Phytoplankton are photosynthetic microorganisms that form the base of the marine food web. Many species swim using flexible flagella and can reach remarkable speeds (~ 1 mm/s) in relation to their small size (often 5-50 microns). Despite being much slower than ocean currents or turbulence, their motility can dramatically affect their spatial distribution: they are not purely at the mercy of the flow! Using a combination of millifluidic experiments and mathematical modeling, we have shown that their motility (biased by inherent asymmetries in their morphology through a process called gyrotaxis) can explain the occurrence of dramatic thin layers of high phytoplankton concentration, often observed in the ocean a few meters below the surface and thought to be precursors of red tides. We also discovered that turbulence results in strong, microscale patchiness in the distribution of phytoplankton, depending on how fast cells swim and how stable they are against overturning by the flow. In a related project, we found that the coupling of shape and flow could change light climate in the ocean and increase the optical backscattering of natural plankton assemblages, because of the preferential alignment of elongated plankton induced by fluid shear. Currently, we are learning more about phytoplankton’s swimming strategies by using digital holographic microscopy to obtain three-dimensional trajectories of individual cells.

Now working on this theme: Mike Barry (previously: Mack Durham).