Luciferin in Scintillons at Sea

by Janie
 

Figure 1. Bioluminescent tide at Del Mar North Beach, CA. CC BY-NC-SA 2.0 slwo­r­king2

Summer through au­tumn is the time of year for ma­rine di­no­­fla­gel­lates' famed light fes­ti­vals. If the con­di­tions are just right, the roll of an ocean wave or the push of a ka­yak paddle or the sway of your legs as you wade in deep­er is enough to set the wa­ters alight with lu­min­ous blue.
 
 

Now it's winter, past bioluminescence season, but with the early nightfall and all of the glittering holiday lights illumi­nating the suburban dark, I'm reminded of nighttime ocean bioluminescence all over again. A question reoccurred the other night while returning home: Why is it that running your hand through the seawater causes its microbial occupants to glow?
 
 

Figure 2. Lingulodinium polyedrum (left) and Noctiluca scintillans (right). Source and source

Turns out the responsible organelle is one I had never heard of. Mechanical disturbances wiggle the cell mem­brane of dinoflagellates like Lingulodinium polyedrum and Noctiluca scintillans (the latter also colloquially known as the sea sparkle). This shear stress triggers an action poten­tial. There is first a depolarization of the dinoflagellates' vacuole membrane, which in turn triggers the opening of channels that allow protons to cascade from the acidic vacuole into adjacent spherical organelles called scintillons. These organelles are unique to dinoflagellates. Within these electron-dense cytoplasmic drops loosely enveloped by a vacuole-derived membrane, that lowering of the pH simultaneously switches the conformation of luciferase into an active form and re­leases the luciferin substrate from an inhibitory protein. Now luciferase can oxidize luciferin, and this process shoots out photons responsible for the bright blue flashes.
 

Figure 3. A scintillon (Sc), heavily labeled, hanging as a drop in the vacuolar space (enlarged in inset, with the narrow neck indicated by an arrow). Source

Why the molecular acrobatics for the sake of light-produc­tion? It's speculated that this bioluminescence is a defense mechanism for the dinoflagellates, perhaps intended to startle off intruding predators, or to lure even larger fish that will pick off the intruders.
 

Pacifica Pier and Elkhorn Slough are famed hotspots for bioluminescence-viewing here in the San Francisco Bay Area (California, USA). We will have to wait a few months for the next season of glowing waves – but appreciating "fairy lights" isn't li­mit­ed to a few months out of the year. From the ocean's dino­flagellates to winter's holiday lights, from muggy summer fireflies to clear-sky nights strewn with stars, there's an abstract, pleasurable wistfulness in watch­ing glittering lights. Whether it's those as far-traveled as starlight filtered through years, or even those as quick as a wink as biofluorescence's jumping photons, it is afterglow of a near or far past illuminating the present. All light is history (and really beautiful history at that).
 

Figure 4. Stargazing at Anza Borrego De­sert State Park. Photo credit: Lilah Blalock