Jellyfish and fireflies; a comparison between biofluorescence and bioluminescence.
One takes flight, the other glides through the water, but both glow mystically. Not surprisingly, they use slightly different mechanisms to do so. One similarity between the two mechanisms is the use of an enzyme. Enzymes, which are supplied by the organism, act as a catalyst in biochemical reactions. In this case, the product or result of the biochemical reaction is a super cool, rave-worthy, form of light.
I was 7 years old, standing with my face pressed up against the aquarium glass, soaking in the green fluorescence of the jellyfish as they swished up and down inside the tank. For weeks, my mind mused at the idea of glowing fish. Surely, my father explained this phenomenon to me at the time, but it took well over a decade before I really came to understand it.
Fluorescence occurs when a protein or chemical absorbs light at one wavelength and emits light at a different wavelength. Wavelength, which corresponds to the color of light or type of radiation, is inversely proportional to the energy of a photon of light. In this case, it is the absorption of a shorter wavelength of light (UV or blue light) and emission of a longer one (green, yellow or red). The overall process might be thought of as the capturing of a photon and release of a photon in a lower energy state, or different color. This process is most commonly observed in organisms containing the Green Fluorescent Protein (GFP) or other fluorescent proteins. Animals have been found to emit green, yellow and red light. Different species of jellyfish- and other marine life- use different mechanisms for emitting light, but the Aequorea victoria is known for its fluorescence by this special protein.
The gene for GFP has been taken from the jellyfish and inserted into other organisms, like bacteria and mice, and used in labs all around the world. Here is an example of GFP being expressed by the bacteria E. coli. The bacteria only fluoresce when I hold a black light near the plate.
When we go to the Aquarium we usually see a set of black lights, which are really just special UV lights, accompanying these beautiful creatures. Why is that? If the UV lights were not gleaming on the jellyfish, we would not see them glow. Blue light in the open ocean and from the enzyme aequorin (more on this later) or Ultra Violet light at the aquarium is absorbed by GFP and causes it to achieve an energetically excited state. In this excited state, GFP emits light with slightly less energy and longer wavelengths, which we see as the color green! The glowing capabilities of jellyfish are not solely attributed to fluorescent proteins and the light source that is available to them.
For Aequorea victoria, the enzymes in action are GFP and aequorin. The latter is able to emit blue light at depths of the ocean where light from the sun does not reach. In the presence of calcium, its prosthetic group containing coelenterazine undergoes oxidation. This reaction has two notable products: carbon dioxide and coelenteramide. Coelenteramide is the excited form of coelenterazine. In its excited state coelenteramide emits blue light! This type of biochemical reaction is recognized as bioluminescence. In this respect, jellyfish are capable of both biofluorescence (by means of GFP) and bioluminescence (through aequorin). It is thought that these two enzymes work together when an external light source is not available. Aquorin emits blue light while GFP almost instantly absorbs that light and re emits it as green light. That’s why commercial aquariums use UV lights to make their jellyfish glow all the time. On the other hand, fireflies only exhibit bioluminescence.
Fireflies contain a system similar to the zero-light requirement system of the protein aequorin in jellyfish. The firefly equivalent of coelenteramide is luciferin. Instead of the enzymatic protein aequorin, they use luciferase. Luciferin, when acting as the substrate for luciferase and in the presence of calcium, oxygen and adenosine triphosphate (ATP), generates cold light otherwise known as bioluminescence. Sound familiar? The light generated by fireflies is usually yellow but can be golden or even reddish sometimes.
Scientists are still researching the exact mechanisms for biofluorescence and bioluminescence every day. So, although it may seen like they have a good grasp on these concepts now, they actually have much to learn still.
On a side note: I want to write for my audience first, so if you think this post could have benefited from more hard science or less technical jargon, please feel free to leave a comment below. I will gladly take your opinion into consideration for future posts. 🙂