Flagellar locomotion is one of the first forms of locomotion on our planet. Microorganisms use their flagella to reorientate or generate movement in search for nutrients or gradients in light intensity. These microscopic swimmers are able to adapt their swimming gait to the circumstances they encounter. Motile microorganisms such as the Chlamydomonas reinhardtii are used as a source of inspiration in biomimetics and used in biological, pharmaceutical and industrial applications to produce for example biofuels or medication. The fluids in which these microscopic cells swim are often of complex rheology, containing macromolecules such as synthetic polymers of proteins, which influences the
motile behaviour of these cells. The motile behaviour is mostly studied in a quasi 2D environment which might be different from the behavior in a more natural 3D environment. Therefore, our aim is to analyse the motility of microorganisms in a 3D environment in fluids with different rheological properties.

To gain more insight in the influence of rheology on the motility of microorganisms, the model microorganism Chlamydomonas reinhardtii is observed in fluids of various levels of elasticity. An in-house 3D microscopic setup, consisting of 4 LaVision sCMOS cameras, is used to observe the motility of the cells. A Lagrangian tracking algorithm is used to detect and follow many individual motile cells simultaneously. We focused on the velocity of the cells, their helical motions (radius and pitch) which they use to scan their surroundings, and their flagellar beating frequency.

We observed a velocity and pitch independent of the Deborah number, whereas decreasing values for these metrics were found in Newtonian fluids of equal viscosity. The radius of the helical motion showed to be independent of elasticity or viscosity. The beating frequency of the cells shows to be constant in a viscoelastic fluid independent on the viscosity or elasticity, whereas it tends to decrease in Newtonian fluids of increasing viscosity.

Chlamydomonas reinhardtii might have developed a more efficient swimming gait to propel themselves in a viscoelastic fluid compared to a Newtonian fluid, especially in high viscosities. Moreover, we argued that motile microorganisms that are allowed to swim freely in a 3D confinement, show a different motile behaviour than when their motility is constricted by a quasi 2D plane. To validate these findings we recommend to perform experiments both in quasi 2D and 3D, while using the same cell culture.