The fluid mechanics of marine microbes: Phytoplankton gyrotaxis and bacterial rheotaxis

mso-ansi-language:EN-US;mso-fareast-language:EN-US;mso-bidi-language:AR-SA">Although we now know that microorganisms rule the oceans - controlling productivity and biogeochemical cycles - we largely ignore how they are affected by typical flow conditions. Here I present microfluidic and millifluidic experiments, combined with mathematical models, to show that fluid flow can have profound effects on the biomechanics of swimming microorganisms. I illustrate this for two cases of directed motility, or 'taxis'. In the first case - 'gyrotaxis' - the coupling of hydrodynamic shear and bottom-heaviness, typical of many phytoplankton species, causes intense clustering of cells in layers and patches, which can have profound effects on population dynamics. In the second case - 'rheotaxis' - the coupling of shear and the chiral shape of bacterial flagella leads to a previously unrecognized torque on cells, which can hamper motility and thus foraging in the sea.