The Bioluminescent Bay

To understand the experience, imagine millions of microscopic aquatic fireflies all glowing at once and swimming in dense formations.

On a moonless night in Vieques, take a ride into one of the most magical places in the world. Look behind the boat and watch the shimmering glow of the wake. When it stops and the guide stomps a foot on the deck, hundreds of luminescent streaks appear in the paths of scattering fish. Imagine taking a dip and finding yourself enveloped in a blue-green glow, then lifting your hand from the water and watching the glowing water drip back into the phosphorescent pool beneath.

Waving your arms in the water makes angel wings appear; the stronger the movement, the brighter the glow. Enter the world that time forgot, the last of its kind to still keep such a high level of luminescence. The phenomenon occurs elsewhere in the world, but nowhere is it nearly as bright as in one of the bioluminescent bays of Vieques, known as Caño Hondo or Puerto Mosquito.

The light is produced by trillions of microorganisms that become luminescent when disturbed. The concentration here is an astounding 720,000 per gallon. Vieques is one of the few places in the world where pollution has not decimated the population; it is fortunate indeed that the island has escaped mainstream tourism.

What makes the bay glow

The glow comes from a physico-chemical reaction that begins when the single-celled dinoflagellate is disturbed. This defense mechanism starts a chemical chain reaction that produces a blue-green glow lasting as long as the organism is disturbed. It is nothing like a glow-in-the-dark toy that has to be held up to a light to charge; the reaction happens because a very specific set of nutrients is available in the water to sustain these little water fireflies.

There is more than one bioluminescent bay on Vieques. The brightest is Mosquito Bay, and the population figures above refer to it. Several things had to come together in one spot for bioluminescence to evolve in Puerto Mosquito and elsewhere:

• Mangroves, or their ancestors, had to surround the bay and provide a habitat for a specific bacterium that produces vitamin B12 in large quantities as a metabolic by-product.
• The mouth of the bay had to be narrow, so its waters would not be washed out to sea, diluting the dinoflagellate population.
• The location had to be remote, and so without pollution.
• The temperature had to stay warm and constant, within a very narrow range.
• The water had to stay calm enough for most of the saltier water to sink to the bottom and be carried back to the ocean by mild undercurrents, since the organisms cannot thrive in saltier ocean water.
• The population of the organism’s natural predators had to remain low.

A more scientific explanation

Bioluminescence is the direct conversion of chemical energy into light energy. In the bay’s dinoflagellates, the luminosity is thought to be a defense mechanism.

The reaction begins with the creation of a packet, or quantum, of light energy (a photon) through a highly specialized physico-chemical process: a recombination of radicals (oxide, hydroperoxide) involving a specific molecule that undergoes a chemical change when acted on by an enzyme (the substrate D-luciferin), ATP (adenosine triphosphate, the energy-storing molecule of all living organisms) and oxygen. The reaction is controlled by luciferase, an enzyme that accelerates and controls the rate of chemical reactions in cells. The chemical energy excites a specific molecule, resulting in the photon that produces the light.

The light emitted is independent of any light the organism absorbs, unlike a glow-in-the-dark object, which has to be charged by direct contact with light. The whole reaction takes less than 1/1000th of a second, but the light lasts as long as the organism stays excited, provided it can absorb enough nutrients from the water to sustain it.

Photons are discrete energy packets, and their energy is constant; the number released, and the intensity with which we perceive them, may vary. Different substrates (luciferins) produce different colors. Dinoflagellates emit light ranging from blue to yellow-green, at a wavelength of about 490 nm. For those with a degree or two in chemistry, here is the mind-numbingly concise version:

FMNH₂ + RCHO + O₂ → FMN + RCOOH + H₂O + light (490 nm)

Most sincere thanks to Barbara Baker, Ph.D., and Jeffrey Baker, Ph.D., without whose help in correcting, clarifying, revising and proofing, this explanation would have been impossible.