Light polarization directs the flow of algae
Algae cell trajectories predicted for the given light polarization patterns (green lines).
S. Yang et al., Phys. Rev. Lett. 126, 058001 (2021)
Bacteria and algae swim in response to external stimuli. Photosynthetic microorganisms, for example, travel toward regions with higher light intensity. Some of those microorganisms even exhibit polarotaxis behavior: They move in response to the polarization of light. Euglena gracilis algae cells in particular have been shown to propel themselves perpendicularly to the polarization of uniform light fields.
Each E. gracilis consists of a single 50-µm-long rod-shaped cell that propels itself at about 60 µm/s by whipping its flagellum. The cell’s photoreceptor modulates the flagellum’s beating frequency based on its polarization-dependent photoresponses. That polarization dependence may be the reason behind polarotaxis behavior: The microorganisms may orient themselves to maximize light absorption.
In uniform light fields, although the Euglena cells swim orthogonal to the polarization, the net flow is zero because half the cells travel in one direction and half in the other. Now Hepeng Zhang of Shanghai Jiao Tong University in China and his colleagues have tracked the paths of Euglena cells in nonuniform light fields with different polarization patterns. For certain patterns, the cells exhibited the first observed photoinduced net flow. That flow successfully transported microscopic objects about the same size as the microorganisms. In the future, E. gracilis cells could potentially be used to transport material in microfluidics or for targeted drug delivery.
Zhang and his colleagues exposed a Euglena cell culture to blue light with polarization patterns determined by different birefringent liquid-crystal plates. A camera caught the cells’ resulting motion, and a standard particle-tracking algorithm monitored each cell’s position, orientation, and velocity.
Depending on the polarization pattern, the cells either circled around a point, traced a star shape, or took a spiral path, with the motions in each case tending to align orthogonal to the polarization field, as expected. The researchers matched their experimental data with a simple model of a Brownian particle with locally modulated orientation.
S. Yang et al., Phys. Rev. Lett. 126, 058001 (2021)
To demonstrate the cells’ ability to transport particles, the researchers added 50-µm-diameter hollow glass spheres on top of the culture and tracked their motion. An example of the glass spheres’ paths is shown by the colored lines in the top half of the figure to the right, and the corresponding calculated flow field is shown by the blue arrows in the bottom half. The researchers found that the spheres spiraled into the center at up to 5 µm/s.
Zhang and his colleagues’ technique can guide cells in their natural environment without any special preparation. What’s more, the light intensity needed is only 100 µW/cm2, less than a thousandth the intensity of an average commercial laser pointer aimed at a wall. (S. Yang et al., Phys. Rev. Lett. 126, 058001, 2021 .)