Study finds plankton use UV light sensors to detect pressure changes and avoid being swept away.

Researchers have discovered how marine plankton respond to changes in pressure and propel themselves through the water, using tiny protrusions called cilia.

The research, presented in eLifeas a revised preprint, has been described by the editors as a fundamental study that addresses the question of how some zooplankton change direction in response to pressure, a phenomenon known as barotaxis.

The authors provide compelling evidence that barotaxis involves UV light sensors in cells, which interact with motor neurons in the brain to activate the rhythmic beating of cilia. The results shed light on how marine plankton detect and respond differently to opposing environmental cues, allowing them to find the optimal habitat between 0 and 20 m below the sea.

Hydrostatic pressure increases as the water deepens, and this pressure can provide plankton with depth information regardless of light availability or time of day. Many marine organisms are known to sense and respond to water pressure, but they use very different pressure-sensing mechanisms.

“Fish have a gas-filled swim bladder that allows them to sense pressure, and some crab species have tiny hairs that are thought to act as pressure detectors,” says lead author Luis Bezares-Calderón, a postdoctoral researcher at Living Systems Institute at the University of Exeter. , UK “It is unclear which, if any, of the different pressure-sensing mechanisms observed in marine life are used by much smaller planktonic animals.”

To solve this problem, the team used larvae of a marine worm, called Platynereis dumerilii, which are equipped with a strip of tiny cilia that they use to swim. The larvae beat their cilia to swim up and down between different water depths, where they eventually settle on seagrass beds near the coast. The mechanism by which larvae are guided by light in their environment is well understood, making it an ideal model for learning more about sensing water pressure.

To monitor how plankton larvae respond to pressure, the team built a custom water chamber in which they could precisely control water pressure. They found that the larvae responded to increased pressure by swimming upward faster and in a straighter trajectory.

Higher pressure levels caused the larvae to move higher in the chamber and increase the straightness of their swimming. Additionally, the increase in upward swimming was directly related to the magnitude of the increase in pressure, suggesting that the larvae detect changes in water pressure rather than a specific pressure level .

To understand the mechanism of this change in swimming behavior, the team studied the effect of pressure on the speed at which the larvae beat their cilia. They found that the average beat frequency increased as soon as pressure was applied, suggesting that the larvae responded to higher pressure by using their cilia to propel them upward.

So how do they feel about this change in pressure? To identify pressure-sensitive cells, the team used imaging and a fluorescent marker to observe nerve activity under a microscope. This identified a group of four cells in the middle of the brain whose nerve activity increased as pressure increased. These cells resembled in shape, number, size and position the previously identified “photoreceptor” cells that help plankton respond to light.

“This led us to the unexpected discovery that light receptors in the cilia, previously sensitive to both UV and green light, are also activated by pressure,” says Bezares-Calderón. “This suggests that a single cell type could integrate signals from light and pressure, where UV light prompts plankton to swim downward, away from the light, and increased pressure prompts it to swim toward the top.”

The team sought to confirm this by deleting an essential gene in photoreceptor cells called opsin. As expected, larvae without opsin responded significantly less to pressure signals and showed defects in sensory cilia.

The remaining question was how photoreceptor cells convert pressure signals into physical movement of cilia. Using an existing brain wiring map for plankton larvae, they found a possible connection to the brain’s serotonin signaling system. When they blocked this signaling, it attenuated the pressure response, revealing that the serotonin signaling network links the plankton’s pressure sensors to the resulting physical swimming response.

“Our work provides insight into the mechanisms of pressure sensation and response in a marine plankton larva,” concludes lead author Gáspár Jékely, professor of molecular biology of organisms at the Center for Organismal Studies at the University of Heidelberg.

“This suggests that increasing pressure – either due to the actions of the sinking or diving larva or due to downdrafts – activates sensory photoreceptor cells, causing the cilia to beat faster in a proportional manner. to the magnitude of the pressure.

“Our results show that plankton receive external signals of light and pressure via a single sensory cell, and that these signals then diverge in the brain to trigger different swimming behaviors that guide the plankton to an optimal position in the water. “