Bacterial swimming strategies and turbulence

Most bacteria in the ocean can be motile. Chemotaxis allows bacteria to det ect nutrient gradients, and hence motility is believed to serve as a method of approaching sources of food. This picture is well established in a stag nant environment. In the ocean a shear microenvironment is associated with turbulence. This shear flow prevents clustering of bacteria around local nu trient sources if they swim in the commonly assumed "run-and-tumble" strate gy. Recent observations, however, indicate a "back-and-forth" swimming beha vior for marine bacteria. In a theoretical study we compare the two bacteri al swimming strategies in a realistic ocean environment. The "back-and-fort h" strategy is found to enable the bacteria to stay close to a nutrient sou rce even under high shear, Furthermore, rotational diffusion driven by ther mal noise can significantly enhance the efficiency of this strategy. The su periority of the "back-and-forth" strategy suggests that bacterial motility has a control function rather than an approach function under turbulent co nditions.