Taxis in density gradients

Many organisms inhabit inhomogeneous environments such as those with gradients of heat, light, nutrients, fluid viscosity or density. They react to these inhomogeneities by reorienting and changing speed, often exhibiting a directed migration termed taxis. For instance, E. coli reorients to swim up nutrient gradients but swims down light or viscosity gradients. Here, we demonstrate a new, relatively unexplored taxis in density gradients, which are prevalent in oceans, lakes, and ponds. This taxi is sensitive to whether the organisms generate thrust in front (so-called pullers) or back (pushers). Pullers, like Chlamydomonas reinhardtii for instance, reorient to swim up or down the gradients, depending on the initial orientation. But pushers, like E. coli, rotate to swim normal to the gradients. This densitaxis could help explain organism behaviors in the ocean or be leveraged to sort or organize a suspension of organisms.