Bioluminescent organisms have evolved dozens of times over the course of life history. What biochemistry is needed to light up the darkness? Various studies are devoted to this issue. Plunge deep enough into the depths of the ocean, and you will see not darkness, but light. 90% of fish and marine life that thrive at depths of 100 or even 1000 meters are capable of producing their own light. Flashlight fish hunt and communicate using a kind of Morse code sent by light pockets under the eyes. Fish of the Platytroctidae family shoot glowing ink at their attackers. Hatchet fish make themselves invisible by emitting light in their abdomens to simulate descending sunlight; predators look at them and see only a continuous glow.
Scientists have indexed thousands of bioluminescent organisms throughout the tree of life and expect to add more. However, they have long wondered how bioluminescence came to be. Now, as recently published studies show, scientists have made significant progress in understanding the origins of bioluminescence - both evolutionarily and chemically. New insights may one day allow bioluminescence to be used in biological and medical research.
One of the long-standing challenges is to determine how many times a single bioluminescence has occurred. How many species came to her independently of each other?
While some of the most famous examples of light in living organisms are terrestrial - think fireflies, for example - most of the evolutionary events associated with bioluminescence took place in the ocean. Bioluminescence is virtually and apparently absent in all terrestrial vertebrates and flowering plants.
In the depths of the ocean, light gives organisms a unique way to attract prey, communicate and defend themselves, says Matthew Davis, a biologist at Saint Cloud State University in Minnesota. In a study published in June, he and his colleagues found that fish that use light to communicate and signal courtship were especially common. Over a period of about 150 million years - not long by evolutionary standards - such fish have spread widely into more species than other fish. Bioluminescent species, which used their light exclusively for camouflage, on the other hand, were not as diverse.
Marriage signals can be changed relatively easily. These changes, in turn, can create subgroups in the population, which eventually split into unique species. In June, Todd Oakley, an evolutionary biologist at the University of California, Santa Barbara, and one of his students, Emily Ellis, published a study showing that organisms using bioluminescence as mating signals had many more species and a faster rate of species accumulation than their close relatives who do not use light. Oakley and Ellis studied ten groups of organisms, including fireflies, octopuses, sharks, and tiny arthropods, ostracods.
Davis and his colleagues' research was limited to ray-finned fish, which comprise approximately 95% of fish species. Davis calculated that even in this one group, bioluminescence developed at least 27 times. Stephen Haddock, a marine biologist at the Monterey Bay Aquarium Research Institute and an expert in bioluminescence, estimated that among all life forms, bioluminescence independently appeared at least 50 times.
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Many ways to ignite
In almost all luminous organisms, bioluminescence requires three ingredients: oxygen, the light-emitting pigment luciferin (from the Latin word lucifer, meaning “carrying light”), and the enzyme luciferase. When luciferin interacts with oxygen - via luciferase - it forms an excited, unstable component that the set emits, returning to a lower-energy state.
Curiously, there are far fewer luciferins than luciferase. Although species tend to have a unique luciferase, very many have the same luciferin. Only four luciferins are responsible for producing most of the light in the ocean. Of the nearly 20 groups of bioluminescent organisms in the world, nine of them emit light from luciferin called coelenterazine.
However, it would be a mistake to believe that all coelenterazine-containing organisms descended from one luminous ancestor. If that were the case, why would they develop such a wide spectrum of luciferase, asks Warren Francis, a biologist at Ludwig Maximilian University in Munich. Presumably, the first pair of luciferin-luciferase should have survived and multiplied.
It is also likely that many of these species do not produce coelenterazine on their own. Instead, they get it from their diet, says Yuichi Oba, a professor of biology at Chubu University in Japan.
In 2009, a team led by Oba discovered that a deep-sea crustacean (copepods) - a tiny, widespread crustacean - was making its coelenterazine. These crustaceans are an extremely plentiful food source for a wide range of marine animals - so abundant that they are called "rice in the ocean" in Japan. He thinks these crustaceans are the key to understanding why so many marine organisms are bioluminescent.
Both and his colleagues took amino acids, which are believed to be the building blocks of coelenterazine, labeled them with a molecular marker, and loaded them into food for copepods. Then they fed this food to crustaceans in the laboratory.
After 24 hours, the scientists extracted the coelenterazine from the crustaceans and looked at the markers that were added. Obviously, they were everywhere - which was the ultimate proof that crustaceans synthesized luciferin molecules from amino acids.
Even the jellyfish that first discovered coelenterazine (and were named after) do not produce coelenterazine of their own. They get their luciferin by eating crustaceans and other small crustaceans.
Mysterious origins
Scientists have found another clue that could help explain the popularity of coelenterazine among deep-sea animals: this molecule is also found in organisms that do not emit light. This struck Jean-François Ries, a biologist at the Catholic University of Leuven in Belgium, as odd. It's surprising that “so many animals rely on the same molecule to produce light,” he says. Perhaps coelenterazine has other functions besides luminescence?
In experiments with rat liver cells, Reese showed that coelenterazine is a potent antioxidant. His hypothesis: Coelenterazine may have first spread among marine organisms living in surface waters. There, the antioxidant could provide the necessary protection against the oxidative effects of harmful sunlight.
When these organisms began to colonize deeper ocean waters, where the need for antioxidants is lower, the ability of coelenterazine to emit light came in handy, Reese suggested. Over time, organisms have developed different strategies - like luciferase and specialized light organs - to enhance this quality.
However, scientists have not figured out how other organisms, not only the Oba copepods, make coelenterazine. The genes that code for coelenterazine are also completely unknown.
Take comb jelly, for example. These ancient sea creatures - considered by some to be the first branch of the animal tree - have long been suspected of producing coelenterazine. But no one has been able to confirm this, let alone identify specific genetic instructions at work.
Last year, however, it was reported that a group of researchers led by Francis and Haddock came across a gene that may be involved in the synthesis of luciferin. To do this, they studied the transcriptomes of ctenophores, which are snapshots of the genes that an animal expresses at a given moment. They looked for genes encoded for a group of three amino acids - the same amino acids that Oba fed to his copepods.
Among 22 species of bioluminescent ctenophores, scientists have found a group of genes that match their criteria. These same genes were absent in two other non-luminescent ctenophore species.
New World
The genetic mechanism of bioluminescence has applications outside of evolutionary biology. If scientists can isolate the genes for luciferin and luciferase pairs, they could potentially make organisms and cells glow, for one reason or another.
In 1986, scientists at the University of California at San Diego modified and incorporated the firefly luciferase gene into tobacco plants. The study was published in the journal Science, featuring one of these plants glowing eerily against a dark background.
This plant does not produce light by itself - it contains luciferase. But in order for this tobacco to glow, it must be watered with a solution containing luciferin.
Thirty years later, scientists have still not been able to create self-luminous organisms using genetic engineering, because they do not know the biosynthetic pathways for most luciferins. (The only exception was found in bacteria: Scientists were able to identify the glow genes that code for the bacterial luciferin-luciferase system, but these genes need to be modified to be useful for any non-bacterial organism.)
One of the biggest potential uses of luciferin and luciferase in cell biology is to incorporate them as bulbs into cells and tissues. This kind of technology would be useful for tracking cell location, gene expression, protein production, says Jennifer Prescher, professor of chemistry at the University of California, Irvine.
The use of bioluminescence molecules will be just as useful as the use of a fluorescent protein, which is already used to monitor the development of HIV infections, to visualize tumors and track nerve damage in Alzheimer's disease.
Currently, scientists using luciferin for imaging experiments must create a synthetic version of it, or buy it at $ 50 per milligram. Introducing luciferin from the outside into the cell is also difficult - it wouldn't be a problem if the cell could make its own luciferin.
Research continues and is gradually defining evolutionary and chemical processes on how organisms produce light. But most of the bioluminescent world is still in the dark.
Ilya Khel
