In this work, we focus on the motility behavior of two model microorganisms widely used in the study of active fluids: Chlamydomonas reinhardtii microalga and Synechocystis sp. Cyanobacterium. Understanding the physiological responses of microorganisms under variable environmental conditions is essential for bioreactor engineering. Yet, most of the previous studies focused on the observation of cellular motility regardless of the growth process. Here, we measure the motility of Chlamydomonas reinhardtii and Synechocystis sp. during their growth when subjected to different intensities of hydrodynamic shear stress. The results demonstrate a significant difference in the motility response of the two species against the applied hydrodynamic shear stress.

Mechanical agitation appears to affect the motility of Chlamydomonas reinhardtii microalgae by stimulating the growth process and increasing the magnitude of the cellular swimming velocity. The motility varies following 3 different phases: the rising phase starting almost at the middle of the exponential growth phase, and the decay and damped phases during the stationary phase. This behavior is described using a linear model for the rising phase and a damped oscillatory model for the decay and damped phases.

The motility of Synechocystis does not follow a well-defined pattern in time. However, it seems that the peak of the swimming velocity occurs always in the middle of exponential phase of growth. Synechocystis cells show a high endurance to the applied shear such that the global effect of agitation intensity on their motility is insignificant.

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