In-depth guide
The science of bioluminescence
When you paddle Laguna Grande on a dark night and the water lights up around your kayak, you are watching a chemical reaction inside millions of single-celled organisms. Here is what is actually happening, why it glows blue-green, and why a lagoon on Fajardo's coast is one of the few places on Earth where you can reliably see it.
Key facts
- Organism
- Pyrodinium bahamense
- Type
- Dinoflagellate plankton
- Light
- Blue-green flash
- Trigger
- Movement in the water
- Best viewing
- Dark, new-moon nights
- Where
- Laguna Grande lagoon
What makes the water glow
The glow comes from Pyrodinium bahamense, a single-celled organism called a dinoflagellate. These are microscopic plankton, and Laguna Grande holds them in enormous numbers. Each one is capable of producing a quick flash of blue-green light when the water immediately around it is disturbed. On their own they are invisible, but when millions react at once to a passing kayak or a trailing hand, the water itself appears to light up.
The chemistry of cold light
The light is the product of a chemical reaction. A molecule called luciferin reacts with oxygen, sped along by an enzyme called luciferase, and the energy that reaction releases comes out almost entirely as light rather than heat. That is why bioluminescence is often described as cold light: unlike a flame or a bulb, it does not warm anything around it. The same basic luciferin and luciferase pairing, in different forms, powers glow in fireflies, deep-sea fish, and many marine organisms.
Why they flash
Why would a tiny organism spend energy making light? The leading explanation is defense. A sudden flash can startle a small predator, and it can also act like a burglar alarm, drawing the attention of something larger that might eat the predator. Either way the flash is a brief, involuntary response to being jostled, not a deliberate display, which is exactly why your own movement through the water sets it off.
Why it is a lagoon, not a bay
People call it the bio bay, but Laguna Grande is technically a lagoon, connected to the open sea only by a long, narrow channel. That distinction matters to the science. The enclosed, calm, mangrove-lined water lets the dinoflagellates concentrate to high densities and stay put, which is part of why the glow here is so dependable. It is one of only a handful of consistently bioluminescent bodies of water in Puerto Rico, alongside Mosquito Bay on Vieques, often considered the brightest, and La Parguera on the south coast.
Seeing it at its best, and keeping it that way
The glow is faint compared with artificial light, so the darker the night, the better it shows. Plan around a new moon and avoid the nights near a full moon, when moonlight washes it out. The lagoon sits in a protected reserve and swimming is not permitted, which protects both the ecosystem and the spectacle, since bioluminescence is sensitive to light pollution and changes in water quality. To plan the visit itself, see the Laguna Grande bioluminescent bay guide and the best time to visit. For more on the town and coast that surround it, read our history of Fajardo and the lighthouse. The brightest of the island's glowing waters, Mosquito Bay, sits off our Vieques day trip.
Bioluminescence FAQ
Single-celled plankton called Pyrodinium bahamense, a type of dinoflagellate. Millions of them live in the lagoon, and each one gives off a brief blue-green flash when the water around it is disturbed, so a paddle stroke or a moving body lights up the water.
It is a chemical reaction. A light-producing molecule called luciferin reacts with oxygen, helped along by an enzyme called luciferase, and the energy is released almost entirely as light rather than heat. That is why it is often called cold light.
The flash is widely thought to be defensive: a sudden burst of light can startle a predator or draw attention to it, an effect sometimes described as a burglar alarm. It is a brief, involuntary response to movement in the water, not something the organism does on purpose.
Despite the bio bay nickname, Laguna Grande is technically a lagoon connected to the sea by a long, narrow channel. That enclosed, calm, mangrove-fringed water is part of why the organisms concentrate so densely and the glow is so strong.
Laguna Grande sits in a protected reserve and swimming is not allowed. The phenomenon is sensitive to light pollution, water quality, and disturbance, so the protections help keep one of only a few consistently bioluminescent waters in Puerto Rico healthy and bright.
We see it as a blue-green flash. Each Pyrodinium bahamense cell releases a brief burst of light when jostled, and when millions react together to a paddle or a hand, the dark water appears to shimmer with that cool blue-green color.
Yes. Laguna Grande is one of only a handful of consistently bioluminescent waters in Puerto Rico, alongside Mosquito Bay on Vieques, often considered the brightest, and La Parguera on the south coast. All three rely on the same dinoflagellates.