Abstract
In this study, we document hydrodynamics-mediated trapping of microorganisms around a moving spherical nutrient source such as a settling marine snow aggregate. There exists a range of size and excess density of the nutrient source, and motility and morphology of the microorganism under which hydrodynamic interactions enable the passive capture of approaching microorganisms onto a moving nutrient source. We simulate trajectories of chemotactic and non-chemotactic bacteria encountering a sinking marine snow particle effusing soluble nutrients. We calculate the average nutrient concentration to which the bacteria are exposed, under regimes of strong and weak hydrodynamic trapping. We find that hydrodynamic trapping can significantly amplify (by ≈40%) the nutrient exposure of bacteria, both chemotactic and non-chemotactic. The subtle interactions between hydrodynamic and chemotactic effects reveal non-trivial variations in this “hydrodynamic amplification,” as a function of relevant biophysical parameters. Our study provides a consistent description of how microorganism motility, fluid flow and nutrient distribution affect foraging by marine microbes, and the formation of biofilms on spherical nutrient sources under the influence of fluid flow.
Highlights
Chemotaxis–the directed motion of bacteria along favorable gradients in chemical concentration–is one of the primary mechanisms through which marine bacteria locate nutrition, from sources like phytoplankton, marine snow and oil drops (Stocker and Seymour, 2012)
The major bio-physical parameters, and their respective dimensionless representations in our study are: the bacterial dipole strength, αD = FD/(8π μb2Vs); the mean run-time of the bacterium, τ ∗ = τ0Vs/b; the rotational diffusivity of the bacterium, D = Drb/Vs; the nutrient’s molecular diffusivity DC, represented by the Schmidt number, Sc = ν/DC, where ν is the kinematic viscosity of the surrounding fluid; the radius of the settling aggregate R = a/b; and the excess density K ρ = 2 ρgb2/(9μVs)
The idea is that fluid flow caused by a bacterium, if strong enough, causes it to rotate toward a nearby rigid surface and approach it
Summary
Chemotaxis–the directed motion of bacteria along favorable gradients in chemical concentration–is one of the primary mechanisms through which marine bacteria locate nutrition, from sources like phytoplankton, marine snow and oil drops (Stocker and Seymour, 2012). Bacteria utilize a number of strategies, like “run-and-tumble” or “run-reverse-flick,” to bias their motion to chemical cues, and find and populate nutrient-rich regions in their environment (Berg, 2004; Stocker et al, 2008; Son et al, 2016; Desai and Ardekani, 2017). These strategies are actively regulated on the level of an individual cell, via chemosensing, i.e., feedback mechanisms involving membrane receptors and intracellular signals (Eisenbach et al, 2004). Microorganisms are translated and rotated by background flows and they undergo changes in their swimming motion by “interacting hydrodynamically” with their surroundings
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