Transient state dynamics of <i>Chlamydomonas reinhardtii</i> cells during phototaxis

The directed movement of microorganisms toward light is called phototaxis—a fundamental process shown by many uni- and multicellular photosynthetic microorganisms for their survival and growth. Here, we determine the time-dependent response of a population of model phototactic single-celled algae to changing light conditions. When the condition changes from dark to light, we show that the response of a dark-adapted isotropic population depends on not just the light intensity but also the cell number density. On the other hand, when conditions change from light to dark, the time dependence of the recovery from an aligned state to an isotropic state is independent of cell density and light intensity. We find that such kinetics emerges from the coupling between flagellar dynamics, cell speed, and incident photon flux. We further show that existing minimal noninteracting models, which robustly describe the steady-state properties of populations of under given light conditions, capture this time-dependent phenomenon only for dilute suspensions.