The detailed structure of the bioconvection spot: the cell-cluster radiation from single sinking region*

The present study focuses on convective flows that appear in Euglena suspensions. Euglena is one of the photosynthetic microorganisms that can alter their locomotion in response to light. It moves towards regions with suitable light intensity and away from too strong light, a behavior known as positive/negative phototaxis. When Euglena suspensions are illuminated from below by strong light, the cell density in the top layer of the suspension becomes higher than that near the bottom due to their negative phototaxis. In this situation, the Rayleigh-Taylor instability occurs. The balance between the upward movement and the downward flow generates a convective flow, called “bioconvection.”

We demonstrated that a single convection cell of Euglena can exist without surrounding convection cells, which is reproducible by the experimental system controlling spatial and temporal light distributions to manipulate the local cell density of the suspension. We termed this single convection cell the “bioconvection spot.” The structure of the bioconvection spot consists of central region with high cell density and surrounding region where clusters of the individuals are radiated periodically.

The structure details and their dependency on the conditions, e.g. the suspension height and the mean cell density, remain unclear. In this presentation, we report the details of the structure including the radiation details. In particular, the bioconvection spot emits more clusters when the height and the density increase.