Diatoms are a group of photosynthetic microorganisms that play important roles in aquatic food webs and global biogeochemical cycles. They reside in the upper layer of oceans and lakes, and their interactions with the dissipation scales of turbulence govern a variety of processes, such as nutrient acquisition, prey–predator interactions, and aggregation. Interactions of cells with turbulence may also alter vertical motions and spatial distributions. However, observations of these interactions remain scarce because of the difficulty of generating ecologically relevant flows in the lab or tracking microscopic cells in the field. Here, we present an experimental system capable of measuring trajectories and spatial distributions of live diatom cells in turbulence. The small-volume turbulence tank uses stochastic forcing to restrict mean flow while producing homogeneous turbulence statistics at relatively high Reynolds number. Individual cell trajectories are tracked in three dimensions with a volumetric particle imager that uses the principle of defocused imaging combined with a double-pinhole aperture mask to obtain particle positions in three dimensions using a single camera. We conducted experiments at two different turbulence intensities and our results show that while root-mean-square velocities of diatoms are similar to those of tracer particles, their spatial distributions indicate enhanced clustering at the dissipation scales in comparison with tracer particles. This clustering behaviour is surprising both because diatom cells are characterised by a very low Stokes number and because clustering decreases when the turbulence intensity (and dissipation rate) increases. Several mechanisms could explain this result, including cell shape effects and active regulation of cell density in response to the ambient turbulence.
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The authors would like to acknowledge funding from the U.S. National Science Foundation (OCE-1334788 to Evan A. Variano and OCE-1334365 to Lee Karp-Boss and Peter Jumars). Additionally, we would like to extend our thanks to Peter Jumars for extensive discussions at the outset of this project and for providing comments on the manuscript, Laura Mazzaro for help with designing and constructing the prototype tank, and Morteza Gharib for fruitful conversations regarding the volumetric particle imager. We also acknowledge useful comments from anonymous reviewers.
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Pujara, N., Du Clos, K.T., Ayres, S. et al. Measurements of trajectories and spatial distributions of diatoms (Coscinodiscus spp.) at dissipation scales of turbulence. Exp Fluids 62, 149 (2021). https://doi.org/10.1007/s00348-021-03240-5