John O. Kessler

Disruption of Vertical Motility by Shear Triggers Formation of Thin Phytoplankton Layers

Thin layers of phytoplankton are important hotspots of ecological activity that are found in the coastal ocean, meters beneath the surface, and contain cell concentrations up to two orders of magnitude above ambient concentrations. Current interpretations of their formation favor abiotic processes, yet many phytoplankton species found in these layers are motile. We demonstrated that layers formed when the vertical migration of phytoplankton was disrupted by hydrodynamic shear. This mechanism, which we call gyrotactic trapping, can be responsible for the thin layers of phytoplankton commonly observed in the ocean. These results reveal that the coupling between active microorganism motility and ambient fluid motion can shape the macroscopic features of the marine ecological landscape.

Growth of bioconvection patterns in a suspension of gyrotactic micro-organisms in a layer of finite depth

The effect of gyrotaxis on the linear stability of a suspension of swimming, negatively buoyant micro-organisms is examined for a layer of finite depth. In the steady basic state there is no bulk fluid motion, and the upwards swimming of the cells is balanced by diffusion resulting from randomness in their shape, orientation and swimming behaviour. This leads to a bulk density stratification with denser fluid on top. The theory is based on the continuum model of Pedley, Hill & Kessler (1988), and employs both asymptotic and numerical analysis. The suspension is characterized by five dimensionless parameters : a Rayleigh number, a Schmidt number, a layer-depth parameter, a gyrotaxis number G, and a geometrical parameter measuring the ellipticity of the micro-organisms. For small values of G, the most unstable mode has a vanishing wavenumber, but for sufficiently large values of G, the predicted initial wavelength is finite, in agreement with experiments. The suspension becomes less stable as the layer depth is increased. Indeed, if the layer is sufficiently deep an initially homogeneous suspension is unstable, and the equilibrium state does not form. The theory of Pedley, Hill & Kessler (1988) for infinite depth is shown to be appropriate in that case. An unusual feature of the model is the existence of overstable or oscillatory modes which are driven by the gyrotactic response of the micro-organisms to the shear at the rigid boundaries of the layer. These modes occur at parameter values which could be realized in experiments.

Co-operative and concentrative phenomena of surmming micro-organisms

Abstract This paper describes linear and co-operative phenomena based on the dynamics of the trajectories of swimming cells. The swimming direction of algal cells can be guided so as to focus the c...

Confinement stabilizes a bacterial suspension into a spiral vortex.

Confining surfaces play crucial roles in dynamics, transport, and order in many physical systems, but their effects on active matter, a broad class of dynamically self-organizing systems, are poorly understood. We investigate here the influence of global confinement and surface curvature on collective motion by studying the flow and orientational order within small droplets of a dense bacterial suspension. The competition between radial confinement, self-propulsion, steric interactions, and hydrodynamics robustly induces an intriguing steady single-vortex state, in which cells align in inward spiraling patterns accompanied by a thin counterrotating boundary layer. A minimal continuum model is shown to be in good agreement with these observations.

A Hydrodynamics Approach to the Evolution of Multicellularity: Flagellar Motility and Germ‐Soma Differentiation in Volvocalean Green Algae

During the unicellular‐multicellular transition, there are opportunities and costs associated with larger size. We argue that germ‐soma separation evolved to counteract the increasing costs and requirements of larger multicellular colonies. Volvocalean green algae are uniquely suited for studying this transition because they range from unicells to multicellular individuals with germ‐soma separation. Because Volvocales need flagellar beating for movement and to avoid sinking, their motility is modeled and analyzed experimentally using standard hydrodynamics. We provide comparative hydrodynamic data of an algal lineage composed of organisms of different sizes and degrees of complexity. In agreement with and extending the insights of Koufopanou, we show that the increase in cell specialization as colony size increases can be explained in terms of increased motility requirements. First, as colony size increases, soma must evolve, the somatic‐to‐reproductive cell ratio increasing to keep colonies buoyant and motile. Second, increased germ‐soma specialization in larger colonies increases motility capabilities because internalization of nonflagellated germ cells decreases colony drag. Third, our analysis yields a limiting maximum size of the volvocalean spheroid that agrees with the sizes of the largest species known. Finally, the different colony designs in Volvocales reflect the trade‐offs between reproduction, colony size, and motility.

ORIENTATION OF SWIMMING FLAGELLATES BY SIMULTANEOUSLY ACTING EXTERNAL FACTORS1

The directionality of phototaxis combined with gravitaxis was investigated experimentally for populations of the swimming alga Euglena gracilis Klebs. Two irradiances were used: a “weak” irradiance to elicit positive phototaxis and a “strong” irradiance to elicit negative phototaxis. In addition, by changing the density of cells in the suspension, the number of collisions between cells was varied to determine the effects of these collisions on the distribution of swimming directions in both the absence and the presence of illumination. We found that positive phototaxis was associated with a broader distribution of swimming directions than was negative phototaxis. In the latter case, the effect of phototaxis dominated over that of gravitaxis. Experiments on another swimming alga, Chlamydomonas nivalis Wille, showed that collisions between cells degraded the directionality of gravitaxis.

Roles of root border cells in plant defense and regulation of rhizosphere microbial populations by extracellular DNA 'trapping'

BackgroundAs roots penetrate soil, specialized cells called ‘border cells’ separate from root caps and contribute a large proportion of exudates forming the rhizosphere. Their function has been unclear. Recent findings suggest that border cells act in a manner similar to that of white blood cells functioning in defense. Histone-linked extracellular DNA (exDNA) and proteins operate as ‘neutrophil extracellular traps’ to attract and immobilize animal pathogens. DNase treatment reverses trapping and impairs defense, and mutation of pathogen DNase results in loss of virulence.ScopeHistones are among a group of proteins secreted from living border cells. This observation led to the discovery that exDNA also functions in defense of root caps. Experiments revealed that exDNA is synthesized and exported into the surrounding mucilage which attracts, traps and immobilizes pathogens in a host-microbe specific manner. When this plant exDNA is degraded, the normal resistance of the root cap to infection is abolished.ConclusionsResearch to define how exDNA may operate in plant immunity is needed. In the meantime, the specificity and stability of exDNA and its association with distinct microbial species may provide an important new tool to monitor when, where, and how soil microbial populations become established as rhizosphere communities.

Gyrotactic Buoyant Convection and Spontaneous Pattern Formation in Algal Cell Cultures

Regular convection patters may form spontaneously in isothermal liquids which contain swimming microorganisms. The energy for this dissipative process is supplied by the swimmers. Individual cell trajectories are guided by gravity and vorticity so that cells accumulate toward regions of the liquid where the downstreaming velocity is a maximum. This concentrative mechanism, named “gyro-taxis”, has been proven by the demonstration that swimming cells focus at the axis of a downward cylindrical Poiseuille flow of the cell culture. Since the density of the cells exceeds that of the liquid in which they swim, gyrotaxis reinforces vorticity. This convection pattern producing system has been named “Gyrotactic Buoyant Convection (GBC). At sufficient average cell concentration, GBC can cause localised intermittent concentration pulses.

Thermotaxis, chemotaxis and age

It has been demonstrated that polymorphonuclear leukocytes migrate in vitro along a temperature gradient, i.e., that they exhibit positive thermotaxis. The effect varies monotonically with the temperature gradient, a fact which suggests that PMN thermotaxis may be important in nonspecific immune amplification. Thermotaxis and chemotaxis are synergistic: simultaneously applied codirectional chemotactic and thermotactic stimuli produce a motile response that is more than double the sum of the separate effects. Conversely, counter-directional stimuli produce inhibition. Chemotaxis correlates positively with age for cell donors less than 35 years old, but there is no significant correlation with age for the cell donors older than 61 years in this small pilot sample. Neither is there a significant correlation between chemotaxis and thermotaxis. Nonetheless, these preliminary data seem to open new avenues for the study of cell response to inflammation.

Motility, mixing, and multicellularity

A fundamental issue in evolutionary biology is the transition from unicellular to multicellular organisms, and the cellular differentiation that accompanies the increase in group size. Here we consider recent results on two types of “multicellular” systems, one produced by many unicellular organisms acting collectively, and another that is permanently multicellular. The former system is represented by groups of the bacterium Bacillus subtilis and the latter is represented by members of the colonial volvocalean green algae. In these flagellated organisms, the biology of chemotaxis, metabolism and cell–cell signaling is intimately connected to the physics of buoyancy, motility, diffusion, and mixing. Our results include the discovery in bacterial suspensions of intermittent episodes of disorder and collective coherence characterized by transient, recurring vortex streets and high-speed jets of cooperative swimming. These flow structures markedly enhance transport of passive tracers, and therefore likely have significant implications for intercellular communication. Experiments on the Volvocales reveal that the sterile flagellated somatic cells arrayed on the surface of Volvox colonies are not only important for allowing motion toward light (phototaxis), but also play a crucial role in driving fluid flows that transport dissolved molecular species. These flows, generated by the collective beating of flagella, confer a synergistic advantage with regard to transport of nutrients and chemical messengers. They allow these species to circumvent a nutrient acquisition bottleneck which would exist if transport were purely diffusive, and thereby evolve to larger multicellular individuals. In both cases, a higher level of organization, specialization and complexity counteract the higher costs inherent to larger groups.