1,391 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
Large scale Micro-Photometry for high resolution pH-characterization during electro-osmotic pumping and modular micro-swimming
Micro-fluidic pumps as well as artificial micro-swimmers are conveniently
realized exploiting phoretic solvent flows based on local gradients of
temperature, electrolyte concentration or pH. We here present a facile
micro-photometric method for monitoring pH gradients and demonstrate its
performance and scope on different experimental situations including an
electro-osmotic pump and modular micro-swimmers assembled from ion exchange
resin beads and polystyrene colloids. In combination with the present
microscope and DSLR camera our method offers a 2 \mu m spatial resolution at
video frame rate over a field of view of 3920x2602 \mu m^2. Under optimal
conditions we achieve a pH-resolution of 0.05 with about equal contributions
from statistical and systematical uncertainties. Our quantitative
micro-photometric characterization of pH gradients which develop in time and
reach out several mm is anticipated to provide valuable input for reliable
modeling and simulations of a large variety of complex flow situations
involving pH-gradients including artificial micro-swimmers, microfluidic
pumping or even electro-convection.Comment: 5 figures, 15 page
Physical constraints on accuracy and persistence during breast cancer cell chemotaxis
Directed cell motion in response to an external chemical gradient occurs in
many biological phenomena such as wound healing, angiogenesis, and cancer
metastasis. Chemotaxis is often characterized by the accuracy, persistence, and
speed of cell motion, but whether any of these quantities is physically
constrained by the others is poorly understood. Using a combination of theory,
simulations, and 3D chemotaxis assays on single metastatic breast cancer cells,
we investigate the links among these different aspects of chemotactic
performance. In particular, we observe in both experiments and simulations that
the chemotactic accuracy, but not the persistence or speed, increases with the
gradient strength. We use a random walk model to explain this result and to
propose that cells' chemotactic accuracy and persistence are mutually
constrained. Our results suggest that key aspects of chemotactic performance
are inherently limited regardless of how favorable the environmental conditions
are
Destabilization of a flow focused suspension of magnetotactic bacteria
Active matter is a new class of material, intrinsically out-of equilibrium
with intriguing properties. So far, the recent upsurge of studies has mostly
focused on the spontaneous behavior of these systems --in the absence of
external constraints or driving--. Yet, many real life systems evolve under
constraints, being both submitted to flow and various taxis. In the present
work, we demonstrate a new experimental system which opens up the way for
quantitative investigations, and discriminating examinations, of the
challenging theoretical description of such systems. We explore the behavior of
magnetotactic bacteria as a particularly rich and versatile class of driven
matter, which behavior can be studied under contrasting and contradicting
stimuli. In particular we demonstrate that the competing driving of an
orienting magnetic field and hydrodynamic flow lead not only to jetting, but
also unveils a new pearling instability. This illustrates new structuring
capabilities of driven active matter
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