Pyrocystis is a genus of dinoflagellate that possesses the remarkable ability to make its own light. Its name derives from the Latin pyro, meaning fire, and cystis, meaning a hollow sac or cavity. Just like a firefly, P. fusiformis is bioluminescent, although for very different reasons. In fact, these dinoflagellates glow using the same mechanism as a firefly despite their evolutionary distance. Both utilize a pigment called luciferin and an enzyme called luciferase 1. When the luciferase enzyme oxidizes the pigment, the energy from the reaction is released in a tiny flash of light2. When P. fusiformis bloom in nature, they’re bright enough to bedazzle the entire surf break with pinpricks of eerie blue-green light. In fact, tourist flock to bioluminescent dinoflagellate hotspots, like Mosquito Bay, Puerto Rico, where you can swim and kayak in the luminous glow.Pyrocystis fusiformis only glows when disturbed, though, whether that be by a breaking wave, the swish of a swimmer’s hand, or the advance of a predator. In fact, its hypothesized that the reason dinoflagellates evolved the ability to glow at all is to avoid being eaten (much to the contrary of fireflies, which flash in intricate patterns to attract a mate). In the dark ocean, it is advantageous not to be seen. When a zooplankton predator takes a mouthful of glowing dinos, it is likely to spit them right back out rather than risk glowing itself. Otherwise, it might be seen by a bigger (more opaque) predator and become fish food. This is called the “burglar alarm” hypothesis and was caught on video in a study dating back to 19923.
This glowing defense mechanism is particularly important for P. fusiformis because they lack flagella and are non-motile, meaning they’re unable to swim away. They can, however, move up and down in the water column by adjusting their buoyancy. Given their druthers, P. fusiformis prefer to float along at about 80-100m deep, where they grow the fastest 4. In culture, P. fusiformis reproduces asexually about every 5-7 days5.
Some dinoflagellates are mixotrophs, meaning they’re able to prey on other plankton (including fellow dinoflagellates!) in addition to harnessing light from the sun. P. fusiformis, though, is strictly a friendly autotroph. The same circadian rhythm that turns on its photosynthesis during the day switches on its bioluminescence abilities at night.
Whether they’re lighting up a Puerto Rican bay or an Erlenmeyer flask, P. fusiformis are truly a sight to behold. They are just one more example of the creativity of nature and the lengths that organisms have gone for survival in this 3.7 billion-year game of cat and mouse.