  Marine plankton are usually modeled as settling elongated micro-swimmers. For
the first time, we consider the torque induced by fluid inertia on such
swimmers, and we discover that they spontaneously swim in the direction
opposite to gravity. We analyze the equilibrium orientation of swimmers in
quiescent fluid and the mean orientation in turbulent flows using direct
numerical simulations. Similar to well-known gyrotaxis mechanisms, the effect
of fluid inertial torque can be quantified by an effective reorientation time
scale. We show that the orientation of swimmers strongly depends on the
reorientation time scale, and swimmers exhibit strong preferential alignment in
upward direction when the time scale is of the same order of Kolmogorov time
scale. Our findings suggest that the fluid inertial torque is a new mechanism
of gyrotaxis that stabilizes the upward orientation of micro-swimmers such as
plankton.
