If you were to stand in the sun of a summer day for a few minutes, or cup your hands over a bright light bulb, stand near a lighted candelabra, or, by chance, stand in the path of the beam from an operating 150-watt helium-argon laser, you would notice certain characteristics common in experiencing all of them. Certainly, each of these sources gives off light, one form of energy. But another form of energy, in each case, is released as well. This is because some of the photons (the infinitesimal particles that make up light) in ordinary light have an energy content too low to emit light. But they do radiate this other form of energy. Scientists call it "infrared radiation". The layman calls it by a much simpler and more familiar term, namely heat.

Well, that's not news. Anyone knows that the sun is hot, as are light bulbs, candles, and lasers. Certainly it seems that heat is a necessary by-product of light. And so it was thought until Aristotle's time, when an exception was found.

That exception is bioluminescence. Bioluminescence, the property of light originating from living matter, is a matter that has intrigued scientists (and non-scientists as well) for thousands of years. The mere idea of a plant or animal emitting light seems "eerie" enough to some. Indeed, scientists have studied the "simple" ability to bioluminesce in great detail. However, it is this "cold-light" property of bioluminescence that has intrigued them possibly to a greater extent.

The process through which it was discovered how bioluminescent organisms are able to luminesce (to create light through the reactions of chemicals — virtually no extra heat is released) instead of incandescing (creating light by heating substances to very high temperatures) is a long one.

It began in the days of the ancient Greeks, when the great philosopher Aristotle (whose methods and ideas in biology, possibly more than in any other field, were remarkably advanced for his time) wrote that he found that certain pieces of dead, decaying wood "seem[ed] to produce light though . . . not by their natures composed of fire itself or indeed of any kind of fire".¹ He had the right idea, and should certainly be commended for his observations. However, he failed to realize that the wood itself was not producing light, but, instead, that the light was being emitted from a parasitic fungus growing on the wood. In fact, it was later found that for wood to luminesce would be virtually impossible, and, as a matter of fact, it does not, anyway.

The basis on which the preceding statement (i.e., ". . . for wood to luminesce would be virtually impossible . . ."), as well as on which much of what is known about bioluminescence today, is an observation made by Robert Boyle, an early "physicist", in 1667. Boyle performed an experiment with the same "shining wood" that spurred Aristotle to experiment (Boyle didn't realize it was a fungus, either).

This classic experiment, which was a major step in revealing a great secret of bioluminescence, consisted simply of placing a piece of the wood into a bell jar and pumping out the air. The luminescence stopped! He tested this with luminescent meat and fish* as well. In each case, the luminescence stopped. It was evident that something present in the air was allowing the "wood", meat, and fish to shine. (Actually, the meat and fish, as such, like the wood, were not shining, but produced light as a result of luminescent bacteria living on the flesh.)
* sound appetizing, don't they?

In time, this "something" in the air was identified as oxygen. So it became apparent that another something was being oxidized to produce light. Now the search for that new something began. And, furthermore, when the process of photosynthesis, the process whereby green plants (including trees) manufacture food and other needed materials from light energy and carbon dioxide, was discovered, it became evident that wood could not luminesce. This is because photosynthesis is an oxygen-releasing cycle, whereas bioluminescence is an oxygen-using cycle, so that for an organism to carry on photosynthesis and luminesce as well would require two different exchange systems. (I must concede here that one organism, a dinoflagellate [a single-celled plant-like organism] called Gonyaulax polyedra, does carry on both! And that's not all it does, as will soon be shown.) In fungi, no problems of this type are encountered, for they do not carry on photosynthesis.

While on the subject of the dinoflagellate Gonyaulax, it might be in order to tell of some of its properties. And before this is done, it should be made clear that, in many ways, Gonyaulax illustrates the exception rather than the rule.* But exceptions should not be shunned because they are exceptions. Gonyaulax is, as described by one author, " . . . one of those contradictions — the kind that stimulate some of the great forward leaps of science, for contradictions cry out for solutions in new directions. . ."²
* among bioluminescent organisms

One of the oddities of Gonyaulax is its built-in "timekeeping device". It luminesces only by night, and does not by day. That in itself is fairly "unexciting", as comparable properties are illustrated by numerous other organisms. But when Gonyaulax is moved into a laboratory where it is constantly dim, it continues this "on-off" cycle. Furthermore, if a different "day" and "night" from those indicated by Gonyaulax's cycle are created in a laboratory, it will make a start toward adjusting to them, but when normal (or constantly dim) conditions return, it will immediately fall back to its "old" cycle. Unusual, it is; contradictory to anything in particular, it isn't. The main contradiction in the bioluminescent properties of Gonyaulax is the way it produces light. It was found that the bodies of Gonyaulax's contain tiny crystalline particles, which were christened scintillons. Analysis of the scintillons showed them to be made up of guanine, an organic compound of the purine group, also found in nucleic acids. It was found that these scintillons would emit light in acid surroundings. That a reaction should occur figures; guanine, being a base, should best react with acid. However, an acid-base reaction is not the usual process whereby bioluminescence is created! Rather, it is almost invariably created by the oxidation of a certain compound with the aid of another certain compound, an enzyme.

These two compounds were discovered and named by Dr. Raphaël Dubois, a professor of physiology at the University of Lyons (France) during the late 1800's and early 1900's. He underwent a great search for the elusive substances that make plants and animals shine. He first researched with Pyrophorus, which is also known as a "click beetle" or "cucujo". These unusual beetles, whose large luminescent spots, located on their heads, look rather like a frightening pair of eyes, and whose females even lay luminescent eggs, proved, unfortunately, to be of little help to Dr. Dubois. But in 1896 and 1897, "armed" with a species of clam called Pholas dactylus, he "struck paydirt". Pholas was long known not only as a shining mollusk, but, perhaps better, as a culinary delicacy. Returning to Dr. Dubois, he decided to squeeze some of Pholas's luminescent juice into cold water, and some into a vessel of very hot water. The results were "curious", to say the least. The cold-water mixture luminesced for a while, but the light gradually faded away. The hot-water mixture, on the other hand, never did shine! But the wonders weren't over. After the light in the cold-water mixture had faded, he mixed together the contents (both now non-luminous) of the vessels. The resulting mixture shone brightly! Dr. Dubois figured, then, that bioluminescence was created by the action of an enzyme upon a substrate. How? Well, Dr. Dubois knew that it is a property of enzymes to be destroyed by great heat. When he squeezed the juice into the hot-water mixture, the enzyme was immediately destroyed. But the substrate, also originally present in the juice, remained intact. In the cold-water mixture, both the enzyme and the substrate started out intact, but the enzyme helped oxygen gradually "eat up" the substrate. So, when Dr. Dubois mixed together the contents of the vessels, he was subjecting the unused substrate from the hot-water mixture to the unaffected enzyme (enzymes do not take part in chemical reactions, but act as catalysts to help the reaction take place) from the cold water, thus, in his own way, creating light.

Dr. Dubois felt it his duty to name these new compounds. Inspired by the word "Lucifer" (which means "he who brings light", and is not a justified name for the devil!), he named the enzyme "luciferase" and the substrate "luciferin". These names are used to this day.

Now that I have gone over some of the physical properties of bioluminescence and their discoverers, I should like to mention some bioluminescent organisms, where they live, how they use their ability to luminesce, etc.

I have already mentioned a few: the luminescent fungi, bacteria, the dinoflagellate Gonyaulax polyedra, the click beetle, and the clam Pholas dactylus. This random group represents only some that have been used in various experiments. But here, I attempt to "fit them all together".

There is a saying that the best way to do anything is to start at the bottom. So it would seem natural to take a look at the dwellers of the sea bottom, called benthos in marine ecologists' lingo. Most of the benthos are animals that resemble plants; they are either immobile or nearly so.

One type of the benthos is the sponge. Of about three thousand species of sponges, none are definitely luminescent. There is one (genus Grantia), however, that seems to be. E. Newton Harvey, "the leading authority on living light",³ said he was "inclined to believe the light of this species of sponge is a true luminescence".⁴ He should be. He discovered it!

There are several genera of horn coral (unlike the more familiar stone coral, of which none are known to be luminescent) that include luminescing species that emit a lilac-colored light.

Long, slender sea pens, which are actually colonies of smaller animals, luminesce a pale green.

Jellyfish-like animals called "hydroids" live on the sea bottom, luminescing a pale white.

Huge nemerteans (unsegmented worms), slither along the bottom, emitting a greenish-white hue.

As can be seen, benthos are not particularly interesting; about all they do is "sit there", casting their eerie lights, which probably serve as a "warning signal" to deter foes from attacking. But some very interesting species are found in the next "layer" up, the home of the deep-sea nekton (marine lingo for free swimmers and fliers). One author describes these depths of the marine world as "not always 'dark as pitch', even though sunlight cannot filter through to reach them. They must resemble, rather, the streets of a great city such as London, on a cloudy moonless night when it was blacked out during the German blitz. From time to time passers-by could be seen with masked flashlights, or a car with covered headlights would cautiously make way along the shadowy streets.

"The lights that move dimly in the ocean depths come from the various luminescent creatures that live there."⁵

There seems to be a "lower limit" upon these bioluminescent creatures in the sea, at a depth of about three miles. Some of the creatures living at such depths are the rather devilish-looking Stomias ferox and the fish of the genus Porichthys. They seem to use their luminescence, like the luminescent benthos, as a protecting agent. Some of the Stomias ferox were captured recently and placed into aquarium tanks. To their disappointment, they would not luminesce unless strongly physically stimulated. But such is the life of the biologist.

At a slightly higher level are found various fish that look slightly more "normal." These are primarily of the class Elasmobranchii, and include dogfish, skates, rays, sharks, and other similar fish. The photophores (light-emitting centers) on these fish point downward, probably to keep the fish from casting a shadow against the sky when seen by hungry attackers from below.

Above the Elasmobranchii level are the mollusks (e.g., the octopus and squid), and annelids (segmented worms) of the genus Odontosyllis. The squid is perhaps the most remarkable of these. Not only are many of its species naturally luminescent, but many can eject luminescent chemicals into the water around them. Sometimes, squid luminesce in more than one color. It must be admitted that almost all luminescent octopi are not self-luminescent, but are provided with this service by bacteria.

Many other fish maintain growths of bacteria as well. Some even have special pockets for them, and some have developed even to the point to which they have shades of a sort that can be opened or closed voluntarily by the fish over these "pockets." Remarkable, I believe.

I have mentioned the unusual Gonyaulax polyedra already, but it should be mentioned that there are various other dinoflagellates, and also "corresponding" one-celled animal-like forms called radiolarians, sometimes nicknamed "wheelers".

And, last, but definitely not least, I should mention the fireflies. In the 1950's, these were the subject of rather extensive and intensive study. Fireflies were caught by local people and sold to the McCollum-Pratt Institute in Baltimore for 25¢ per hundred.

The tails were "chopped off" there, and the luciferin and luciferase were analyzed and their processes studied. It was found that adenosinetriphosphate (ATP) supplied much of the energy necessary for the luciferin-luciferase reaction to take place. It was realized that in most luminescing organisms, bioluminescence is only a "side effect" of basic necessary functions such as digestion, respiration, hormone release, etc.

Bioluminescence is not an "old" science; it is just now opening new horizons. It has practical uses — being non-flammable and non-explosive, it could be used, in the form of dried luciferin-luciferase powder, in places where ordinary forms of lighting would be dangerous or unavailable. The studies linked with bioluminescence have brought new insight to almost all other fields of biology, as well as physics, mathematics, and other sciences.

Bioluminescence has proven to be one of the studies most relevant to science.

Footnotes

Bibliography

Klein, H. Arthur. 1965. Bioluminescence. Lippincott Co. Philadelphia / New York
Harvey, E. Newton. 1952. Bioluminescence. Academic Press. New York