Light-Induced Phase Separation with Finite Wavelength Selection in Photophobic Microalgae

Area of Science:

• Cellular biology
• Microbiology
• Biophysics

Background:

• Motile microalgae, such as Chlamydomonas reinhardtii, exhibit phototaxis, altering movement in response to light.
• Light absorption by dense cell suspensions can lead to self-shading, influencing microalgal behavior.

Purpose of the Study:

• To investigate the instability of photophobic microalgal suspensions to density fluctuations.
• To explore the role of light absorption and shading in driving collective cell behaviors.
• To demonstrate a motility-induced phase separation mechanism in Chlamydomonas reinhardtii.

Main Methods:

• Experimental observation of Chlamydomonas reinhardtii suspensions in a circular illumination geometry.
• Analysis of density fluctuations and pattern formation.
• Development of a drift-diffusion framework to model the system's behavior.

Main Results:

• Photophobic cell suspensions exhibit instability due to shading interactions, leading to phase separation.
• Transient branching patterns emerge, demonstrating finite wavelength selection via motility-induced phase separation.
• A drift-diffusion model successfully predicts wavelength dependence on cell density, light intensity, and viscosity.

Conclusions:

• Collective behaviors, specifically active phase separation, driven by phototaxis can provide photoprotection.
• Motility-induced phase separation is a mechanism for microalgae to mitigate the effects of high light intensity on short timescales.