Motility and self-organization of gliding Chlamydomonas populations

Cellular appendages such as cilia and flagella represent universal tools enabling cells and microbes, among other essential functionalities, to propel themselves in diverse environments. In its planktonic, i.e. freely swimming, state the unicellular bi-flagellated microbe Chlamydomonas reinhardtii employs a periodic breaststroke-like flagellar beating to displace the surrounding fluid. Another flagella-mediated motility mode is observed for surface-associated Chlamydomonas cells, which glide along the surface by means of force transduction through an intraflagellar transport machinery. Experiments and statistical motility analysis demonstrate that this gliding motility enhances clustering and supports self-organization of Chlamydomonas populations. We employ Minkowski functionals to characterize the spatiotemporal organization of the surface-associated cell monolayer. We find that simulations based on a purely mechanistic approach cannot capture the observed non-random cell configurations. Quantitative agreement with experimental data however is achieved when considering a minimal cognitive model of the flagellar mechanosensing.Comment: 6 pages, 4 figure

Light-regulated adsorption and desorption of Chlamydomonas cells at surfaces

Microbial colonization of surfaces represents the first step towards biofilm formation, which is a recurring phenomenon in nature with beneficial and detrimental implications in technological and medical settings. Consequently, there is a current interest in elucidating the fundamental aspects of the initial stages of biofilm formation of microorganisms on solid surfaces. While most of the research is oriented to understand bacterial surface colonization, such observations at a fundamental level using photosynthetic microalgae are thus far elusive. Recent single-cell studies showed that the flagellar adhesion of Chlamydomonas is switched on in blue light and switched off under red light [Kreis et al., Nature Physics, 2018, 14, 45-49]. Here, we study this light-switchable surface association of C. reinhardtii on the population level and measure the kinetics of adsorption and desorption of suspensions of motile cells on glass surfaces using bright field optical microscopy. We observe that both processes exhibit a response lag relative to the time at which the blue- and red-light conditions are set and model this feature using time-delayed Langmuir-type kinetics. We find that cell adsorption occurs significantly faster than desorption, which we attribute to the protein-mediated molecular adhesion mechanism of the cells. Adsorption experiments using phototactically blind Chlamydomonas mutants demonstrate that phototaxis does not affect the cell adsorption kinetics. Hence, this method can be used as an assay for characterizing the dynamics of the surface colonization of microbial species exhibiting light-regulated surface adhesion.Comment: 10, pages, 6 figure

Toroidal instabilities in the presence of charge and non-Newtonian fluids

Cylindrical jets break into spherical droplets due to surface-tension driven, Rayleigh-Plateau instabilities. Interestingly, toroidal droplets can also transform into a spherical droplet via a shrinking instability whereby the handle of the torus progressively disappears. We study this instability using particle image velocimetry and determining the velocity field inside the droplet. Using the experiments as a guide, we theoretically analyze the problem and account for the discrepancy between previous theoretical and simulation work. This allows elucidating which of the many possible modes controlling the toroidal droplet evolution are needed to capture the evolution and deformation of the droplet as seen experimentally. We then apply a voltage difference across the droplet and a controlled ground to charge the toroidal droplet. In this case, surface tension stresses compete with electrostatic stresses due to the presence of surface charge; qualitatively changing the behavior of the droplet, which, for sufficiently high voltages, is able to transition from a shrinking torus to an expanding torus. Remarkably, an additional transition happens at even higher voltages; in this case, the torus produces finger-like structures reminiscent of the Saffman-Taylor instability. Despite the three-dimensional character of our experiments, charge and geometry both combine to allow observing an instability that is most often seen in quasi-two dimensional situations. We study and model all these transitions, and identify the essential physics needed to describe them. Finally, we exploit that thin toroidal droplets approach the cylindrical limit to also study the effect of charge over jet break-up. We do this by comparing the experimentally determined wavelength associated to the fastest unstable mode to theoretical expectation for charged cylindrical jets. Furthermore, we study the break-up dynamics in the presence of rheologically non-linear materials. In this case, the droplets resist break-up for long times compare to when we use simple liquids. We show that we can explain our data by incorporating the non-linearities into a linear treatment of the problem through the strain-rate-dependent viscosity.Ph.D

Motility and self-organization of gliding <i>Chlamydomonas</i> populations

Cellular appendages such as cilia and flagella represent universal tools enabling cells and microbes, among other essential functionalities, to propel themselves in diverse environments. In its planktonic, i.e., freely swimming, state the unicellular biflagellated microbe Chlamydomonas reinhardtii employs a periodic breaststroke-like flagellar beating to displace the surrounding fluid. Another flagella-mediated motility mode is observed for surface-associated Chlamydomonas cells, which glide along the surface by means of force transduction through an intraflagellar transport machinery. Experiments and statistical motility analysis demonstrate that this gliding motility enhances clustering and supports self-organization of Chlamydomonas populations. We employ Minkowski functionals to characterize the spatiotemporal organization of the surface-associated cell monolayer. We find that simulations based on a purely mechanistic approach cannot capture the observed nonrandom cell configurations. Quantitative agreement with experimental data, however, is achieved when considering a minimal cognitive model of the flagellar mechanosensing.</p

Supplementary information files for Motility and self-organization of gliding Chlamydomonas populations

Supplementary files for article Motility and self-organization of gliding Chlamydomonas populations Cellular appendages such as cilia and flagella represent universal tools enabling cells and microbes, among other essential functionalities, to propel themselves in diverse environments. In its planktonic, i.e., freely swimming, state the unicellular biflagellated microbe Chlamydomonas reinhardtii employs a periodic breaststroke-like flagellar beating to displace the surrounding fluid. Another flagella-mediated motility mode is observed for surface-associated Chlamydomonas cells, which glide along the surface by means of force transduction through an intraflagellar transport machinery. Experiments and statistical motility analysis demonstrate that this gliding motility enhances clustering and supports self-organization of Chlamydomonas populations. We employ Minkowski functionals to characterize the spatiotemporal organization of the surface-associated cell monolayer. We find that simulations based on a purely mechanistic approach cannot capture the observed nonrandom cell configurations. Quantitative agreement with experimental data, however, is achieved when considering a minimal cognitive model of the flagellar mechanosensing. </p

Dynamic Assembly of Ultrasoft Colloidal Networks Enables Cell Invasion Within Restrictive Fibrillar Polymers

In regenerative medicine, natural protein-based polymers offer enhanced endogenous bioactivity and potential for seamless integration with tissue, yet form weak hydrogels that lack the physical robustness required for surgical manipulation, making them difficult to apply in practice. The use of higher concentrations of protein, exogenous cross-linkers, and blending synthetic polymers has all been applied to form more mechanically robust networks. Each relies on generating a smaller network mesh size, which increases the elastic modulus and robustness, but critically inhibits cell spreading and migration, hampering tissue regeneration. Here we report two unique observations; first, that colloidal suspensions, at sufficiently high volume fraction (ϕ), dynamically assemble into a fully percolated 3D network within high-concentration protein polymers. Second, cells appear capable of leveraging these unique domains for highly efficient cell migration throughout the composite construct. In contrast to porogens, the particles in our system remain embedded within the bulk polymer, creating a network of particle-filled tunnels. Whereas this would normally physically restrict cell motility, when the particulate network is created using ultralow cross-linked microgels, the colloidal suspension displays viscous behavior on the same timescale as cell spreading and migration and thus enables efficient cell infiltration of the construct through the colloidal-filled tunnels

DataSheet1_Quantifying epithelial cell proliferation on curved surfaces.PDF

Out-of-plane curvature is an important, but poorly explored geometric parameter that influences cell behavior. We address the impact of curvature on epithelial proliferation through monitoring how MDCK cells proliferate on planar and curved toroidal hydrogel substrates with a broad range of Gaussian curvatures. We illustrate in detail the imaging processing methodology to characterize curved surfaces and quantify proliferation of cells. We find that MDCK cells grow readily on both curved and flat surfaces and can cover the entire surface of the toroidal structure as long as the initial seeding is uniform. Our analysis shows that proliferation does not depend on Gaussian curvature within the range probed in our experiment, but rather on cell density. Despite epithelial proliferation is insensitive to the curvature range presented in this study, the toroidal-construct fabrication technique and image processing methodology may find utility for probing cell processes like collective migration, as it involves long-range force transmission.</p