In two papers appearing in March issues of the Proceedings of the National Academy of Sciences, Professor Roman Stocker and co-authors explore how the environment influences the movements of marine plankton. In “Low-Reynolds-Number Swimming at Pycnoclines” (layers of ocean water where density increases sharply with depth), the researchers describe how buoyancy increases the energy expenditure of motile microbes and stifles their nutrient uptake, but also increases their stealth, making their movements more difficult for predators to detect. In “Bacterial Rheotaxis,” the scientists discovered that a subtle interplay between a flow velocity gradient and the helical shape of a bacterium’s flagella generates a torque that alters the bacterium’s swimming direction, driving it off course in a microscale version of a baseball pitcher’s curveball.