Abstract
Plankton live in dynamic fluid environments. Their ability to change in response to different hydrodynamic cues is critical to their energy allocation and resource uptake. This study used a microfluidic device to evaluate the rheotactic behaviors of a model dinoflagellate species, Karlodinium veneficum, in different flow conditions. Although dinoflagellates experienced forced alignment in strong shear (i.e. “trapping”), fluid straining did not play a decisive role in their rheotactic movements. Moderate hydrodynamic magnitude (20 < |uf| < 40 µm s−1) was found to induce an orientation heading towards an oncoming current (positive rheotaxis), as dinoflagellates switched to cross-flow swimming when flow speed exceeded 50 µm s−1. Near the sidewalls of the main channel, the steric mechanism enabled dinoflagellates to adapt upstream orientation through vertical migration. Under oscillatory flow, however, positive rheotaxis dominated with occasional diversion. The varying flow facilitated upstream exploration with directional controlling, through which dinoflagellates exhibited avoidance of both large-amplitude perturbance and very stagnant zones. In the mixed layer where water is not steady, these rheotactic responses could lead to spatial heterogeneity of dinoflagellates. The outcome of this study helps clarify the interaction between swimming behaviors of dinoflagellates and the hydrodynamic environment they reside in.
Highlights
Our results suggest that dinoflagellates K. veneficum actively changed their rheotactic movement in response to a highly variable microenvironment, and the unsteadiness of the hydrodynamic condition played a critical role in their swimming behaviors, which could spontaneously contribute to spatial heterogeneity
Comparing the dinoflagellate trajectories and the distribution of flow field led to an approximate threshold of flow speed around 75 μm s −1 that divided the two types of trajectories, above which the hydrodynamic effect on the movement of dinoflagellates was apparent
Since dinoflagellates located in the near-wall region of the main channel experienced low fluid inertia, their preferential orientation favored to sustain vertical migration that weakly positive rheotaxis could be maintained with the aid of the steric mechanism (Fig. 6A)
Summary
Plankton can even form aggregation at frontal zones by swimming against vertical c urrents[14,15] This seems to suggest that the rheotactic reaction, i.e. the responsive orientation of a microorganism to the flow, can contribute significantly to spatial heterogeneity of planktonic microorganisms. We investigate the rheotactic characteristics of a model dinoflagellate, Karlodinium veneficum (CCMP426), by examining their swimming behaviors and orientation dynamics in various flow conditions. Our results suggest that dinoflagellates K. veneficum actively changed their rheotactic movement in response to a highly variable microenvironment, and the unsteadiness of the hydrodynamic condition played a critical role in their swimming behaviors, which could spontaneously contribute to spatial heterogeneity
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