70 research outputs found
A brief introduction to the model microswimmer {\it Chlamydomonas reinhardtii}
The unicellular biflagellate green alga {\it Chlamydomonas reinhardtii} has
been an important model system in biology for decades, and in recent years it
has started to attract growing attention also within the biophysics community.
Here we provide a concise review of some of the aspects of {\it Chlamydomonas}
biology and biophysics most immediately relevant to physicists that might be
interested in starting to work with this versatile microorganism.Comment: 16 pages, 7 figures. To be published as part of EPJ S
Universal entrainment mechanism governs contact times with motile cells
Contact between particles and motile cells underpins a wide variety of
biological processes, from nutrient capture and ligand binding, to grazing,
viral infection and cell-cell communication. The window of opportunity for
these interactions is ultimately determined by the physical mechanism that
enables proximity and governs the contact time. Jeanneret et al. (Nat. Comm. 7:
12518, 2016) reported recently that for the biflagellate microalga
Chlamydomonas reinhardtii contact with microparticles is controlled by events
in which the object is entrained by the swimmer over large distances. However,
neither the universality of this interaction mechanism nor its physical origins
are currently understood. Here we show that particle entrainment is indeed a
generic feature for microorganisms either pushed or pulled by flagella. By
combining experiments, simulations and analytical modelling we reveal that
entrainment length, and therefore contact time, can be understood within the
framework of Taylor dispersion as a competition between advection by the no
slip surface of the cell body and microparticle diffusion. The existence of an
optimal tracer size is predicted theoretically, and observed experimentally for
C. reinhardtii. Spatial organisation of flagella, swimming speed, swimmer and
tracer size influence entrainment features and provide different trade-offs
that may be tuned to optimise microbial interactions like predation and
infection.Comment: New analytical entrainment theory; includes Supplementary
informations as Appendix; Supplementary movies available upon reques
A Python based automated tracking routine for myosin II filaments
The study of motor protein dynamics within cytoskeletal networks is of high interest to physicists and biologists to understand how the dynamics and properties of individual motors lead to cooperative effects and control of overall network behaviour. Here, we report a method to detect and track muscular myosin II filaments within an actin network tethered to supported lipid bilayers. Based on the characteristic shape of myosin II filaments, this automated tracking routine allowed us to follow the position and orientation of myosin II filaments over time, and to reliably classify their dynamics into segments of diffusive and processive motion based on the analysis of displacements and angular changes between time steps. This automated, high throughput method will allow scientists to efficiently analyse motor dynamics in different conditions, and will grant access to more detailed information than provided by common tracking methods, without any need for time consuming manual tracking or generation of kymographs
Transitions in synchronization states of model cilia through basal-connection coupling
Despite evidence for a hydrodynamic origin of flagellar synchronization
between different eukaryotic cells, recent experiments have shown that in
single multi-flagellated organisms, coordination hinges instead on direct basal
body connections. The mechanism by which these connections leads to
coordination, however, is currently not understood. Here we focus on the model
biflagellate {\it Chlamydomonas reinhardtii}, and propose a minimal model for
the synchronization of its two flagella as a result of both hydrodynamic and
direct mechanical coupling. A spectrum of different types of coordination can
be selected, depending on small changes in the stiffness of intracellular
couplings. These include prolonged in-phase and anti-phase synchronization, as
well as a range of multistable states induced by spontaneous symmetry breaking
of the system. Linking synchrony to intracellular stiffness could lead to the
use of flagellar dynamics as a probe for the mechanical state of the cell.Comment: 14 pages, 9 figure
Ciliary contact interactions dominate surface scattering of swimming eukaryotes
Interactions between swimming cells and surfaces are essential to many
microbiological processes, from bacterial biofilm formation to human
fertilization. However, in spite of their fundamental importance, relatively
little is known about the physical mechanisms that govern the scattering of
flagellated or ciliated cells from solid surfaces. A more detailed
understanding of these interactions promises not only new biological insights
into structure and dynamics of flagella and cilia, but may also lead to new
microfluidic techniques for controlling cell motility and microbial locomotion,
with potential applications ranging from diagnostic tools to therapeutic
protein synthesis and photosynthetic biofuel production. Due to fundamental
differences in physiology and swimming strategies, it is an open question
whether microfluidic transport and rectification schemes that have recently
been demonstrated for pusher-type microswimmers such as bacteria and sperm
cells, can be transferred to puller-type algae and other motile eukaryotes, as
it is not known whether long-range hydrodynamic or short-range mechanical
forces dominate the surface interactions of these microorganisms. Here, using
high-speed microscopic imaging, we present direct experimental evidence that
the surface scattering of both mammalian sperm cells and unicellular green
algae is primarily governed by direct ciliary contact interactions. Building on
this insight, we predict and verify experimentally the existence of optimal
microfluidic ratchets that maximize rectification of initially uniform
Chlamydomonas reinhardtii suspensions. Since mechano-elastic properties of
cilia are conserved across eukaryotic species, we expect that our results apply
to a wide range of swimming microorganisms.Comment: Preprint as accepted for publication in PNAS, for published journal
version (open access) and Supporting Information see
http://dx.doi.org/10.1073/pnas.121054811
Light control of localized photobioconvection
Microorganismal motility is often characterized by complex responses to environmental physico-chemical stimuli. Although the biological basis of these responses is often not well understood, their exploitation already promises novel avenues to directly control the motion of living active matter at both the individual and collective level. Here we leverage the phototactic ability of the model microalga Chlamydomonas reinhardtii to precisely control the timing and position of localized cell photoaccumulation, leading to the controlled development of isolated bioconvective plumes. This novel form of photobioconvection allows a precise, fast, and reconfigurable control of the spatiotemporal dynamics of the instability and the ensuing global recirculation, which can be activated and stopped in real time. A simple continuum model accounts for the phototactic response of the suspension and demonstrates how the spatiotemporal dynamics of the illumination field can be used as a simple external switch to produce efficient bio mixing
Phototaxis beyond turning: persistent accumulation and response acclimation of the microalga Chlamydomonas reinhardtii
Phototaxis is an important reaction to light displayed by a wide range of
motile microorganisms. Flagellated eukaryotic microalgae in particular, like
the model organism Chlamydomonas reinhardtii, steer either towards or away from
light by a rapid and precisely timed modulation of their flagellar activity.
Cell steering, however, is only the beginning of a much longer process which
ultimately allows cells to determine their light exposure history. This process
is not well understood. Here we present a first quantitative study of the long
timescale phototactic motility of Chlamydomonas at both single cell and
population levels. Our results reveal that the phototactic strategy adopted by
these microorganisms leads to an efficient exposure to light, and that the
phototactic response is modulated over typical timescales of tens of seconds.
The adaptation dynamics for phototaxis and chlorophyll fluorescence show a
striking quantitative agreement, suggesting that photosynthesis controls
quantitatively how cells navigate a light field.Comment: Six pages, three figures, plus supplementary materia
Entrainment dominates the interaction of microalgae with micron-sized objects
The incessant activity of swimming microorganisms has a direct physical effect on surrounding microscopic objects, leading to enhanced diffusion far beyond the level of Brownian motion with possible influences on the spatial distribution of non-motile planktonic species and particulate drifters. Here we study in detail the effect of eukaryotic flagellates, represented by the green microalga Chlamydomonas reinhardtii, on microparticles. Macro- and micro-scopic experiments reveal that microorganism--colloid interactions are dominated by rare close encounters leading to large displacements through direct entrainment. Simulations and theoretical modelling show that the ensuing particle dynamics can be understood in terms of a simple jump-diffusion process, combining standard diffusion with Poisson-distributed jumps. This heterogeneous dynamics is likely to depend on generic features of the near-field of swimming microorganisms with front-mounted flagella
Antiphase Synchronization in a Flagellar-Dominance Mutant of Chlamydomonas
Groups of beating flagella or cilia often synchronize so that neighboring
filaments have identical frequencies and phases. A prime example is provided by
the unicellular biflagellate Chlamydomonas reinhardtii, which typically
displays synchronous in-phase beating in a low-Reynolds number version of
breaststroke swimming. We report here the discovery that ptx1, a flagellar
dominance mutant of C. reinhardtii, can exhibit synchronization in precise
antiphase, as in the freestyle swimming stroke. Long-duration high-speed
imaging shows that ptx1 flagella switch stochastically between in-phase and
antiphase states, and that the latter has a distinct waveform and significantly
higher frequency, both of which are strikingly similar to those found during
phase slips that stochastically interrupt in-phase beating of the wildtype.
Possible mechanisms underlying these observations are discussed.Comment: 5 pages, 4 figure
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