Gyrotactic plankton cells in turbulence: the effects of motility, shape, fluid acceleration and inertia

A detailed understanding of the physical mechanisms driving gyrotactic species to migrate vertically towards the surface allows better quantification of biogeochemical fluxes across the ocean. We focus on marine phytoplankton cells that are motile under gyrotactic forcing. Some species spontaneously swim in the direction opposite to gravity [1]. Gyrotaxis is originating either from morphological aspects (elongated shape, density heterogeneity) or the coupled effect of swimming and settling which results in an inertial torque. Indeed, fluid inertial torque may have a potential impact on the gyrotaxis for elongated planktonic swimmers, especially for those forming long chains and thus having large swimming and settling speeds
Based on numerical simulations of hundreds of thousands of micro-organisms swimming in homogeneous isotropic turbulence, we will comment on the different sources of gyrotactic induced spatial clustering [2, 3] and vertical migration [4]. Some specific configurations lead to the accumulation of elongated plankton cells in upwelling flow regions enhancing their ability to move across turbulence through the water column.  

[1] Kessler J.O. (1985), Nature - 313, 218–220.
[2] Durham W. M., et al. (2013) Nat. Commun. - 4, 2148.
[3] De Lillo F., et al. (2014) Phys. Rev. Lett. – 112, 044502
[4] Lovecchio S., et al. (2019) Sci. Adv. - 5: eaaw7879