Author: Maria Zagorulya
Institution: University of Rochester
Humans are not the only species to experience fatigue and even illness when moving from one time zone to another. The same hormone responsible for causing our internal clocks to fall out of sync with actual time also controls the vertical migration of plankton in the ocean, according to new research published September 25th in the journal Cell. The discovery may fill in one of the missing links in our understanding of the evolution of daily rhythms and sleeping patterns in animals.
Every day, as they travel up and down in the water, the marine larvae of the annelid worm Platynereis dumerilii perform what may be the world’s biggest migration in terms of biomass. During the day, the larvae stay deep in the water avoiding the damaging UV light from the sun. At dusk, they migrate up to reach the water’s surface. Finally, during the night, they sink back down again. The plankton carry out their daily journey using microscopic hairlike structures called cilia, which propel them vertically. When cilia beat, the plankton migrate upwards, and when they stop beating, the larvae sink. But what controls the initiation of this process every day?
“We found that a group of multitasking cells in the brains of these larvae that sense light also run an internal clock and make melatonin at night,” says Detlev Arendt, PhD, senior scientist leading the research at the European Molecular Biology Laboratory (EMBL) in Heidelberg, Germany. “So we think that melatonin is the message these cells produce at night to regulate the activity of other neurons that ultimately drive day-night rhythmic behavior.”
Melatonin—“the hormone of darkness”—is a hormone is responsible for the pattern of daily activities in vertebrates. It controls sleep and has been linked to jet lag in humans, but melatonin’s function is not as well understood in other organisms.
Studying plankton and their mechanisms of swimming, Maria Antonietta Tosches, PhD, a postdoc in Arendt’s lab, noticed a particular neuron in the brain that causes the cilia to sometimes beat and sometimes stop. With the help of modern molecular sensors that allow 3D imaging of the tissue in the living organism, Tosches visualized the activity of the neuron at different times of the day. In the daytime, this neuron fires sporadically, causing few pauses in the rhythm of cilia’s beating; at night, in the presence of melatonin, it fires rhythmically and constantly, resulting in long pauses. As a result, during the day when melatonin is not produced, the cilia beat more, and the plankton travel toward the surface. At night, when melatonin is produced, the cilia beat less and the plankton sink back to the depths.
Sure enough, when the researchers treated the larvae with melatonin at various times of the day, the plankton ceased swimming and sank. “It’s as if they were jet lagged,” says Tosches. Then, when the researchers introduced a known melatonin-blocking agent, the effect of melatonin was reversed, and the plankton resumed their normal daytime behavior. The results suggest that melatonin controls the circadian rhythms of plankton, just as it does in vertebrates.
“Step by step we can elucidate the evolutionary origin of key functions of our brain,” Arendt says. “The fascinating picture emerges that human biology finds its roots in some deeply conserved, fundamental aspects of ocean ecology that dominated life on Earth since ancient evolutionary times.”
This piece was published under the guidance of Science Writing Mentor Jennifer Akst.