  We present a method to differentiate organisms solely by their motion based
on the generalized Langevin equation (GLE) and use it to distinguish two
different swimming modes of strongly confined unicellular microalgae
Chlamydomonas reinhardtii (CR). The GLE is the most general model for active or
passive motion of organisms and particles and in particular includes
non-Markovian effects, i.e., the trajectory memory of its past. We extract all
GLE parameters from individual cell trajectories and perform an unbiased
cluster analysis to group them into different classes. For the specific cell
population employed in the experiments, the GLE-based assignment into the two
different swimming modes works perfectly, as checked by control experiments.
The classification and sorting of single cells and organisms is important in
different areas, our method that is based on motion trajectories offers
wide-ranging applications in biology and medicine.
