Gyrotactic bioconvection at pycnoclines

Abstract

Bioconvection is an important phenomenon in aquatic environments, affecting the spatial distribution of motile micro-organisms and enhancing mixing within the fluid. However, stratification arising from thermal or solutal gradients can play a pivotal role in suppressing the bioconvective flows, leading to the aggregation of micro-organisms and growth of their patchiness. We investigate the combined effects by considering gyrotactic motility where the up-swimming cells are directed by the balance of the viscous and gravitational torques. To study this system, we employ a continuum model consisting of Navier-Stokes equations with the Boussinesq approximation coupled with two conservation equations for the concentration of cells and stratification agent. We present a linear stability analysis to determine the onset of bioconvection for different flow parameters. Also, using large-scale numerical simulations, we explore different regimes of the flow by varying the corresponding boundary conditions and dimensionless variables such as Rayleigh number and Lewis number (Le) and we show that the cell distribution can be characterized using the ratio of the buoyancy forces as the determinant parameter when Le < 1 and the boundaries are insulated. But, in thermally stratified fluids corresponding to Le > 1, temperature gradients are demonstrated to have little impact on the bioconvective plumes provided that the walls are thermally insulated. In addition, we analyse the dynamical behaviour of the system in the case of persistent pycnoclines corresponding to constant salinity boundary conditions and we discuss the associated inhibition threshold of bioconvection in the light of the stability of linearized solutions.

Growth and diel vertical migration patterns of the toxic dinoflagellate Protoceratium reticulatum in a water column with salinity stratification: the role of bioconvection and light

“…We could not observe any morphological changes during the experiments, such as resting stages and/or cysts, which could otherwise have stopped the bioconvection pattern due to the immobility of resting stages (Han et al 2002) or led to increased descent/sinking of cysts down below the halocline (Anderson et al 1985). Karimi & Ardekani (2013) applied a continuum model based upon Navier-Stokes equations together with linear stability analysis and large-scale 3-dimensional numerical simulations to foresee how gyrotactic bioconvection could affect spatial distribution of micro organisms in a water column with a constant linear salinity gradient. They concluded that the downwards extension of the bioconvection plume is regulated by the pycnocline.…”

“…A prerequisite in this context is that the swimming cells are bottom heavy (like P. reticulatum) and thereby are able to orient themselves by a balance between viscous and gravitational torque (Pedley & Kessler 1992, Karimi & Ardekani 2013, Zhan et al 2014. The viscous torque is due to the vertical shear force formed between the dense falling plume and the upwards swimming cells.…”

“…Bioconvection and fluid mechanics theory have been applied to explain such phenomena (Pedley & Kessler 1992, Vincent & Hill 1996, Ghorai & Hill 2005, 2008, Karimi & Ardekani 2013, Zhan et al 2014, and gyrotaxis is considered to be the main regulatory mechanism for upwardsswimming cells (Kessler 1985, Mitchell et al 1990. Bioconvection therefore describes a type of convection set up by density instability at the surface due to dense cell aggregations formed by upwardsswimming microalgae.…”

Nutrient Transport Driven by Microbial Active Carpets

“…The collective motion of microorganisms and active colloids has sparked great interest, as biological functions can emerge from self-organisation of local power injection [1][2][3][4][5][6][7][8][9]. To sustain these processes, self-propelled particles increase nutrient uptake [10][11][12][13] and redistribute oxygen [14] by hydrodynamically enhanced mixing [15][16][17], bioconvection [18][19][20], and particle entrainment [21][22][23][24]. The vast majority of these flow-driving swimmers accumulate at surfaces [25][26][27][28][29][30][31][32][33][34][35], at concentrations an order of magnitude larger than in the bulk [25,27,30], and thus form 'active carpets'.…”

“…. (a) Spatial correlation functions of vertical flows generated by a carpet of bacterial turbulence, gv z (ρ), for heights z ∈[2,20] µm (blue-red). Same asFig.…”