“Come on back here! I’m gonna getcha! I’m gonna swim with you…I’m gonna be your best friend!”
Little did Marlin and Dory know that the light — and the hungry anglerfish attached to it — already had millions of best friends without whom it couldn’t glow. Thanks to bioluminescent bacteria like Vibrio fischeri and Photobacterium phosphoreum, many species have found a way to shine in the most unlikely of habitats.
“I SEE A LIGHT”
Bioluminescence is the production and emission of light by a living thing. This light has a wide variety of uses, from confusing prey, to startling predators, to attracting a mate. Bioluminescent organisms can be found on most of the branches of the tree of life, from bacteria all the way up to fish.
Bioluminescence probably originated in the ocean, and is more commonly found in marine species than in ones on land. Bioluminescent marine species live across all of the ocean depths; the greatest number of bioluminescent species can be found in the ocean’s dimly lit twilight zone, but of the organisms that have adapted to life in the ocean’s deep, dark midnight zone, about 90% are bioluminescent!
Unlike the light generated by an incandescent light bulb, bioluminescent light comes not from heat, but from energy released by a chemical reaction in the organism. All bioluminescent reactions involve a “luciferin” compound and a “luciferase” protein, both from the Latin word “lucis,” meaning “of light.” When a luciferin and a luciferase come together with oxygen and ATP (a cell’s fuel source) the reaction releases energy that we see as colored light.
Luciferins and luciferases come in many different flavors — over a dozen chemical luminescence systems are known — suggesting that bioluminescence has evolved many different times under different conditions. The luciferins and luciferases usually seen in marine bioluminescent species react to produce green or blue light, as these wavelengths of light travel well through seawater without getting absorbed or scattered.
“FISH ARE FRIENDS, NOT FOOD”
Many bioluminescent species are able to produce light on their own, but others, like our toothy anglerfish friend, have had to get more creative. A large number of bioluminescent species are not bioluminescent in their own right, but have evolved a symbiotic relationship with bioluminescent bacteria.
A symbiotic relationship is one in which two living organisms live in or on each other in a close, physical way (from the Greek sym, “together,” and bio, “life”). Such relationships aren’t always good ones; the luminous bacteria that live symbiotically with the Tanner crab, for example, are parasitic and damage the crab’s legs. In the case of the deep-sea anglerfish and its photobacteria, however, the relationship is a mutualistic one, meaning both the fish and its glowy roommates benefit from their partnership.
Luminous bacteria living in the seawater, like Vibrio fischeri or Photobacterium phosphorum, float through pores in the bulbous sac, or esca, on the end of an anglerfish’s lure and colonize it. This lure is a completely mobile appendage whose luminescence helps female anglerfish lure charming, anthropomorphized prey to their mouths and attract their unfortunate potential mates.
“I WISH I COULD SPEAK WHALE”
While photobacteria can glow on their own, they just…don’t. Before they’ll start to glow, photobacteria like V. fischeri need a little help from their friends.
Bacteria are surprisingly social organisms; instead of communicating using words or howls, bacteria send signals to each other by releasing chemicals into their environment. The more bacteria there are in an area, the more signaling molecules will be floating around them. Many bacteria regulate the production of some proteins — like those that make them glow — in response to changes in levels of these signaling molecules in their surrounding environment.
Photobacteria use this kind of communication, known as quorum sensing, as a sort of light switch. Once the number of other photobacteria reaches a certain level, the bacteria detect the resulting elevated levels of signaling molecules around them. In response, the genes that encode the luciferin and luciferase proteins get turned on and the bacteria begin to glow. If the numbers of bacteria drop, however, those genes get switched off again and the glowing stops.
Quorum sensing was first observed in V. fischeri — the glowing made it easy to see the correlation between bacterial density and regulation of the genes responsible for light production. Eventually, it became clear that the process wasn’t specific to V. fischeri and its bioluminescent abilities, but was common to the regulation of many different types of proteins across the bacterial family tree.
“WE WERE LIKE, ‘WOAAAAH!'”
Whether glowing bacteria or luminescent fish, the ocean’s bioluminescent organisms are stunning, sometimes literally. The next time you’re on a deep sea adventure and see a pretty light, perhaps don’t stand in awe too long—you might just go from ardent admirer to snack in a flash.