Reverse Bioconvection of Chlamydomonas in The Hyper-Density Medium

Abstract

Chlamydomonas is known to form bioconvection as the result of its gravitactic swimming behavior. Several mechanisms have been discussed as the main cause of the preferential reorientation of the microorganisms with respect to gravity vector. In this study, we assessed the gravitactic orientation mechanism of Chlamydomonas by observing the bioconvective motion in the hyper-density medium containing Percoll. Observation of the bioconvection at the right angle to the convective motion (vertical observation) revealed that Chlamydomonas formed the reverse bioconvection in the hyper-density medium. Cells accumulated at the bottom of the chamber and then formed upward-moving plumes from the bottom accumulation. This reverse bioconvection indicates that the gravitactic orientation of Chlamydomonas is primarily due to the torque generated by the morphological fore-aft asymmetry of the cell. It is, therefore, highly likely that the orientation mechanism based on the drag-gravity model makes greater contribution in the gravitactic orientation of Chlamydomonas than other mechanisms, which include gravity sensing and gravity-buoyancy torque.