Phytoplankton in flow: How cells adapt migration strategies to tackle turbulence
Phytoplankton are the basis of oceanic life and play a fundamental role in biogeochemical cycles. A commonly observed motility trait in phytoplankton is gravitaxis, the tendency of cells to align their swimming direction with (or against) gravity. Gravitaxis results from a reorienting torque that realigns organisms with the vertical direction. Different explanations for this phenomenon have been proposed, such as a non-homogeneous distribution of body mass (i.e., heavier or lighter organelles located off-center), an asymmetry in cell shape (which causes a realigning torque due to asymmetric drag), or an active gravi-sensing mechanism. Phytoplankton are ubiquitously exposed to turbulence, which is a primary determinant of their fitness, yet, mechanisms by which phytoplankton may adapt to turbulence have remained unknown. To answer to this question, we combined experiments with a mechanistic model of gravitactic cells exposed to turbulent conditions. We engineered a new experimental setup that allowed us the study of the toxic marine alga Heterosigma akashiwo, known to exhibit negative gravitaxis, under turbulent environments. To mimic the effect of Kolmogorov-scale turbulent eddies, which expose cells to repeated reorientations, we observed H. akashiwo in a ``flip chamber,'' whose orientation could be automatically flipped.
We present experiments that demonstrate how phytoplankton are capable of rapid adaptive behavior in response to fluid flow disturbances that mimic turbulence. Tracking of single cells revealed a striking, robust behavioral adaptation, whereby within tens of minutes half of the population reversed its direction of migration to swim downwards, demonstrating an active response to fluid flow. Our mechanistic model provides a rationalization of this phenomenon, and show how morphological adaptation of H. akashiwo cells underpins behavioral adaptation in turbulent flows. We further speculate on the motives for this bet-hedging-type strategy, which has considerable implications for some of the ocean's most important organisms.