Glow-in-the-dark jellyfish, glow-in-the-dark pigs, and why are there glow-

Produced by: Science Popularization China

Author: Wulianhuakai (young biologist)

Producer: China Science Expo

Bioluminescence is a very common phenomenon. The dancing of fireflies on summer nights and the "blue tears" caused by the gathering of Noctiluca scintillans on the beach are unique sights that countless people check in and take photos of. Glowing objects look very beautiful, but if you put them in your mouth as food, you have to doubt the safety issues. There have been pictures of glow-in-the-dark sushi circulating on the Internet. Sushi that looks normal under strong light emits a faint blue light in a dark environment.

Glowing sushi

(Image source: Facebook screenshot)

In July 2020, Thai media reported the news about glow-in-the-dark sushi: A Thai guy bought a box of sushi. As he walked out of the supermarket, the light on the shrimp on the sushi gradually dimmed, and then it glowed brightly in the dark. He recorded this strange phenomenon, shot a video and uploaded it to social networks, which sparked widespread discussion.

Why does this shrimp, which has been made into food, emit a cold blue light? Can this fluorescent food still be eaten? To answer these questions, we need to start with the fluorescence phenomenon in nature.

Fluorescent protein that makes pigs glow

Fluorescent proteins are very common factors that can cause bioluminescence. When stimulated by light of a specific wavelength, they emit light of the corresponding color. The first fluorescent protein discovered was the wild-type green fluorescent protein (GFP) found in the Victoria jellyfish (Aequorea victoria) in 1962.

Aequorea victoria

(Image source: Wikipedia)

The principle of jellyfish luminescence is relatively complicated. First, aequorin and coelenterazine in the jellyfish covalently combine to produce a stable intermediate with luminescence ability. The covalent bond formed is a peroxide bond. Under the influence of calcium ions, the covalent bond breaks, and an oxidation reaction occurs while releasing blue light. These blue lights can activate the fluorescent protein in the jellyfish.

The GFP luminescent group is then activated under the irradiation of blue light and emits green fluorescence in the form of energy, which makes the Victoria Aequorea appear green.

In short, its natural luminescence process requires the generation of blue light to excite GFP, which then emits green light.

Green fluorescent protein luminescence principle

(Image source: Reference 1, translated by the author)

Based on the above phenomenon, modern biology uses DNA recombination technology to clone fluorescent protein genes into suitable cells for expression, and then uses fluorescence microscopy to observe the labeled proteins in vivo. Fluorescence microscopy artificially simulates the above blue light emitting process, so that when using a fluorescence microscope, green fluorescence will be seen.

With the modification of the GFP gene, enhanced GFP and fluorescent proteins of more colors were produced, allowing people to track cell activities more easily and clearly. Therefore, fluorescent proteins are also hailed as "illuminating the future of biological research."

More than ten years ago, Chinese scientists bred "luminous pigs". These cloned pigs can emit four kinds of fluorescence under the excitation light of specific wavelengths: red, yellow, green and cyan.

Transgenic cloned pigs bred by Guangzhou Institute of Biomedicine and Health, Chinese Academy of Sciences

(Image source: cas.cn)

So, is the reason why the sushi shrimp in the hands of the Thai guy in the news emits a faint light because it also has fluorescent proteins in its body? Unfortunately, this is not the case. Fluorescent proteins are mostly found in cnidarians, and there are currently no reports of fluorescent proteins being found in commonly edible shrimps, so we can basically rule out the possibility of fluorescent proteins.

Plankton under the "luciferin-luciferase" luminescence system

In summer, the "blue tears" phenomenon appears on the coasts of many places in my country. The waving sea surface is covered with dots of blue light, just like blue tears flowing from the sea. This scene is formed by the luminescence of the Noctiluca scintillans. The reason why the Noctiluca scintillans glows is based on the "luciferin-luciferase" luminescence enzyme reaction.

Blue Tears photographed by the author on the coast of Rizhao, Shandong Province, on the evening of 2022-03-13

The color of "blue tears" is similar to the color of the glow-in-the-dark sushi we saw at the beginning! As for why they are all blue, it is probably because the propagation of light in water is very different from that in the atmosphere. Blue and green light have the strongest penetration in seawater, so most marine luminous organisms emit blue or green light. This also creates an interesting phenomenon. In the dark deep sea, in addition to black, red also has an excellent stealth effect, which can also explain why many deep-sea creatures have a red appearance.

Propagation of light in water

(Image source: Wikipedia)

In luminous plankton, luciferin undergoes an oxidation reaction under the catalysis of luciferase to generate excited luciferin and emit light. Many marine organisms such as luminous dinoflagellates, jellyfish, comb jellies, sea fireflies and fireflies emit light due to this. Terrestrial fireflies also emit light based on this principle. It is worth noting that the luciferin and luciferase used by different species are not the same, and their reaction processes are also different.

The process of enzyme reaction of different species A Firefly B White-line earthworm C Sea firefly D Luminescent freshwater limpet E Luminescent bacteria F Krill

(Image source: Reference 3)

Different from the passive luminescence of fluorescent proteins, the luciferase reaction belongs to the true luminescence of organisms. Because of this, this luminescence system sometimes requires external perturbations or neural control to stimulate. And it has a certain diurnal or seasonal periodicity.

However, the luminous plankton is mainly attached to the shrimp's cephalothorax, appendages and shell, and it is unlikely that the muscles can still glow after the shell is removed. The sushi shrimp in the news has been peeled, so it may not be this enzymatic reaction that makes it glow.

Glowing krill

Is it possible that the shrimp on this sushi is a luminous shrimp itself?

All organisms in the Euphausiidae family can bioluminesce. Krill bioluminescence has a seasonal cycle, usually in the fall, and can also bioluminesce when frightened.

Left: Krill Right: Glowing krill

(Image source: Wikipedia)

Krill have an organ called a photophore, which is a golden spherical body with a slightly reddish color. It contains crystals, photophores, reflectors and nerves. Krill also use the "luciferin-luciferase" luminescence system to emit light. Unlike luminous dinoflagellates, krill cannot produce luciferin by themselves. They can only obtain luciferin from the outside world by eating luminous dinoflagellates to complete the enzymatic reaction.

Schematic diagram of krill structure and anatomical structure of light organ

(Image source: Wikipedia Reference 2 Author's Chinese translation)

The light organs of krill are mainly distributed below the eyestalks and below the legs and abdomen. The light-emitting parts are inconsistent with the scene presented by the glow-in-the-dark sushi. And unfortunately, the physiological characteristics of krill are not suitable for making sushi, so the possibility that the glowing sushi is krill is unlikely.

Bioluminescent marine bacteria

Now there is another possibility, have the sushi shrimps been invaded by luminous bacteria?

Bacteria that can perform bioluminescence are called luminous bacteria. Most of them are marine bacteria. Marine luminous bacteria can coexist in squids and bony fish. More than 20 species from 5 genera have been discovered so far.

Some luminous bacteria and their hosts

(Image source: Wikipedia)

In luminescent bacteria, the Lux operon controls the expression of luminescence-related genes. The wild luminescence gene system includes the structural genes LuxC, D, A, B, and E. LuxA and B encode the α and β subunits of bacterial luciferase, respectively. LuxC, D, and E encode NADPH-dependent fatty acid reductase, acyltransferase, and ATP synthase, respectively.

All luminous bacteria have similar luminescent reaction mechanisms. Under the action of oxygen and luciferase, the reduced flavin mononucleotide (FMNH2) and long-chain fatty aldehydes are oxidized to oxidized flavin mononucleotide (FMN) and long-chain fatty acids, and emit blue-green light with a wavelength of 490 nanometers. The optimal temperature for bacterial luciferase reaction is 18°C, and it is rapidly inactivated when it exceeds 25°C.

Gene expression and biochemical principles of luminescent bacteria

(Image source: Reference 4)

Based on the above information, it can be inferred that the sushi bought by the Thai boy was glowing most likely because it was contaminated by luminous bacteria. The local quarantine department also detected luminous bacteria in subsequent tests.

How did the luminous bacteria in the contaminated shrimp survive from the ocean to people's mouths? It should be that the bacteria were not thoroughly cleaned after fishing, and the sushi was made of raw meat and rice and stored in a low temperature environment, which are conducive to the survival of luminous bacteria. After a period of reproduction, the sushi finally emits a cold blue light in the dark night.

Coincidentally, there have been news reports of cases of raw pork emitting blue light in many places abroad. After testing by local quarantine departments, it was confirmed that it was caused by the luminous bacteria Pseudomonas fluorescens.

Glowing bacteria

(Image source: Wikipedia)

Conclusion

Finally, we only analyzed the reasons for the existence of luminous sushi from the perspective of natural luminescence of marine organisms. Regarding the luminescence phenomenon of other food ingredients in life, other possibilities cannot be ruled out, such as the mixing of phosphorus and other luminous substances during food processing. In addition, we need to remind everyone that if you encounter luminous food in life, you can hand it over to the relevant department for testing, and please do not eat it.

Editor: Guo Yaxin

References

[1] Bhuckory, S., Kays, JC, & Dennis, AM (2019). In vivo biosensing using resonance energy transfer. Biosensors, 9(2), 76.

[2] Zheng Zhong, Li Shaojing. Review and Prospect of Marine Plankton Bioluminescence Research - New Trends in Marine Plankton Biology XIII[J]. Nature Magazine, 1987, 6: 31-36.

[3] Yan, Y., Wang, S., Xie, F., Fang, X., Zhang, YM, & Zhang, SXA (2019). Firefly-inspired approach to develop new chemiluminescence materials. Iscience, 13, 478-487.

[4] Sun, S., Yang, X., Wang, Y., & Shen, X. (2016). In vivo analysis of protein–protein interactions with bioluminescence resonance energy transfer (BRET): progress and prospects. International journal of molecular sciences, 17(10), 1704.

[5] Cui Qingyu, Wang Mingyu, Xu Hai. Research progress of bacterial lux fluorescence reporter system[J]. China Biotechnology, 2017, 37(8): 66-71.

(Note: Latin text should be italicized.)