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On most beaches, the only light you’ll see in the water at night is the reflection of stars or streetlights. But in a few special places around the world, the ocean lights up at night with countless tiny blue flashes.

These bioluminescent bays look pretty magical, but it really all comes down to science. Specifically, this stunning natural phenomenon is caused by tiny creatures called dinoflagellates and some cool chemistry inside them. Dinoflagellates are single-celled organisms called protists, which are a ragtag bunch of eukaryotes.

Their DNA is neatly organized in a nucleus, unlike bacteria. But protists don’t fit nicely into other eukaryotic groups like plants, animals, and fungi. Many species of dinoflagellates are microscopic, although some can grow up to 2 millimeters in diameter, like Noctiluca scintillans, also known as the sea sparkle.

And that’s pretty big for a single cell! Dinoflagellates live in marine and freshwater environments all over the world. Some are photosynthetic and need sunlight for energy.

And others are heterotrophic, which means they eat other organisms. Of the hundreds of species of dinoflagellates, relatively few are bioluminescent. About 70 light-producing species have been discovered so far.

It takes energy to do chemical reactions that emit light, so there needs to be some sort of survival benefit that makes it worth it. And if you’re a single-celled organism, you’re at the bottom of the food chain. It’s hard work to avoid getting eaten.

So scientists think that bioluminescence is actually a defense mechanism to help them escape predators like copepods, which are small shrimp-like creatures. When the dinoflagellate senses something moving in the water, whether it’s a copepod, a human hand, or even the force generated by waves on the beach, it lets off a flash. The tiny flash hopefully startles any nearby predators, letting the dinoflagellate get away.

Or the glow might be like a neon diner sign and attract larger predators that ignore dinoflagellates, but would consider copepods just the right size for a snack. Scientists still don’t know all the biochemical details about how dinoflagellates glow, but they’re puzzling it out. They think that certain kinds of motion stretches the organism’s cell membrane and activates mechanical receptors.

Those receptors send a signal to scintillons, which are basically tiny pods inside bioluminescent dinoflagellate cells that hold all of the compounds needed to produce light. That signal seems to make each scintillon open ion channels, to let in hydrogen ions from an adjoining sack called the acidic vacuole. This lowers the pH and sets a chemical reaction in motion, in which an enzyme called luciferase combines oxygen with a compound called luciferin.

And when this reaction happens, it releases energy in the form of a flash of light. Although different forms of luciferin can produce different colors of light, dinoflagellates and many other bioluminescent marine species emit blue light. That’s probably because blue light can travel farthest in water, since water more quickly absorbs the other colors in the spectrum.

Now, bioluminescent dinoflagellates can be beautiful, but some of them can also be deadly. Species like Pyrodinium bahamense have a dangerous neurotoxin called saxitoxin. In humans, saxitoxin binds to sodium channels in neurons to mess them up.

This can cause symptoms like numbness, drowsiness, paralysis, and even death in extreme cases. Saxitoxin is harmful to marine animals as well as humans, and it can build up in fish and shellfish that eat dinoflagellates. And since it’s not easy to tell when a clam is sick, people can unknowingly eat toxin-filled seafood.

But many other bioluminescent dinoflagellates aren’t toxic at all, like the sea sparkle. So you shouldn’t necessarily be afraid of bioluminescent bays or anything. There are just a handful of bays in the world that consistently glow, located in tropical areas like Jamaica, Vietnam, and Puerto Rico.

They’re delicate ecosystems because the dinoflagellates need specific conditions to thrive, like enough warmth, salinity, and nutrients. The surrounding landscape can be important too. Pesticides and heavy metals from land can kill dinoflagellates, while decaying organic matter can be an important source of nutrients.

Natural occurrences like storms or changes in wind and water patterns can blow the dinoflagellates out of bays, leading to a drop in bioluminescence. For instance, Laguna Grande in Puerto Rico still hasn’t fully recovered from the effects of last year’s Hurricane Maria and it's still quite dim. Other areas have occasional outbreaks of bioluminescence, including Australia, the US, the UK, and the Maldives.

Scientists still don’t fully understand why, but they think that certain combinations of nutrients, salinity, and warm weather may cause these critters to overgrow. But a bunch of dinoflagellates can also cause red tides, where there are so many of these tiny creatures that the water becomes red or brown. Some, but not all, red tides are bioluminescent.

And some can be incredibly toxic to the local wildlife. So bioluminescent dinoflagellates can be an essential food source for marine ecosystems, or produce deadly neurotoxins that harm fish and people alike. But, regardless, if you get to see a spectacularly glowing bay, you should count yourself lucky.

Thanks for watching this episode of SciShow, where we feel lucky to be supported by so many wonderful patrons on Patreon. If you want to help us create free educational videos 7 days a week, you can go to patreon.com/scishow. [ ♪ Outro ].