Biological Light in the Ocean Darkness

by Michael I. Latz
Scripps Institution of Oceanography

The setting sun fortells an incredible transformation 
occurring beneath the ocean surface. As darkness descends, 
the water becomes alive with displays of bioluminescence -- 
living light produced by a myriad of organisms -- that has a 
major impact on virtually all biological communities. If you 
study bioluminescence as I do, your day is just beginning. 

In the ocean, bioluminescent organisms are everywhere, 
inhabiting all depths covering all the world's oceans. 
Remember the spectacular red tide last year? A giant bloom of 
phytoplankton -- plant-like microscopic organisms -- 
discolored the water brown due to their immense numbers, as 
many as 2 million cells per liter. To me the most memorable 
feature occurred not during the day but at night. These cells 
produced bioluminescence that highlighted breaking waves with 
streaks of electric blue light, and traced the paths of 
swimming fish. The red tide phytoplankton use their flashes 
as a burglar alarm so they won't get eaten; in this case, the 
"burglar" is the animal trying to eat them. In doing so, it 
stimulates the cells to make flashes of light, attracting 
still other predators which try to eat the burglar. So if 
you're not careful around these luminescent plankton, you'll 
end up in someone's stomach! It's enough to make you lose 
your appetite, which is exactly its purpose.

Even those animals not interested in eating dinoflagellates 
must be wary of their movements, because the ocean is like a 
luminescent minefield. Any inadvertent motion may set off an 
"explosion" of plankton luminescence which exposes the 
unlucky animal to hungry predators.

In the perpetual darkness of the deep sea, where sunlight 
never reaches, bioluminescence also serves other purposes. 
Angler fish grow luminescent bacteria in a special structure 
which dangles at the end of a stalk projecting from their 
forehead. Just as fisherman use a glowing lure for night 
fishing, in the perpetual darkness of the deep sea these fish 
attract prey by their glowing lures. Still other fish produce 
far-red beams of light from areas on their cheeks. Because 
most deep-sea animals can only see blue colors, the red 
luminescence serves as an invisible searchlight for finding 
prey or mates. Jellyfish so delicate that they disintegrate 
when touched emit brilliant displays of light when disturbed. 
Their message is leave me alone.

Whatever its purpose, bioluminescence is produced as a result 
of a chemical reaction which releases lots of energy. Unlike 
a light bulb, in which electrical energy is converted into 
light, with some energy lost as heat, the bioluminescent 
reaction is 100% efficient in channeling all the energy into 

Bioluminescence serves man as well. The jellyfish biochemical 
system is used to measure calcium levels, while that of the 
firefly measures ATP, the primary energy source of all cells. 
The genes for the bacterial, firefly, and jellyfish 
luminescent proteins can be spliced into the genes of other 
organisms to monitor gene activity. When that gene is turned 
on, the cell glows! Trace amounts of chemicals and pollutants 
are detected using a bioluminescence test.

Interested in seeing bioluminescence in the San Diego area? 
Don't bother looking for fireflies, which fly around on balmy 
summer evenings using their light flashes to attract mates; 
they don't live here. Instead, view the flashlight fish at 
the Scripps Aquarium. These fish harbor luminescent bacteria 
in special organs in their cheeks. They shutter the light to 
make a Morse code of flashing for signaling their friends and 
attracting prey. More adventurous? Kayak in San Diego Bay and 
experience the glowing mating dance of thousands of swimming 
worms when the moon is right. Find a dark beach and check out 
the plankton bioluminescence stimulated by the breaking 
waves. Or go for that midnight swim and watch the sparkles of 
living light as you move through the water. And remember that 
bioluminescence is a natural part of how organisms interact 
with their environment and each other. 

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