Bioluminescent plankton

Those that are lucky enough to have seen a bioluminescent beach say it is a fantastic spectacle, like few on Earth. The sparks of blue or green light that accompany the water in the winding breaking of the waves captivate young and old alike. I, unfortunately, have never seen it, although I have seen occasional sparks in the night water, and I have experienced the fun of stirring some bioluminescent protozoan culture. Just for the record: do not do that; they die after stirring them if you do it too often or too energetically.

Despite the low presence of bioluminescence on the coast – it only occurs on some beaches and from time to time – it is a well-studied and relatively widespread phenomenon among sea creatures. Even the U.S. military has been investigating its usefulness in detecting enemy submarines. What matters to us here, however, is not the military applications of this curiosity of nature but its origin and causes.

Which organisms emit light?

There are many types of bioluminescent organisms (i.e., they emit light by biological mechanisms), both marine and terrestrial: bacteria, protists, jellyfish and ctenophores, squid, worms, mollusks, crustaceans, echinoderms, fish, etc. However, what interests us are those belonging to plankton. That is, mostly unicellular protozoa and algae (especially dinoflagellates), some jellyfish and ctenophores, and some crustaceans, mainly from the copepod group.

The dinoflagellate Noctiluca scintillans is responsible for many of the bioluminescent beach shows worldwide. Photo by Albert Calbet.

Why do they do it?

Concerning plankton, the main reason for bioluminescence is to prevent predation. Imagine that you are a copepod, and suddenly you notice the suction of a fish’s mouth. Although you may have time to escape, and you are skilled enough to propel yourself at high speed, if you can also confuse your predator by releasing a flash of light so that it cannot follow you, the success of your escape will be significantly improved. Other organisms in terrestrial groups, such as insects, use luminescence to attract mates. There are also abyssal fish that emit light so that curious prey approach them like fools that go to the light and end up in their mouths. In this case, the light comes from symbiotic bacteria grown in different fish cavities. 

Marine bioluminescent bacteria are a curious case. Many groups of bacteria are symbionts of higher organisms, such as cephalopods or fish, where they perform functions of attracting prey (as we have mentioned) or repelling predators, or even to communicate. However, there are also free-living bioluminescent bacteria. Until recently, it was believed that bioluminescence was linked to the density of these organisms and was a response to high population densities. It has recently been shown that bioluminescence can also occur in individual bacteria. In these cases, it is supposed that emitting light is a mechanism for attracting predators, especially in places where nutrients are scarce. When ingested, the bacteria, which are also resistant to digestion, find a richer environment inside their host and increase their chances of survival.

How do they do it?

Bioluminescence occurs when the enzyme luciferase reacts with oxygen and a protein called luciferin. The process is as follows: Oxygen oxidizes luciferin with the help of luciferase and, using the energy stored in the ATP, they produce energy in the form of light and water as a by-product. Sounds complex, doesn’t it? But even a bacterium can do it!

 
The copepod Pleuromamma abdominalis emits flashes of light when it feels in danger. Drawing from Miquel Alcaraz

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