Biological Light in the Ocean Darkness

By Michael I. Latz

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.