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.

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 ...

Interactions between swimming cells and surfaces are essential to many microbiological processes, from bacterial biofilm formation to human fertilization. However, despite 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 of 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 pullertype algae and other motile eukaryotes, because 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 experimentally verify the existence of optimal microfluidic ratchets that maximize rectification of initially uniform Chlamydomonas reinhardtii suspensions. Because mechano-elastic properties of cilia are conserved across eukaryotic species, we expect that our results apply to a wide range of swimming microorganisms.algal surface accumulation | swimming rectification S urface interactions of motile cells play crucial roles in a wide range of microbiological phenomena, perhaps most prominently in the formation of biofilms (1) and during the fertilization of mammalian ova (2). However, despite their widely recognized importance, the basic physical mechanisms that govern the response of swimming bacteria, algae, or spermatozoa to solid surfaces have remained unclear. This predicament is exemplified by the current debate (3-6) about the relevance of hydrodynamic long-range forces and steric short-range interactions for the accumulation of flagellated cells at liquid-solid interfaces. From a general perspective, improving our understanding of cell surface scattering processes promises not only new insights into structure, dynamics, and biological functions of flagella and cilia, it will also help to advance microfluidic techniques for controlling microbial locomotion (7,8), with potential applications in diagnostics (9), therapeutic protein synthesis (10), and photosynthetic biofuel production (11-14). That microfluidic circuits provide an excellent test bed for developing and assessing new strategies for the control of cell motility was recently demonstrated by the rectification of ...

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.

Flows generated by ensembles of flagella are crucial to development, motility and sensing, but the mechanisms behind this striking coordination remain unclear. We present novel experiments in which two micropipette-held somatic cells of Volvox carteri, with distinct intrinsic beating frequencies, are studied by high-speed imaging as a function of their separation and orientation. Analysis of time series shows that the interflagellar coupling, constrained by lack of connections between cells to be hydrodynamical, exhibits a spatial dependence consistent with theory. At close spacings it produces robust synchrony for thousands of beats, while at increasing separations synchrony is degraded by stochastic processes. Manipulation of the relative flagellar orientation reveals in-phase and antiphase states, consistent with dynamical theories. Flagellar tracking with exquisite precision reveals waveform changes that result from hydrodynamic coupling. This study proves unequivocally that flagella coupled solely through a fluid can achieve robust synchrony despite differences in their intrinsic properties.DOI: http://dx.doi.org/10.7554/eLife.02750.001