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Light of the Sea

by Moselio Schaechter, 1/4/07

Waves on the Beach by Allen Conant, Courtesy of

On a 1843 voyage, 29 days out from Philadelphia on the brig Childe Harold, Henry Morse Olmsted noted in his log:

"Those who go down to the sea in ships and do business upon the great waters, see the wonderful works of the Lord. And truly we saw last night one of his wonderful works. It was a grand view of the phosphorescence of the Ocean.. The vessel was going thro' the water at abt 4 knots and as she broke thru' the sea, one blaze of silver light lit up her forward sails so much that one could almost see to read. The wake was a straight path of light in the midst of the dark waves. As far as the eye could reach, around for miles, every crest of a ware gave forth that same, clear, lovely light."

This experience is familiar to people who go for a night walk on the beach at certain times of the year. As the waves crash on the shore, they suddenly light up with great intensity, emitting a grenish-bluish light that dissipates as suddenly as it arises. A wave later, the phenomenon repeats itself. Any other disturbance of the sea surface, including passing ships and swimming fish leads to the appearance of such light.

Angler Fish, courtesy of NOAAIt turns out that the seas contain an abundance of living forms that are bioluminescent, that is, capable of emitting light. Just like fireflies on land, certain fish, squid, and jellyfish are bioluminescent. In the depth of the sea, light is used as a signaling device, sometimes as a lure to possible prey. The "angler fish" has a light emitting structure dangling from the end of a stalk on its forehead, reminding us of the lights used by fishermen for night fishing. Other sea creatures, such as hatchetfish, use light to make themselves hard to distinguish by predators: they emit just enough light to balance that of moonlight, thus they cast no shadow that on moonlit nights could be seen from below by eager foragers.

Dinoflagellates/Red Tide, courtesy of NASAThe bioluminescence seen in the crashing surf is due to microscopic algae called dinoflagellates, the same organisms that are responsible for the poisonous red tides. There can be as many as several million cells of these organisms in one teaspoonful, which explain why the light can be so intense. Here light is probably used as an alarm to ward off predators. The dinoflagellates advertise their presence, thus announcing that they be best avoided. But why is the light brightest when the water surface is disturbed? The reason is that bioluminescence requires oxygen, a chemical abundant in air but relatively poorly soluble in water. Thus, the crashing of waves results in the mixing of air with water, which temporarily increases the oxygen concentration.

Much of the bioluminescence at sea is a microbial phenomenon. Dinoflagellates are microbes, and so are bioluminescent bacteria that inhabit certain organs of fish and squid. These animals have specialized organs that carry large numbers of these bacteria. In these organs, the bacteria are packed so tightly that it's wall-to-wall bacteria. In addition, free-living bioluminescent bacteria can also be present, sometimes in such enormous numbers that they literally light up huge areas of the ocean, as big as the state of Connecticut. These "milky seas" have been described by countless mariners and, with modern tools, can be detected from space.Milky sea satellite imagery, Courtesy of Steven Miller, Naval Research Laboratory

Scientists know a great deal about the chemistry of light emission by living things. All bioluminescent organisms have an enzyme called luciferase that acts with oxygen to change a compound known as luciferin (who says scientists don't have a sense of humor), a process that results in light emission. There is no heat loss, so this process is far more efficient than the light made in a light bulb.

Microbes, small as they are, are responsible for stunning natural phenomena, sometimes on a grand scale. In providing nutrients, such as utilizable nitrogen, the activities of microbial are essential for life on Earth to proceed. Most of these activities are not seen with the naked eye, but some are readily visible as well as dazzling.

For more intersting topics by Moselio Schaechter visit his blog, Small Things Considered.