All Together Now (Sometimes) Motile cilia and flagella protrude from the surface of many eukaryotic cells. Understanding how cilia and flagella operate is important for understanding ciliated cells in metazoans, the ecology and behavior of motile microorganisms, and the mechanisms of molecular motors and signal transduction. Using very-high-speed video microscopy, Polin et al. (p. 487 ; see the Perspective by Stocker and Durham ) discovered that the biflagellated cells of the single-cell alga Chlamydomonas rheinhartii switch between synchronous beating, which keeps the cells traveling forward, and asynchronous beating, which allows the organisms to make sharp turns. This random progression occurs in the dark and allows cells to diffuse, and it may underpin directional movement toward light in the same way that the run-and-tumble behavior of prokaryotes allows them to move up chemical gradients.

Aerobic bacteria often live in thin fluid layers near solid-air-water contact lines, in which the biology of chemotaxis, metabolism, and cell-cell signaling is intimately connected to the physics of buoyancy, diffusion, and mixing. Using the geometry of a sessile drop, we demonstrate in suspensions of Bacillus subtilis the self-organized generation of a persistent hydrodynamic vortex that traps cells near the contact line. Arising from upward oxygentaxis and downward gravitational forcing, these dynamics are related to the Boycott effect in sedimentation and are explained quantitatively by a mathematical model consisting of oxygen diffusion and consumption, chemotaxis, and viscous fluid dynamics. The vortex is shown to advectively enhance uptake of oxygen into the suspension, and the wedge geometry leads to a singularity in the chemotactic dynamics near the contact line.bioconvection ͉ chemotaxis ͉ singularity ͉ Bacillus subtilis T he interplay of chemotaxis and diffusion of nutrients or signaling chemicals in bacterial suspensions can produce a variety of structures with locally high concentrations of cells, including phyllotactic patterns (1), filaments (2), and concentrations in fabricated microstructures (3). Less well explored are situations in which concentrating hydrodynamic f lows actually arise from these ingredients. Here we report a detailed experimental and theoretical study of an intriguing mechanism termed the ''chemotactic Boycott effect.'' Described brief ly before (4), it is intimately associated with buoyancy-driven f lows, metabolite diffusion, and slanted air-water menisci. The ubiquity of contact lines and their transport singularities (5) suggest importance of these observations in biofilm formation (6). The large-scale stirring created by these f lows illustrate important advective contributions to intercellular signaling, as in quorum sensing (7).The chemotactic Boycott effect takes its name from a phenomenon in sedimentation (8) that occurs when the chamber containing a fluid with settling particles is tilted from vertical. Settling depletes the fluid near the upper wall, making it buoyant relative to nearby fluid, whereupon it rises. This boundary flow stirs up the entire medium, greatly accelerating the settling process. In the chemotactic version, negatively buoyant aerobic bacteria swim up to the free surface of a sessile drop and slide down the slanted meniscus, producing high concentrations of cells near the three-phase contact line. In earlier work where this was observed (4), the detailed nature of hydrodynamic flows near the contact line was unclear. Here, by direct visualization and particle-imaging velocimetry (PIV), we show that the sliding surface layer drives a circulating hydrodynamic vortex in the meniscus region that is central to the microecology. Although counterintuitive in viscous flows, persistent circulation driven by forcing at the free surface is consistent with the classic analysis for vortex generation in wedge geometry (9).The initial discussion of the chemota...

Swimming microorganisms create flows that influence their mutual interactions and modify the rheology of their suspensions. While extensively studied theoretically, these flows have not been measured in detail around any freely-swimming microorganism. We report such measurements for the microphytes Volvox carteri and Chlamydomonas reinhardtii. The minute (∼ 0.3%) density excess of V. carteri over water leads to a strongly dominant Stokeslet contribution, with the widelyassumed stresslet flow only a correction to the subleading source dipole term. This implies that suspensions of V. carteri have features similar to suspensions of sedimenting particles. The flow in the region around C. reinhardtii where significant hydrodynamic interaction is likely to occur differs qualitatively from a "puller" stresslet, and can be described by a simple three-Stokeslet model. [6,7] and rheological properties of suspensions [8], and the interaction of organisms with surfaces [9, 10]. As hydrodynamics surely plays a key role in these effects, a detailed knowledge of the flow field around freely swimming microorganisms is needed, both in the near-field and far away. Here we present the first such measurements.The linearity of the Stokes equations implies that the far-field flow around a microorganism can be expressed as a superposition of singularity solutions [11], with the slowest decaying mode dominating sufficiently far away. Theories of fluid-mediated interactions and collective behavior typically assume neutrally buoyant swimmers which exert no net force on the fluid. The thrust T of their flagella and the viscous drag on their body are displaced a distance d apart (often comparable to the cell radius R), and balance to give the far-field flow of a force dipole, or stresslet [12], which decays with distance r as T d/ηr 2 , where η is the fluid's viscosity. The contribution from a suspension of such stresslets to the fluid stress tensor is central to some of the most promising approaches to collective behavior of microorganisms [13].The force-free idealization of swimmers requires precise density-matching [9] not generally realized in nature. To appreciate the striking effects of gravity, one need only consider the buoyancy-driven plumes of bioconvection [14]. Models of this instability express the contribution of cells to the Navier-Stokes equations as a sum of force monopoles (Stokeslets), coarse-grained as a body force proportional to the cell concentration and gravitational force F g per cell [14]. As the flow around a Stokeslet decays as F g /ηr, it is clear, if not appreciated previously, that there is a distance Λ ∼ T d/F g at which the nearby stresslet contribution crosses over to the distant Stokeslet regime. This is one of several crossover lengths relevant to swimmers; for ciliates, unsteady effects become important on scales smaller than the viscous penetration depth [15]. For a given organism, the relevance of the length Λ to a particular physical situation depends on the cell concentration and the observable of ...

The spherical alga Volvox swims by means of flagella on thousands of surface somatic cells. This geometry and its large size make it a model organism for studying the fluid dynamics of multicellularity. Remarkably, when two nearby Volvox colonies swim close to a solid surface, they attract one another and can form stable bound states in which they "waltz" or "minuet" around each other. A surface-mediated hydrodynamic attraction combined with lubrication forces between spinning, bottom-heavy Volvox explains the formation, stability, and dynamics of the bound states. These phenomena are suggested to underlie observed clustering of Volvox at surfaces.Long after he made his great contributions to microscopy and started a revolution in biology, Antony van Leeuwenhoek peered into a drop of pond water and discovered one of nature's geometrical marvels [1]. This was the freshwater alga which, years later, in the very last entry of his great work on biological taxonomy, Linnaeus named Volvox [2] for its characteristic spinning motion about a fixed body axis. Volvox is a spherical colonial green alga (Fig. 1), with thousands of biflagellated cells anchored in a transparent extracellular matrix (ECM) and daughter colonies inside the ECM. Since the work of Weismann [3], Volvox has been seen as a model organism in the study of the evolution of multicellularity [4][5][6].Because it is spherical, Volvox is an ideal organism for studies of biological fluid dynamics, being an approximate realization of Lighthill's "squirmer" model [7] of self-propelled bodies having a specified surface velocity. Such models have elucidated nutrient uptake at high Péclet numbers [6,8] by single organisms, and pairwise hydrodynamic interactions between them [9]. Volvocine algae may also be used to study collective dynamics of selfpropelled objects [10], complementary to bacterial suspensions (E. coli, B. subtilis) exhibiting large-scale coherence in thin films [11] and bulk [12].While investigating Volvox suspensions in glass-topped chambers, we observed stable bound states, in which pairs of colonies orbit each other near the chamber walls. Volvox is "bottomheavy" due to clustering of daughter colonies in the posterior, so an isolated colony swims

It has long been conjectured that hydrodynamic interactions between beating eukaryotic flagella underlie their ubiquitous forms of synchronization; yet there has been no experimental test of this connection. The biflagellate alga Chlamydomonas is a simple model for such studies, as its two flagella are representative of those most commonly found in eukaryotes. Using micromanipulation and high-speed imaging, we show that the flagella of a C. reinhardtii cell present periods of synchronization interrupted by phase slips. The dynamics of slips and the statistics of phase-locked intervals are consistent with a low-dimensional stochastic model of hydrodynamically coupled oscillators, with a noise amplitude set by the intrinsic fluctuations of single flagellar beats.

Experimental work in developmental biology has recently shown in mice that fluid flow driven by rotating cilia in the node, a structure present in the early stages of growth of vertebrate embryos, is responsible for determining the normal development of the left-right axis, with the heart on the left of the body, the liver on the right, and so on. The role of physics, in particular, of fluid dynamics, in the process is one of the important questions that remain to be answered. We show with an analysis of the fluid dynamics of the nodal flow in the developing embryo that the leftward flow that has been experimentally observed may be produced by the monocilia driving it being tilted toward the posterior. We propose a model for morphogen transport and mixing in the nodal flow and discuss how the development of left-right asymmetry might be initiated.

This work reviews the present position of and surveys future perspectives in the physics of chaotic advection: the field that emerged three decades ago at the intersection of fluid mechanics and nonlinear dynamics, which encompasses a range of applications with length scales ranging from micrometers to hundreds of kilometers, including systems as diverse as mixing and thermal processing of viscous fluids, microfluidics, biological flows, and oceanographic and atmospheric flows.

Along the evolutionary path from single cells to multicellular organisms with a central nervous system are species of intermediate complexity that move in ways suggesting high-level coordination, yet have none. Instead, organisms of this type possess many autonomous cells endowed with programs that have evolved to achieve concerted responses to environmental stimuli. Here experiment and theory are used to develop a quantitative understanding of how cells of such organisms coordinate to achieve phototaxis, by using the colonial alga Volvox carteri as a model. It is shown that the surface somatic cells act as individuals but are orchestrated by their relative position in the spherical extracellular matrix and their common photoresponse function to achieve colony-level coordination. Analysis of models that range from the minimal to the biologically faithful shows that, because the flagellar beating displays an adaptive down-regulation in response to light, the colony needs to spin around its swimming direction and that the response kinetics and natural spinning frequency of the colony appear to be mutually tuned to give the maximum photoresponse. These models further predict that the phototactic ability decreases dramatically when the colony does not spin at its natural frequency, a result confirmed by phototaxis assays in which colony rotation was slowed by increasing the fluid viscosity.adaptation | evolution | flagella | fluid dynamics | multicellularity