1,721,152 research outputs found
Reverse and flick: Hybrid locomotion in bacteria
No full textMany bacteria are motile. They use one or more helical flagella as propellers, rotating them like the corkscrew on a wine bottle opener. Despite the limited morphological repertoire of the propulsive system, radically different movement strategies have evolved, likely reflecting the diversity of physicochemical conditions among bacterial habitats. In PNAS, Xie et al. (1) report on a newly discovered mechanism for turning used by Vibrio alginolyticus, an inhabitant of the coastal ocean: These monotrichous (“single-haired”) bacteria change direction with a “flick” of their flagellum. Intriguingly, Xie et al. (1) show that less can be more when it comes to bacterial flagella: With its single flagellum, V. alginolyticus outperforms the multiflagellated Escherichia coli in climbing nutrient gradients (“chemotaxis”), suggesting that the flick is part of an advanced chemotaxis system.National Institutes of Health (U.S.) (1-R21-EB008844)National Science Foundation (U.S.) (OCE-0744641-CAREER
Low-Reynolds-number swimming at pycnoclines
No full textMicroorganisms play pivotal functions in the trophic dynamics and biogeochemistry of aquatic ecosystems. Their concentrations and activities often peak at localized hotspots, an important example of which are pycnoclines, where water density increases sharply with depth due to gradients in temperature or salinity. At pycnoclines organisms are exposed to different environmental conditions compared to the bulk water column, including reduced turbulence, slow mass transfer, and high particle and predator concentrations. Here we show that, at an even more fundamental level, the density stratification itself can affect microbial ecology at pycnoclines, by quenching the flow signature, increasing the energetic expenditure, and stifling the nutrient uptake of motile organisms. We demonstrate this through numerical simulations of an archetypal low-Reynolds-number swimmer, the “squirmer.” We identify the Richardson number—the ratio of buoyancy forces to viscous forces—as the fundamental parameter that quantifies the effects of stratification. These results demonstrate an unexpected effect of buoyancy on low-Reynolds-number swimming, potentially affecting a broad range of abundant organisms living at pycnoclines in oceans and lakes.National Science Foundation (U.S.) (Grant CBET-1066545
Microfluidics Expanding the Frontiers of Microbial Ecology
Full text linkMicrofluidics has significantly contributed to the expansion of the frontiers of microbial ecology over the past decade by allowing researchers to observe the behaviors of microbes in highly controlled microenvironments, across scales from a single cell to mixed communities. Spatially and temporally varying distributions of organisms and chemical cues that mimic natural microbial habitats can now be established by exploiting physics at the micrometer scale and by incorporating structures with specific geometries and materials. In this article, we review applications of microfluidics that have resulted in insightful discoveries on fundamental aspects of microbial life, ranging from growth and sensing to cell-cell interactions and population dynamics. We anticipate that this flexible multidisciplinary technology will continue to facilitate discoveries regarding the ecology of microorganisms and help uncover strategies to control microbial processes such as biofilm formation and antibiotic resistance.National Science Foundation (U.S.) (Grant OCE-0744641-CAREER)National Science Foundation (U.S.) (Grant IOS-1120200)National Science Foundation (U.S.) (Grant CBET-1066566)National Science Foundation (U.S.) (Grant CBET-0966000)National Institutes of Health (U.S.) (NIH grant 1R01GM100473-0)Human Frontier Science Program (Strasbourg, France)Human Frontier Science Program (Strasbourg, France) (award RGY0089)Gordon and Betty Moore Foundation (Microbial Initiative Investigator Award
Speed-dependent chemotactic precision in marine bacteria
Full text linkChemotaxis underpins important ecological processes in marine bacteria, from the association with primary producers to the colonization of particles and hosts. Marine bacteria often swim with a single flagellum at high speeds, alternating "runs" with either 180° reversals or ∼90° "flicks," the latter resulting from a buckling instability of the flagellum. These adaptations diverge from Escherichia coli's classic run-And-Tumble motility, yet how they relate to the strong and rapid chemotaxis characteristic of marine bacteria has remained unknown. We investigated the relationship between swimming speed, run-reverse-flick motility, and high-performance chemotaxis by tracking thousands of Vibrio alginolyticus cells in microfluidic gradients. At odds with current chemotaxis models, we found that chemotactic precision the strength of accumulation of cells at the peak of a gradient is swimming-speed dependent in V. alginolyticus. Faster cells accumulate twofold more tightly by chemotaxis compared with slower cells, attaining an advantage in the exploitation of a resource additional to that of faster gradient climbing. Trajectory analysis and an agent-basedmathematicalmodel revealed that this unexpected advantage originates from a speed dependence of reorientation frequency and flicking, which were higher for faster cells, and was compounded by chemokinesis, an increase in speedwith resource concentration. The absence of any one of these adaptations led to a 65-70% reduction in the populationlevel resource exposure. These findings indicate that, contrary to what occurs in E. coli, swimming speed can be a fundamental determinant of the gradient-seeking capabilities of marine bacteria, and suggest a new model of bacterial chemotaxis. Keywords: ocean; motility; run-reverse-flick; chemotaxis; chemokinesisNational Institutes of Health (U.S.) (Grant 1R01GM100473)Gordon and Betty Moore Foundation (Award GBMF3783
Generalized receptor law governs phototaxis in the Phytoplankton Euglena gracilis
Full text linkPhototaxis, the process through which motile organisms direct their swimming toward or away from light, is implicated in key ecological phenomena (including algal blooms and diel vertical migration) that shape the distribution, diversity, and productivity of phytoplankton and thus energy transfer to higher trophic levels in aquatic ecosystems. Phototaxis also finds important applications in biofuel reactors and microbiopropellers and is argued to serve as a benchmark for the study of biological invasions in heterogeneous environments owing to the ease of generating stochastic light fields. Despite its ecological and technological relevance, an experimentally tested, general theoretical model of phototaxis seems unavailable to date. Here, we present accurate measurements of the behavior of the alga Euglena gracilis when exposed to controlled light fields. Analysis of E. gracilis’ phototactic accumulation dynamics over a broad range of light intensities proves that the classic Keller–Segel mathematical framework for taxis provides an accurate description of both positive and negative phototaxis only when phototactic sensitivity is modeled by a generalized “receptor law,” a specific nonlinear response function to light intensity that drives algae toward beneficial light conditions and away from harmful ones. The proposed phototactic model captures the temporal dynamics of both cells’ accumulation toward light sources and their dispersion upon light cessation. The model could thus be of use in integrating models of vertical phytoplankton migrations in marine and freshwater ecosystems, and in the design of bioreactors
Chemotaxis toward phytoplankton drives organic matter partitioning among marine bacteria
No full textThe microenvironment surrounding individual phytoplankton cells is often rich in dissolved organic matter (DOM), which can attract bacteria by chemotaxis. These “phycospheres” may be prominent sources of resource heterogeneity in the ocean, affecting the growth of bacterial populations and the fate of DOM. However, these effects remain poorly quantified due to a lack of quantitative ecological frameworks. Here, we used video microscopy to dissect with unprecedented resolution the chemotactic accumulation of marine bacteria around individual Chaetoceros affinis diatoms undergoing lysis. The observed spatiotemporal distribution of bacteria was used in a resource utilization model to map the conditions under which competition between different bacterial groups favors chemotaxis. The model predicts that chemotactic, copiotrophic populations outcompete nonmotile, oligotrophic populations during diatom blooms and bloom collapse conditions, resulting in an increase in the ratio of motile to nonmotile cells and in the succession of populations. Partitioning of DOM between the two populations is strongly dependent on the overall concentration of bacteria and the diffusivity of different DOM substances, and within each population, the growth benefit from phycospheres is experienced by only a small fraction of cells. By informing a DOM utilization model with highly resolved behavioral data, the hybrid approach used here represents a new path toward the elusive goal of predicting the consequences of microscale interactions in the ocean.National Science Foundation (U.S.) (Ocean Sciences Postdoctoral Fellowship)Gordon and Betty Moore Foundation (Marine Microbial Initiative Investigator Award GBMF 3783
Enhanced drag of a sphere settling in a stratified fluid at small Reynolds numbers
No full textWe present a combined experimental and numerical investigation of a sphere settling in a linearly stratified fluid at small Reynolds numbers. Using time-lapse photography and numerical modelling, we observed and quantified an increase in drag due to stratification. For a salt stratification, the normalized added drag coefficient scales as Ri[superscript 0.51], where R i = a[superscript 3]N[superscript 2]/(νU) is the viscous Richardson number, a the particle radius, U its speed, ν the kinematic fluid viscosity and N the buoyancy frequency. Microscale synthetic schlieren revealed that a settling sphere draws lighter fluid downwards, resulting in a density wake extending tens of particle radii. Analysis of the flow and density fields shows that the added drag results from the buoyancy of the fluid in a region of size (ν/N)[superscript 1/2] surrounding the sphere, while the bulk of the wake does not influence drag. A scaling argument is provided to rationalize the observations. The enhanced drag can increase settling times in natural aquatic environments, affecting retention of particles at density interfaces and vertical fluxes of organic matter.National Science Foundation (U.S.) (OCE-0526241)National Science Foundation (U.S.) (OCE-6917641 CAREER
Filtration of submicrometer particles by pelagic tunicates
Full text linkAuthor Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 107 (2010): 15129-15134, doi:10.1073/pnas.1003599107.Salps are common in oceanic waters and have higher per individual filtration rates than any other
zooplankton filter feeder. Though salps are centimeters in length, feeding via particle capture
occurs on a fine, mucous mesh (fiber diameter d ~ 0.1 μm) at low velocity (U = 1.6 ± 0.6 cm s-1,
mean ± SD) and is thus a low-Reynolds number (Re ~ 10-3) process. In contrast to the current
view that particle encounter is dictated by simple sieving of particles larger than the mesh
spacing, a low-Re mathematical model of encounter rates by the salp feeding apparatus for
realistic oceanic particle size distributions shows that submicron particles, due to their higher
abundances, are encountered at higher rates (particles per time) than larger particles. Data from
feeding experiments with 0.5, 1 and 3 μm diameter polystyrene spheres corroborate these results.
Though particles larger than 1 μm (e.g. flagellates, small diatoms) represent a larger carbon pool,
smaller particles in the 0.1–1 μm range (e.g. bacteria, Prochlorococcus) may be more quickly
digestible because they present more surface area, and we find that particles smaller than the
mesh size (1.4 μm) can fully satisfy salp energetic needs. Furthermore, by packaging
submicrometer particles into rapidly sinking fecal pellets, pelagic tunicates can substantially
change particle size spectra and increase downward fluxes in the ocean.This work was supported by the National Science
Foundation (OCE-0647723 to LPM and OCE-074464- CAREER to RS) and the WHOI Ocean
Life Institute
A coral-on-a-chip microfluidic platform enabling live-imaging microscopy of reef-building corals
Full text linkCoral reefs, and the unique ecosystems they support, are facing severe threats by human activities and climate change. Our understanding of these threats is hampered by the lack of robust approaches for studying the micro-scale interactions between corals and their environment. Here we present an experimental platform, coral-on-a-chip, combining micropropagation and microfluidics to allow direct microscopic study of live coral polyps. The small and transparent coral micropropagates are ideally suited for live-imaging microscopy, while the microfluidic platform facilitates long-term visualization under controlled environmental conditions. We demonstrate the usefulness of this approach by imaging coral micropropagates at previously unattainable spatio-temporal resolutions, providing new insights into several micro-scale processes including coral calcification, coral-pathogen interaction and the loss of algal symbionts (coral bleaching). Coral-on-a-chip thus provides a powerful method for studying coral physiology in vivo at the micro-scale, opening new vistas in coral biology
Intermittent turbulence in flowing bacterial suspensions
No full textDense suspensions of motile bacteria, possibly including the human gut microbiome, exhibit collective dynamics akin to those observed in classic, high Reynolds number turbulence with important implications for chemical and biological transport, yet this analogy has remained primarily qualitative. Here, we present experiments in which a dense suspension of Bacillus subtilis bacteria was flowed through microchannels and the velocity statistics of the flowing suspension were quantified using a recently developed velocimetry technique coupled with vortex identification methods. Observations revealed a robust intermittency phenomenon, whereby the average velocity profile of the suspension fluctuated between a plug-like flow and a parabolic flow profile. This intermittency is a hallmark of the onset of classic turbulence and Lagrangian tracking revealed that it here originates from the presence of transient vortices in the active, collective motion of the bacteria locally reinforcing the externally imposed flow. These results link together two entirely different manifestations of turbulence and show the potential of the microfluidic approach to mimic the environment characteristic of certain niches of the human microbiome
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