Abstract
ABSTRACTSwimming microalgae show various taxes, such as phototaxis and gravitaxis, which sometimes result in the formation of a cell-rich layer or a patch in a suspension. Despite intensive studies on the effects of shear flow and turbulence on the inhomogeneous distribution of microalgae, the effect of a bubble plume has remained unclear. In this study, we used Chlamydomonas as model microalgae, and investigated the spatial distribution of cells in a cylindrical container with a bubble plume. The results illustrate that cells become inhomogeneously distributed in the suspension due to their motility and photo-responses. A vortical ring distribution was observed below the free surface when the bubble flow rate was sufficiently small. We performed a scaling analysis on the length scale of the vortical ring, which captured the main features of the experimental results. These findings are important in understanding transport phenomena in a microalgae suspension with a bubble plume.
Highlights
Microalgae photosynthesize; they contribute to the ecosystem of a pond, a lake, or the ocean as a primary producer that synthesizes organic substances from inorganic ones using the energy of light, simultaneously generating oxygen, through the chemical reactions of photosynthesis
When Q=500 μl/min (Fig. 6C,D), the vortical ring expanded to the size of D. These results indicate that the length scale of the vortical ring of cells increased as the bubble flow rate increased
The results show that L of the vortical ring of cells was smaller than that of the background flow, especially in the small Q regime
Summary
Microalgae photosynthesize; they contribute to the ecosystem of a pond, a lake, or the ocean as a primary producer that synthesizes organic substances from inorganic ones using the energy of light, simultaneously generating oxygen, through the chemical reactions of photosynthesis. The spatial distribution of motile microalgae in a suspension is not uniform. ‘red tides’ in the ocean are caused by accumulation of microalgae, resulting in large-scale coloration of the surface water (Wyatt and Horwood, 1973). In such an accumulated region, cells intensively uptake nutrients and disturb the transmission of light. Algal growth rates would be expected to be strongly affected by their own spatial distribution
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