3D tracking of plankton with single-camera stereoscopy.

We introduce a device developed to perform a 3D tracking of passive or active particles under flow, confined in a medium hundreds of micrometers wide. Micro-objects are placed inside a vertical glass capillary, and two mirrors are positioned behind it at a certain angle, making it possible to have the two reflections of the capillary on the same optical plane. A 3D reconstruction of the trajectories, captured with a single camera, is carried out along the vertical axis with micrometer-scale precision. To investigate the interaction between shear, the role of the gravity field, and motile micro-organisms, we track a model puller-type microalgae, Chlamydomonas reinhardtii, under a Poiseuille flow, using first its natural fluorescence and then bright-field imaging. Understanding how confinement influences motility is crucial, and we show that this 3D tracking setup enables a full description of interactions between a motile organism and a solid border.