Migration of Gyrotactic Micro-Organisms in Water

Xiao Chen,Li Zeng,Yihong Wu

doi:10.3390/w10101455

Xiao Chen, Li Zeng + Show 1 more

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https://doi.org/10.3390/w10101455

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Abstract

Understanding the swimming characteristics of micro-organisms is significant for modelling the migration of motile cells and corresponding ecological risk assessments associated with harmful algae in oceans and estuaries. Presented in this paper is an experimental and numerical investigation of swimming characteristics of a typical gyrotactic micro-organism, Heterosigma akashiwo (H. akashiwo) in water, based on the technology of planar laser-induced fluorescence and the finite volume method. Two-dimensional swimming velocity of algal cells are obtained by analyzing cells’ trajectories in the vertical plane, and three-dimensional swimming velocity is reconstructed based on the assumption that cells’ swimming is isotropic in the horizontal plane. Four important parameters are given to reflect the swimming characteristics of gyrotactic cells in still water, including the mean swimming speed (Vs = 146 μm/s), the relative strength of reorientation by gravitational torque to rotational diffusion (λ = 1.96), the time scale of reorientation (B = 5.6 s), and rotational diffusivity (Dr = 0.046 rad2/s). A database of the ambient vorticity, mean swimming velocity and diffusivity tensor is established, by solving Fokker-Planck equation for the probability density function of cells’ swimming under the combined action of gravity, rotational diffusion, and the ambient vorticity. The mean swimming velocity and translational diffusion tensor of H. akashiwo are found to change with the horizontal and vertical vorticity. It is also shown that gyrotactic cells swim in a given direction for a weak horizontal vorticity, in contrast to cells’ tumbling and being trapped for a strong horizontal vorticity.

Highlights

Gyrotactic micro-organisms are one type of motile micro-organisms widely existing in aqueous environments, such as oceans, lakes, rivers, and reservoirs, which can swim in a particular direction in fluid flowing with a weak horizontal vorticity [1]
The swimming characteristics of H. akashiwo in still water and a three-dimensional vorticity field were investigated based on the laser-based tracking method and the finite volume method
The results showed that the peak of fluorescence spectrum of H. akashiwo locates at 685 nm under the illumination of 450 nm laser

Summary

Introduction

Gyrotactic micro-organisms are one type of motile micro-organisms widely existing in aqueous environments, such as oceans, lakes, rivers, and reservoirs, which can swim in a particular direction in fluid flowing with a weak horizontal vorticity (or tumble unsteadily in the fluid with strong horizontal vorticity) [1]. The swimming characteristics of gyrotactic micro-organisms are essential to understand various ecological phenomena associated with bioconvection [1,2,3,4], algal patches [5,6], thin phytoplankton layers [7], harmful algal blooms [8,9,10], etc. Regarding the swimming characteristics of gyrotactic micro-organisms, there have been various theoretical, numerical, and experimental studies with focus on typical species, such as Heterosigma akashiwo [11,12], Chlamydomonas nivalis (C. nivalis) [1,2,13,14], Chlamydomonas reinhardtii (C. reinhardtii) [15,16,17], Dunaliella salina (D. salina) [18], etc. H. akashiwo is one typical gyrotactic micro-organism in red tide, which is a serious threat for fish, shellfish, etc. Previous studies showed that the swimming behavior of H. akashiwo, such as, swimming modes, swimming speed, vertical swimming velocity, and turning rate were influenced by light, halocline [21], pH [22], salinity [8,23], predators [12,23,24], and strains [25,26].

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