Dynamic force measurements on swimming Chlamydomonas cells using micropipette force sensors

Abstract

Flagella and cilia are cellular appendages that inherit essential functions of microbial life including sensing and navigating the environment. In order to propel a swimming microorganism they displace the surrounding fluid by means of periodic motions, while precisely-timed modulations of their beating patterns enable the cell to steer towards or away from specific locations. Characterizing the dynamic forces, however, is challenging and typically relies on indirect experimental approaches. Here, we present direct in vivo measurements of the dynamic forces of motile Chlamydomonas reinhardtii cells in controlled environments. The experiments are based on partially aspirating a living microorganism at the tip of a micropipette force sensor and optically recording the micropipette's position fluctuations with high temporal and sub-pixel spatial resolution. Spectral signal analysis allows for isolating the cell-generated dynamic forces associated to the periodic motion of the flagella from background noise. We provide an analytic elasto-hydrodynamic model for the micropipette force sensor and describe how to obtain the micropipette's full frequency response function from a dynamic force calibration. Using this approach, we find dynamic forces during the free swimming activity of individual Chlamydomonas reinhardtii cells of 23 ± 5 pN resulting from the coordinated flagellar beating with a frequency of 51 ± 6 Hz. This dynamic micropipette force sensor (DMFS) technique generalises the applicability of micropipettes as force sensors from static to dynamic force measurements, yielding a force sensitivity in the piconewton range. In addition to measurements in bulk liquid environment, we study the dynamic forces of the biflagellated microswimmer in the vicinity of a solid/liquid interface. As we gradually decrease the distance of the swimming microbe to the interface, we measure a significantly enhanced force transduction at distances larger than the maximum extend of the beating flagella, highlighting the importance of hydrodynamic interactions for scenarios in which flagellated microorganisms encounter surfaces.

Light-switchable deposits from evaporating drops containing motile microalgae

“…Secondly, we compare the drag force F d exerted on the algae cells due to the capillary outwards flow to the dynamic propulsion force F p , arising from the beating of the flagella of the algae, which has been measured recently in vivo for motile C. R. and found to be 26 AE 5 pN. 39 Using Stokes' law we find F d = 3pZdhvi = 0.25 AE 0.13 pN with hvi being the maximum averaged velocity of the outward flow measured in the non-motile case (Fig. 4b).…”

Control of helical navigation by three-dimensional flagellar beating

“…Simulations.-We input this estimate of β into our 3-bead model with cell-realistic parameters to determine if beat non-planarity can indeed generate axial rotation and helical swimming. All lengths are nondimensionalised by (= 10 µm, typical cell size), forces by the average tangential flagellar force F 0 (= 30 pN, typical force produced by a flagellum [38]), and η by 10 −3 pNµm −2 (viscosity of water). When β = 0 (no tilt), pitch and roll (θ 2,3 ) are suppressed, and our model reduces to the purely planar case investigated by other authors [24,39].…”

Chlamydomonas reinhardtii swimming in the Plateau borders of 2D foams

“…Despite this, the fitted dimensions are at the same ballpark as the literature values for algae of d = 10± 2.5 µm, which is roughly the size of the algae in the present experiments, as confirmed by our high magnification microscope image ( Figure 2). Also the literature values for forces measured with various methods range between 5 to 50 pN 34,35 . We may only speculate that here the drag coefficient might be compensated, for instance, by the solid-liquid boundary conditions at the algae-liquid interface, which may differ from those for passive particles.…”