were able to have such a high flash rate as breathed-in oxygen is not supplied as fast. Researchers recently learned that nitric oxide gas (NO) plays an important role in firefly flash control. Briefly told, when the firefly’s light is off, no nitric oxide is produced. In this situation, oxygen that enters the light organ is bound to the mitochondria and is thereby not available for transport within the firefly’s body. The presence of nitric oxide, which binds itself to the mitochondria, allows oxygen to flow into the light organ where it combines with the other chemicals needed to produce bioluminescent illumination.

Other many examples of bioluminescent organisms are the phytoplankton. We will specifically talk about the Dinoflagellates. This plankton, unlike the fireflies who use their illumination ability to communicate, use their bioluminescence to defend themselves and fend off predators who come to feast on them. When disturbed, the plankton will give off a brief flash of light that will scare the predator and make them less likely to approach the dinoflagellates.

They are responsible for the blue bioluminescent glow we see in the ocean. The glow is produced through a similar chemical reaction as the firefly’s illumination. Light is produced through a chemical reaction in the presence of oxygen involving a substrate termed luciferin and the enzyme luciferase. But unlike fireflies, whose primary colour of illumination is yellow, the glowing phytoplankton emit is blue. As spoken earlier, the colours of bioluminescence are different based on the luciferases and corresponding luciferins.

Concluding this topic, this wonderful natural phenomenon is often overlooked and has acquired little attention from researchers and scientists alike. Considering how much energy is wasted in the form of heat in our everyday sources of illumination, perhaps we can research more about the “cold light” and conserve energy for sustainable causes.

The speed of light isn’t a limitation, it’s just the beginning.

- Niyatee