Sharp turns and gyrotaxis modulate surface accumulation of microorganisms

Abstract

The accumulation of swimming microorganisms at surfaces is an essential feature of various physical, chemical, and biological processes in confined spaces. To date, this accumulation is mainly assumed to depend on the change of swimming speed and angular velocity caused by cell-wall contact and hydrodynamic interaction. Here, we measured the swimming trajectories of Heterosigma akashiwo (a biflagellate marine alga) near vertical and horizontal rigid boundaries. We observed that the probability of sharp turns is greatly increased near a vertical wall, resulting in significant changes in the distributions of average swimming speed, angular velocity, and rotational diffusivity near the wall (a quantity that has not previously been investigated) as functions of both distance from the wall and swimming orientation. These cannot be satisfactorily explained by standard hydrodynamic models. Detailed examination of an individual cell trajectory shows that wall contact by the leading flagellum triggers complex changes in the behavior of both flagella that cannot be incorporated in a mechanistic model. Our individual-based model for predicting cell concentration using the measured distributions of swimming speed, angular velocity, and rotational diffusivity agrees well with the experiment. The experiments and model are repeated for a cell suspension in a vertical plane, bounded above by a horizontal wall. The cell accumulation beneath the wall, expected from gyrotaxis, is considerably amplified by cell-wall interaction. These findings may shed light on the prediction and control of cell distribution mediated by gyrotaxis and cell-wall contact.

Dispersion of a gyrotactic micro-organism suspension in a vertical pipe: the buoyancy–flow coupling effect

“…Although somewhat disappointed, we still advocate using the Smoluchowski equation in relevant problems, at least as a benchmark test, because we have shown the inaccuracy of the GTD model when the assumptions associated with orientation-position separation are not met. The fact that the experimental results do not agree satisfactorily with the GTD model and the Smoluchowski equation is open to many possible explanations, such as heterogeneity of micro-organisms (Bees, Hill & Pedley 1998), cell-cell interactions in the central core and the microscopic kinematic model of the cell (Omori et al 2022;Walker et al 2022;Zeng, Jiang & Pedley 2022), which deserve in-depth research.…”

Pre-asymptotic dispersion of active particles through a vertical pipe: the origin of hydrodynamic focusing

“…Third, idealised reflective boundary conditions are adopted, which guarantees only a zero probability flux normal to walls. For realistic cell–wall interaction, more complicated behaviour is observed for micro-organisms near the wall (Zeng, Jiang & Pedley 2022). Appropriate boundary conditions and dynamical description for the cell–cell, cell–wall and cell–fluid interactions are still open questions.…”

Streamwise dispersion of soluble matter in solvent flowing through a tube

“…Our generalisation lies in incorporating the local corrections of spatial concentration moments other than mean concentration moments, and Edgeworth modifications to determine the coefficients of comparable magnitude, with the understanding that the interchange of averaging operation and differential operator make remarkable differences. Since this spirit comes from first principles, it could have wide adaptability to the exiting dispersion models (Chatwin 1970;Gill et al 1970;Frankel & Brenner 1989) and generalised transport problem (Christov & Stone 2012;Golestanian 2012;Masoud & Stone 2019;Morris 2020;Zeng, Jiang & Pedley 2022), including diffusion of granular materials, heat transfer, and pre-asymptotic active dispersion of active colloids and micro-organisms.…”

“…Since this spirit comes from first principles, it could have wide adaptability to the exiting dispersion models (Chatwin 1970; Gill et al. 1970; Frankel & Brenner 1989) and generalised transport problem (Christov & Stone 2012; Golestanian 2012; Masoud & Stone 2019; Morris 2020; Zeng, Jiang & Pedley 2022), including diffusion of granular materials, heat transfer, and pre-asymptotic active dispersion of active colloids and micro-organisms.…”