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A glowing sea

Q: This past Sunday [Sep. 4, 2005], my boyfriend and I were at the end of a dock on Mission Bay in San Diego. When we stepped hard on the dock, it made the fish jump. They looked like little glow sticks underwater! I then stuck my foot in the water and moved it about. The water glowed green in the direction of the movement. My boyfriend jumped in, moved his arms and legs, and he looked like a glowing angel. It was the most amazing thing I have seen. What caused this and why? We asked the hotel but they said they had never seen anything like it. (Sarah, Birmingham, Michigan)

A: Little microscopic creatures (called Lingulodinium polyedrum) that glow in the dark caused the alluring strange display that night.

Lingulodinium polyedrum, a well armored organism. Scanning electron microscope image. [Courtesy of J.Woodland Hastings ©, used with permission]

The phenomenon you saw happened something like this. During this past winter, the creatures — single-celled algae that convert light to chemical energy through photosynthesis, much like plants — lived in modest numbers about 30 feet (10 m) below the surface. They rose to the surface daily, captured the Sun’s energy, and sunk at night to the 30-foot depth. But then, perhaps in February, they sensed conditions were ripe for growth. We do not know what triggers the change. We cannot predict when it will happen. We do not understand what motivates these creatures.

Slowly, over the next few months, the population grew. Most individuals split to create two where one formerly existed. A few mated by two cells fusing together to form a fertilized ovum and then later dividing to reproduce. The population surged ("bloomed" as the biologists say) reaching, by June, up to hundreds or thousands of L. polyedrum cells in each milliliter of seawater. The population increased by a factor of about 250.

As the sea warmed with warm summer days, the organisms ascended to form a dense layer near the surface. Millions of cells swam in the dock waters by the time you arrived in September. A single teaspoon contained thousands.

Let’s zoom in on one of these golden-brown creatures — one that is approximately the size of a ragweed pollen (50 microns) and covered with a sheath of cellulose plates. It looks like a porous armor-plated soccer ball and is a species of dinoflagellates. See figure above.

That September night, this fellow — flailing his two flagella (whip-like appendages) in a counterclockwise direction — swam near the dock.

It knew it was night and controls its luminescence through a 24-hour day-night rhythm. The one-celled creature tells time and was ready to flash.

Your foot plopped in beside it (and a few million of its close friends), roiling the waters. The tiny L. polyedrum glowed a blue light as they always do when disturbed. See figure. That’s the light you saw. They flash when a predator touches them. Moreover, they emit blue light — a color that can be seen farther than most other colors underwater.

The bluish halo around the organisms in this fantastic image is similar to the glow they create through bioluminescence. Hastings, however, generated the imaged glow by shining a light on the creatures (luminescence). [Courtesy of J.Woodland Hastings ©, used with permission]

Why? Presumably as a burglar alarm to either warn away predators (for example, shrimp) or to attract visual predators (for instance, fish) to eat the shrimp that are eating them, says Peter JS Franks, professor of biological oceanography at Scripps Institution of Oceanography.

"Bioluminescence may be thought of as a bag of tricks..." says biologist J. Woodland Hastings, a Paul C. Mangelsdorf , Professor of Natural Sciences at Harvard University. Different creatures use it in different ways for different functions:

The L. polyedrum species create a cold chemical light in a manner similar to how fireflies generate light but with notable differences.

"The phenomenon is not only rare: in the different groups that do emit light, the biochemical and physiological mechanisms responsible for it are very different...," says Hastings.

Most bioluminescent creatures make light by combining oxygen with two important chemicals:

• Luciferin, a water-soluble pigment, which is present in the cells of all organisms that make light. In dinoflagellates, it’s related to chlorophyll.
• Luciferase, which starts the chemicals combining and increases the speed with which they combine; it's an enzyme that acts as a catalyst.

The luciferin and luciferase chemicals, however, are chemically different among the various creatures, since the different organisms evolved their ability to create light separately and at different times.

Initially, luciferin reacts with specie-dependent chemicals and oxygen to form intermediate luciferase-bound peroxides that break down to give a product in an excited state — an extremely high-energy, unstable chemical. The new chemical does not last long but falls to a lower-energy state, and emits light.

Bioluminescence is a "curiosity of nature", involving genes and proteins that are mostly unrelated. "How many times this [bioluminescence] may have occurred is difficult to say, but it has been estimated that present-day luminous organisms come from as many as 30 different evolutionarily distinct organisms," says Hastings.

Only some dinoflagellates (like our lambent friend, L. polyedrum), bacteria, mushrooms, cnidaria (jellyfish), annelids (earthworms), mollusks, crustacea (shrimp), insects, and fish can luminesce. Bioluminescence occurs mainly among marine life. We don’t know why. Moreover, at midocean depths (200 to 1200 m) over 95% of individual fish, shrimp, and squid are bioluminescent.

Mammals, birds, reptiles, amphibians, and true plants lack the ability.

Further Reading:

Scripps Institution of Oceanography: Patterns in dense phytoplankton blooms by Peter Franks

Hastings Lab: Summary of Research by J. Woodland Hastings

Hastings, J.W. (1998) Bioluminescence. (N. Sperelakis, ed.), In: Cell Physiology, 2nd Edition, Academic Press, NY., pp.984-1000.

Hastings, J. W., Liu, L., and Schultz, W. (2005) Dinoflagellate Bioluminescence and its Circadian Regulation In, Bioluminescence (John Lee, Editor), The Digital Photobiology Compendium (Dennis P. Valenzeno, Editor), click "Preview Modules" then "Bioluminescence: Ocean"

University of Liverpool: Lingulodinium polyedrum (Stein) Dodge, 1989

Wikipedia: Dinoflagellate

Center for biological timing

University of Calgary: Dinoflagellates by Andrew MacRae

The bioluminescence web page

New Scientist: Sea’s eerie glow seen from space

(Answered Nov. 22, 2005)