Being jolted awake every morning by an alarm clock is plenty annoying, but at least that alarm doesn’t actually live in your body.
The Hawaiian bobtail squid (Euprymna scolopes) has an internal alarm clock that’s run by a species of glowing bacteria known as Vibrio fischeri. This bacterium and the squid are symbiotic, which means the two species live together for mutual benefit. (See “Large, ‘Glamorous’ New Glowing Squid Species Found.”)
Now a recent study published in the journal mBio shows that V. fischeri are required for the squid’s daily circadian rhythm.
“There’s been a lot of work looking at how circadian rhythm in the host can affect symbiosis, but not many people have looked whether symbionts could affect the circadian rhythm in the host,” said study leader Elizabeth Heath-Heckman, a Ph.D. candidate in the lab of Margaret McFall-Ngai at the University of Wisconsin.
An organism’s sleep-wake cycle is perhaps the most dramatic example of a circadian rhythm. But these rhythms also regulate 98 to 99 percent of our body’s genes, and have strong effects on everything from eating and digestion to how the immune system works.
“An organism has different stresses at different times of the day, and pretty much every group of organisms out there, from bacteria up through us, has evolved the capability of keeping time,” said Heath-Heckman, who has been studying these squid and bacteria for over two decades. (Read how even vegetables have internal clocks.)
Humans are diurnal, which means daylight triggers our brains to wake us up and get us out and about. For nocturnal animals like the bobtail squid, it’s the lack of light that prods it to emerge from its burrow and start to feed. (Also see “How a Rooster Knows to Crow at Dawn.”)
But the squid doesn’t emerge alone: It has a specialized light organ on its body that’s inhabited by the bioluminescent V. fischeri. In exchange for a home and a diet of sugars and amino acids provided by the squid, the bacterium helps protect E. scolopes from predators by illuminating it with a blue glow. This counter-illumination hides the squid’s silhouette by helping it blend in with its surroundings.
Blue light, Heath-Heckman explained, is a potent activator of the body clock genes that govern circadian rhythm. Since the Vibrio only glow at night, when the squid is active and feeding, she wondered whether the light might also help regulate the squid’s circadian rhythm.
Cycles of Expression
Heath-Heckman and colleagues started by identifying a set of genes known as cryptochromes in the squid. Cryptochromes help to “wind” the biological clock, she said.
“The clock can run without the cryptochromes, but what these proteins do in invertebrates is to allow sunlight to tell them what is going on,” Heath-Heckman said. These proteins keep the squid’s internal clocks in sync with the natural patterns of daylight.
In invertebrates like squid, these genes are switched on by blue light, like the kind produced by Vibrio. The cryptochrome proteins then activate the internal clock genes. In the head of the squid, which is not colonized by V. fischeri, the cryptochrome genes are activated by ambient light in the environment. (See more pictures of glowing animals.)
Cryptochrome gene expression in the bacteria-colonized light organ, however, is very different. There, Heath-Heckman and colleagues found, the bacteria-made light switched on the genes. What’s more, the cryptochrome genes of young, lab-raised squid that weren’t colonized by the bacteria didn’t cycle at all—meaning they need the bacteria to work.
The cryptochrome genes were only able to cycle when the squid were exposed to the wavelength of light produced by the bacteria and certain bacterial proteins.
Understanding something as complicated as the circadian rhythm has long perplexed researchers, but in recent years, the study of circadian rhythm—known formally as chronobiology—has really taken off.
Heath-Heckman said that bacteria may dictate circadian rhythms in other animals—including us.
“We have an extremely large and important consortium of bacteria in our guts. The immune cells and the cells that line our intestines both turn genes on and off in a circadian manner. Maybe the bacteria are saying something to these cells and entraining their circadian rhythm the same way that we see in squid,” Heath-Heckman concluded. (Also see “Night Owls Stay Alert Longer Than Early Birds.“)
So the next time your alarm rings and you blindly fumble for a way to make that awful racket stop, just think of the Hawaiian bobtail and be glad that you have a snooze button.