Most phytoplankton are slightly heavier than seawater, which presents a challenge for staying near the surface for photosynthesis. "We decided to work on things that seemingly have no appendages to swim," said Manu Prakash, the senior author of the study and a bioengineer at Stanford University. "What we have discovered in this paper is that these 'P. noctiluca' cells are like little submarines that can control their density so precisely that they can choose where they want to be in the water column."
During an expedition near Hawaii, Prakash and postdoctoral researcher Adam Larson, one of the study's first authors, observed blooms of 'P. noctiluca'. "It took a while to piece together why we were catching two different sizes of these cells, until we saw the videos of the inflation process," said Larson. "It can happen suddenly, so you might miss it if you leave the microscope for just 10 minutes."
The research team used a novel tool, the "gravity machine," to simulate the effects of deep-sea environments and study the plankton's movement. "The gravity machine allows us to see a single cell at subcellular resolution in an infinite water column," Prakash explained. This machine mimics ocean conditions and revealed that when the cells inflate, they become less dense than seawater, allowing them to float to the surface.
Further analysis showed that this expansion occurs as part of the plankton's natural cell cycle. When a cell divides, its vacuole - a structure that stores water - swells with freshwater, making the daughter cells buoyant enough to rise. "This is a clever way to slingshot in the ocean during cell division," Prakash said. The cycle, which takes about seven days, aligns with the plankton's search for sunlight and nutrients.
"This is the first time we have clear evidence that the cell cycle could be regulated by an ecological factor," Prakash noted. By applying a theoretical framework, the team identified gravity as a key ecological force shaping this evolutionary adaptation. "All cells experience a downward gravitational pull. If they don't fight back, they will sink to the ocean floor," said co-author Rahul Chajwa.
Looking forward, Prakash's lab aims to explore similar behaviors in other species of plankton. "We have roughly 600 species in our Behavioral Atlas so far, and we're systematically studying various mechanisms," Prakash said. "There are millions of plankton species in the ocean, so this is just the beginning."
Research Report:Inflation induced motility for long-distance vertical migration
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