Flow and concentration fields in a suspension of light-guided microalgae

Many photosynthetic microorganisms display the ability known as phototaxis to move towards optimal light intensity. In suspension of microalgae Chlamydomonas Reinhardtii which are denser than water, it was recently demonstrated that phototaxis can be exploited to generate macroscopic convection flows using a localized light source that directs the horizontal motion of the cells. This is reminiscent of bioconvection which is a hydrodynamic phenomenon in suspension of motile micro-organisms heavier than water. Self-generated macroscopic convection flows arise from unstable density gradients. However, the associated concentration patterns are the most striking side of bioconvection and studies on bioconvection mainly focused on pattern formation.

In the recent light-controlled bioconvection experiments in suspensions of Chlamydomonas Reinhardtii, fluid flows were evidenced as they were used as hydrodynamic tweezers or as they induced a recirculation of the microalgae cells. In the present study, we develop a 2D-geometry fluorescence imaging system to track small passive tracers to visualize fluid flows for the first time in bioconvection in addition to the concentration field. We investigate the relationship between the bioconvection flows and their associated concentration patterns.