Microscopic marine plankton are not helplessly adrift in the ocean. They can perceive cues that indicate turbulence, rapidly respond to regulate their behavior and actively adapt. Now, Moore Foundation grantees at ETH Zurich, led by Roman Stocker, have demonstrated for the first time how plankton cope with turbulence.
Plankton in the ocean are constantly on the move. By day, these tiny organisms, one-tenth the diameter of a human hair, actively migrate toward the sunlit ocean surface to carry out photosynthesis. At night, they make their way to depths of tens of meters, where the supply of nutrients is greater. During their regular trips between well-lit and nutrient-rich zones, plankton cells frequently encounter turbulent layers, which disrupt this essential migratory pattern.
It is still a mystery how these minute organisms can navigate through the dangers of turbulent waters. Plankton cells are whirled around by turbulence — particularly by the smallest, millimeter-sized flow vortices — as if they were in a miniature washing machine, which can induce permanent damage to their propulsion appendages and cell envelope. In the worst case, they can perish in turbulence.
Certain microalgae have, however, developed a sophisticated response to such turbulent cues. Postdoctoral researchers Anupam Sengupta and Francesco Carrara, together with their advisor Stocker, professor at the ETH Zurich Institute of Environmental Engineering and an investigator through the foundation's Marine Microbiology initiative, have shown this in a study published March 15 in the journal Nature.
Using laboratory experiments, the team "brought the ocean into the lab" and examined the migratory behavior of Heterosigma akashiwo, an alga known for forming toxic algal blooms. To examine their swimming behavior, the researchers used a microfabricated chamber, just a few cubic millimeters in volume, in which they introduced the Heterosigma cells.
The chamber could be rotated along its axis using a computer-controlled motor, exposing cells to periodic flips in orientation replicating how tiny turbulent vortices flip the cells upside down in the ocean. The scientists were able to observe that an algal population moving upwards split into two equally sized groups over a period of 30 minutes after the chamber was repeatedly flipped by 180 degrees.
One group of cells continued to strive upward, while the other group switched behavior and began to swim in the opposite direction. This population split did not occur with algae in stationary chambers, in which all swam continuously upwards and accumulated near the top surface.
By zooming into single cells, the researchers discovered the reason for the change in swimming behavior. When exposed to the turbulence-like cues, the cells were able to actively and rapidly change their shape from asymmetric pear-shaped cells swimming up into egg-shaped structures swimming down. Strikingly, this shift involved changes of less than a micrometer (about the diameter of a human hair).
Stocker does not view this mechanism as just a coincidence: "The algae have adapted perfectly to their ocean habitat: they can actively swim, they perceive a range of different environmental signals, including turbulence, and they rapidly adapt and regulate their behavior accordingly," he said.
The team now plans to observe the algae in a larger tank, where they will expose the cells not only to flipping but also to real turbulence. Understanding how these minute cells respond to turbulence holds great importance for our understanding of the ocean.
Read the full press release here.
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