In this work, we focus on the motility behavior of two model microorganisms widely used in the study of active fluids: C*hlamydomonas reinhardtii* microalga and *Synechocystis* *sp.* Cyanobacterium.

One of the major behaviors mostly associated to the growth of various species of living organisms on this planet is motility or cells movement. Motility is defined by the ability of an organism to move by itself. Human cells, animal cells and even plants and trees with large organelles and chloroplasts are constantly moving within cellular boundaries. Mainly, microorganisms used in biotechnological applications are naturally motile, thus understanding their physiological responses under variable environmental conditions in bioreactors is essential for efficient biofuel production.

Different models were suggested to predict the cells movement in different types of flows. Yet, most of the previous studies focused on the observation of cellular motility regardless of the growth, meaning that, without taking in to account the culture’s growth process. Here, we study the motility behavior of C*hlamydomonas reinhardtii* and *Synechocystis* *sp.* during their growth when subjected to different intensities of hydrodynamic shear stress (1-20 s^-1^). For each specie, four agitated photobioreactors (APBR) batch cultures of 150 ml (75 ml for the non-agitated reference culture) are prepared with 1/300 dilution factor. Using a 33 ms resolution video microscopy, the cells movements are observed and recorded. Then, the cells swimming velocities are measured as a function of the population growth dynamics.

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 average velocity *V* was related to the growth, such that it follows 3 different phases, the rising phase starting almost at the middle of the exponential growth phase and the decay and damped phase during the stationary phase. The variation in the average velocity with time, was modeled using a linear model for the rising phase and a damped oscillatory model for the decay and damped phase.

Regarding the results obtained for *Synechocystis,* the average velocity does not follow a well-defined pattern in time. However, it seems that the peak of the velocity occurs always in the middle of exponential phase. The average velocity was not significantly affected by the agitation intensity during the growth, however agitation seemed to alter and disturb the behavior of the cells in such a way that the small perturbations between the peaks became more significant as the agitation intensity increased, suggesting that *Synechocystis* cells are resistant and shows high endurance to the applied shear.

These observations demonstrate the difference in the motility response of the two species against the applied hydrodynamic shear stress, this is mainly attributed to their different physiological and morphological characteristics.


Motility Response to Hydrodynamic Stress During the Growth Cycle in Active Fuid Suspensions

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

HT-ICNMM2020 - Virtual Conference, SHTC - Virtual Conference, FEDSM2020 - Virtual Conference

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