Algal Swimming Patterns Change with Light Intensity
Many microorganisms adjust their swimming trajectories in response to environmental signals such as nutrients or light. Researchers have now discovered a new mode of such behavior in a species of green algae [1]. The microbes swim in wide circles when illuminated and switch from counterclockwise (CCW) to clockwise (CW) swimming when the light intensity is above a threshold value. The researchers determined how this change is generated by the algae’s two whip-like flagella. They say that the results reveal a new navigation strategy that microorganisms can use to find optimal environments.
The single-celled green alga Chlamydomonas reinhardtii is photosynthetic and moves toward light by beating its two flagella, situated close together on its front surface, in a breaststroke pattern. In 2021, Kirsty Wan and Dario Cortese of the University of Exeter in the UK figured out the beating pattern that produces the microbe’s typical corkscrew-shaped trajectory, which follows a tight helix [2]. They showed how changing the frequency, amplitude, and synchronization of the flagellar beating allows the cell to change the overall direction of motion, perhaps to steer it toward or away from a light source and optimize the intensity of light it receives.
“The 3D corkscrewing underlies the basic swimming pattern” of the cell, Wan says. “In the absence of any [environmental] cues, it always rotates as it swims.” The earlier work suggested that in the presence of a light source, the axis of this helix tends to point toward the light. But now Wan, Alan Tsang of the University of Hong Kong, and their colleagues have found that the cell’s behavior is more complicated. They observed cells under a range of illumination levels and found that the axis of the helix follows a wide, circular trajectory when a cell is illuminated, and the center of this circle gradually drifts. Under low light from above, the motion is CCW, and if the light is intense enough, the cell’s circulation direction reverses.
By observing individual cells closely, the researchers determined the changes in beating patterns that produce these differences in motion. The two flagella are not equivalent: One (denoted cis) sits closer to the light-sensing organelle on the surface—the eyespot—than the other (denoted trans). While making CCW circles at low light intensities, the cis flagellum dominates, with a more extended stroke than the trans flagellum. At high light intensities, both flagella beat with equal extension but with a difference in phase, which causes the reversal in the direction of the microbes’ circular motion.
To investigate these beating patterns more carefully, the researchers held a single cell in place at the tip of a glass pipette. They found that the flagella beat back and forth in a particular plane, like rowing oars, and that this plane tilts toward the eyespot in brighter light. The team conducted hydrodynamic computer simulations of the motion and confirmed that these changes in flagellar beating produce the observed changes in trajectories of the cells.
These switching processes “allow a cell with a remarkably simple geometry to produce extremely versatile 3D maneuvers in response to light,” says Wan. “This maneuverability enables the cells to effectively navigate the complex light fields they will typically encounter in nature.” Exactly how the circular paths, and their different rotation senses, are beneficial is not clear. But the researchers point out that CCW circling might be considered a “search mode” for optimal light levels, while the circling of the CW motion has a somewhat smaller radius that might help to prolong the cells’ time in regions of bright light.
Tsang adds that the findings might assist in the design of artificial robotic “microswimmers” for biomedical and technological applications. Analyzing the principles and strategies involved in the swimming behavior of different microorganisms, he says, “may uncover general biophysical laws for microswimmer navigation.”
The results “add to the growing body of evidence that microorganisms exploit multiple swimming modes in response to stimuli,” says Raymond Goldstein of the University of Cambridge in the UK, a specialist in microorganismal swimming. He agrees that the importance of the switching from CCW to CW is not so much in the direction of rotation but in the differences in overall motion over time—that is, how quickly the centers of the circular trajectories diffuse. Changing this factor could confer clear benefits to the organisms, he says.
–Philip Ball
Philip Ball is a freelance science writer in London. His next book, The Man Who Broke Reality, a biography of Niels Bohr, will be published in December.
References
- Z. Wang et al., “Light-dependent switching of circling handedness in microswimmer navigation,” Phys. Rev. Lett. 136, 078301 (2026).
- D. Cortese and K. Y. Wan, “Control of helical navigation by three-dimensional flagellar beating,” Phys. Rev. Lett. 126, 088003 (2021).