Glowing Algae is a bioluminescent organism that appears in the How to Train Your Dragon Franchise. To avoid confusion, it is classified as "Flora" (or plant) here.
Glowing Algae is seen in multiple Dreamworks Dragons episodes in the Franchise and is a fictitious organism based in real life.
In the Franchise, glowing algae is able to survive in both freshwater and saltwater as it typically flows through rivers and occasionally or eventually flows out to sea. It glows bright blue and appears as droplets of glowing blue blobs when separated from the water. It also appears to be sticky, as it was able to stick to the side of a ship as well as sticking to Ruffnut's and Tuffnut's faces for a while.
When Glowing algae is first seen in the Franchise in "Fright of Passage", Hiccup hypothesizes that its glow was a reaction to "Aurvandil's Fire" (the Northern Lights) which appears every ten years on Berk, along with the Flightmare. However, the algae seems to glow under its own power in the episode "Maces and Talons, Part 1".
It is possible that the Glowing Algae is somewhat based on bioluminescent micro-organisms called dinoflagellates, specifically Noctiluca scintillans more commonly known as "Sea Sparkle". The Sea Sparkle's bluish glow is a result of a chemical reaction when they are "disturbed", so it is not continuously glowing as the glowing algae appears to be. Sea Sparkle can be found mostly along coastlines around the world, so it is possible historic Vikings did come across this organism. There are many other Genera and species of bioluminescent "algae" or dinoflagellates, but Sea Sparkle is the most notable.
In the Franchise, glowing algae is most noted as a food source for the Flightmare, and appears to contribute to this dragon's bluish glow. It has also been demonstrated that if dragons other than Flightmares eat glowing algae, that the glowing effect is also imparted to them, but in colors roughly corresponding with their hide color.
In the episode, "Fright of Passage", Astrid Hofferson awaits the arrival of the Flightmare, which tarnished her family's name by causing Fearless Finn Hofferson to freeze with fear. Later, Astrid and Hiccup discover that the Flightmare can breathe a paralyzing mist, and that this dragon is appearing on Berk because it follows its food source: glowing algae.
Glowing algae is seen again in the episode, "Maces and Talons, Part 1" when Viggo Grimborn and the Dragon Riders clash at an Unnamed Island. Viggo is attempting to capture a Flightmare, while the Dragon Riders attempt to stop it. The Flightmare is attracted to this island because it has glowing algae.
- The Glowing Algae seems to have a strong effect on the Flightmare's shot limit. According to Dragonpedia, the Flightmare has a shot limit of 4, which is significantly low and inaccurate, when the Flightmare produced a long trail of its toxic mist to surround the Dragon Riders in "Maces and Talons, Part 1". It is impossible for the Flightmare's original shot limit to be 4 for it to produce such a long trail of its mist without any signs of regeneration. The only explainable reason is that the Glowing Algae has another side effect that increases the shot limit of the Flightmare, or even the toxicity in the mist, since they are both the same color and is somewhat "sticky" as the Flightmare's mist can create a "shiny" aspect on its paralyzed victim.