100 research outputs found
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
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
Opening up Fractal Structures of Three Dimensional Flows via Leaking
7 pages, 3 figures.-- PACS nrs.: 05.45.Df, 47.53.+n, 47.52.+j.We study the behavior of time periodic three-dimensional incompressible flows modelled by three-dimensional volume preserving maps in the presence of a leakage. The distribution of
residence times, and the chaotic saddle together with its stable and unstable invariant manifolds are described and characterized. They shed light on typical filamentation of chaotic flows whose
local stable and unstable manifolds are always of different character (plane or line). A possible application to magma flows is discussed.OP and IT acknowledge the Spanish Ministerio de Ciencia y Tecnologia, Proyecto CONOCE, contract BFM2000-1108 and Proyecto IMAGEN, contract REN2001-0802-C02-01. JS acknowledges financial support from the International Max Planck Research School for Biomimetic Systems (IMPRS). TT is supported by OTKA T032423.Peer reviewe
Microalgae Scatter off Solid Surfaces by Hydrodynamic and Contact Forces
Interactions between microorganisms and solid boundaries play an important role in biological processes, such as egg fertilization, biofilm formation, and soil colonization, where microswimmers move within a structured environment. Despite recent efforts to understand their origin, it is not clear whether these interactions can be understood as being fundamentally of hydrodynamic origin or hinging on the swimmer's direct contact with the obstacle. Using a combination of experiments and simulations, here we study in detail the interaction of the biflagellate green alga Chlamydomonas reinhardtii, widely used as a model puller microorganism, with convex obstacles, a geometry ideally suited to highlight the different roles of steric and hydrodynamic effects. Our results reveal that both kinds of forces are crucial for the correct description of the interaction of this class of flagellated microorganisms with boundaries.We acknowledge the support of a Ph.D. studentship from the Engineering and Physical Sciences Research Council (MC), the Spanish Ministry of Economy and Competitiveness Grant No. FIS2013-48444-C2-1-P, and the subprogram Ramón y Cajal (IT)Peer Reviewe
Runaway electrification of friable self-replicating granular matter
We establish that the nonlinear dynamics of collisions between particles
favors the charging of a insulating, friable, self-replicating granular
material that undergoes nucleation, growth, and fission processes; we
demonstrate with a minimal dynamical model that secondary nucleation produces a
positive feedback in an electrification mechanism that leads to runaway
charging. We discuss ice as an example of such a self-replicating granular
material: We confirm with laboratory experiments in which we grow ice from the
vapor phase in situ within an environmental scanning electron microscope that
charging causes fast-growing and easily breakable palm-like structures to form,
which when broken off may form secondary nuclei. We propose that thunderstorms,
both terrestrial and on other planets, and lightning in the solar nebula are
instances of such runaway charging arising from this nonlinear dynamics in
self-replicating granular matter
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
Microalgae scatter off solid surfaces by hydrodynamic and contact forces
Interactions between microorganisms and solid boundaries play an important role in biological processes, such as egg fertilization, biofilm formation, and soil colonization, where microswimmers move within a structured environment. Despite recent efforts to understand their origin, it is not clear whether these interactions can be understood as being fundamentally of hydrodynamic origin or hinging on the swimmer’s direct contact with the obstacle. Using a combination of experiments and simulations, here we study in detail the interaction of the biflagellate green alga Chlamydomonas reinhardtii, widely used as a model puller microorganism, with convex obstacles, a geometry ideally suited to highlight the different roles of steric and hydrodynamic effects. Our results reveal that both kinds of forces are crucial for the correct description of the interaction of this class of flagellated microorganisms with boundaries
Confinement-induced accumulation and de-mixing of microscopic active-passive mixtures
Understanding the out-of-equilibrium properties of noisy microscale systems and the extent to which they can be modulated externally, is a crucial scientific and technological challenge. It holds the promise to unlock disruptive new technologies ranging from targeted delivery of chemicals within the body to directed assembly of new materials. Here we focus on how active matter can be harnessed to transport passive microscopic systems in a statistically predictable way. Using a minimal active-passive system of weakly Brownian particles and swimming microalgae, we show that spatial confinement leads to a complex non-monotonic steady-state distribution of colloids, with a pronounced peak at the boundary. The particles’ emergent active dynamics is well captured by a space-dependent Poisson process resulting from the space-dependent motion of the algae. Based on our findings, we then realise experimentally the de-mixing of the active-passive suspension, opening the way for manipulating colloidal objects via controlled activity fields
Confinement-induced accumulation and de-mixing of microscopic active-passive mixtures
Understanding the out-of-equilibrium properties of noisy microscale systems and the extent to which they can be modulated externally, is a crucial scientific and technological challenge. It holds the promise to unlock disruptive new technologies ranging from targeted delivery of chemicals within the body to directed assembly of new materials. Here we focus on how active matter can be harnessed to transport passive microscopic systems in a statistically predictable way. Using a minimal active-passive system of weakly Brownian particles and swimming microalgae, we show that spatial confinement leads to a complex non-monotonic steady-state distribution of colloids, with a pronounced peak at the boundary. The particles’ emergent active dynamics is well captured by a space-dependent Poisson process resulting from the space-dependent motion of the algae. Based on our findings, we then realise experimentally the de-mixing of the active-passive suspension, opening the way for manipulating colloidal objects via controlled activity fields
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