Bioluminescent bacteria glow in the ocean for the same reason roadside eateries display neon signs: They want to attract hungry diners.
New laboratory experiments bolster the longstanding theory that marine bacteria light up to get themselves a free ride to other parts of the ocean in the digestive tracts of larger beasts, scientists from Israel and Germany report online December 27 in the Proceedings of the National Academy of Sciences.
“It’s terrific to see this experiment,” says J. Woodland Hastings, a bioluminescence expert at Harvard University who was not involved in the research. “It’s nice to see these ideas confirmed.”
Many deep-sea creatures, from bacteria to fish to squid, are bioluminescent — meaning they generate light inside their bodies through chemical reactions. Different organisms glow for different reasons; the anglerfish, for instance, can light up a lure to attract prey, while some plankton glow when disturbed to attract predators of whatever is stirring them up.
Bioluminescent bacteria live throughout the ocean, and may have several reasons to explain their built-in glow. More than three decades ago, researchers suggested that one such reason could be to mark the presence of a floating food particle, so that a passing fish would see it and eat it. But no one had tracked this idea all the way to its logical conclusion — until now.
Margarita Zarubin, a graduate student at the Interuniversity Institute for Marine Sciences in Eilat, Israel, started with a type of luminescent bacterium, Photobacterium leiognathi, found 600 meters deep in the Red Sea. She put one bag of glowing bacteria at one end of a seawater tank, and at the other end she put another bag of bacteria that had a genetic change that kept the microbes dark. Shrimp and other small animals clustered around only the glowing bacteria.
Next she let brine shrimp swim in water with the luminescent bacteria. After two and a half hours, the shrimp themselves began to glow from their microbial dinner. “We could see the luminescence from inside their guts,” says Zarubin, who did the work while at the University of Oldenburg in Germany and is now with the Hebrew University of Jerusalem.
Then she dropped both glowing and dark shrimp into a flume so they were swept past a hungry cardinalfish; the fish ate only the luminescent shrimp. Finally, the scientists tested the fish feces, and found that the bacteria had passed unscathed through the fish guts and came out intact. The whole process spreads the bacteria through the water faster than they could move otherwise, Zarubin says.
For their part, the shrimp must balance the benefit of eating a food particle that happens to glow against the drawback of becoming luminescent themselves, thus making themselves more vulnerable to predators. But in deep dark waters where food is scarce, the advantage of getting a snack probably outweighs the disadvantage of potentially being eaten, Zarubin says.
Some animals have pigment in their guts that can block light emission as they digest glowing particles, says Michael Latz, a marine biologist at the Scripps Institution of Oceanography in La Jolla, Calif. Only when the animal pops out a glowing fecal pellet do the bacteria become visible again, signaling another creature to eat them and keep the microbes on the move.
Such deep-sea bacterial recycling could be important for more than just understanding bioluminescence, Latz says. The guts of shrimp and other small marine creatures may serve as a highway for spreading bacterial pathogens throughout the sea, like the one that causes cholera.