Also showcasing the patterns formed by active matter, the video presented by Isabelle Eisenmann at the University of Amsterdam and her colleagues stars a single-celled green alga called Chlamydomonas reinhardtii. Somewhat like a microscopic rowing boat, C. reinhardtii propels itself through the water using two tiny arms, or flagella. Green algae depend on light for their energy, but too much can be harmful. Each individual C. reinhardtii cell therefore uses its single eye to compare the light level on all sides, and swims in the direction of least illumination.
I. Eisenmann/University of Amsterdam
The light-avoiding instinct of the single-celled alga Chlamydomonas reinhardtii causes a suspension of the organisms to adopt complex patterns under illumination.
The researchers placed a suspension of C. reinhardtii in a petri dish that was lit from all sides. With no other shade available, the algae sought shelter from the light in each other’s shadows. This collective behavior amplified random initial density fluctuations, and the swarming cells formed an array of branching radial spokes that then retracted into a central blob. By varying the population density, light intensity, and medium viscosity over a series of experiments, Eisenmann and colleagues derived a model that predicted the properties of the patterns formed by the algae.
Eisenmann compares the light-avoiding behavior of C. reinhardtii with that of emperor penguins, which crowd together to escape the cold. But she notes an important difference. Whereas huddling penguins can be driven by purely local interactions, the patterns formed by C. reinhardtii require the algae to influence each other over many cell lengths. Such nonlocal interactions could be important in many more phase-separating active-matter systems, Eisenmann says.