Fluid effects in spatial gradient sensing

The ability to measure shallow gradients in a chemical concentration plays an important role in a number of cellular processes, from chemotaxis to wound healing and development. Berg and Purcell were the first to demonstrate that diffusion sets a fundamental physical limit to the accuracy with which a cell can measure chemical concentrations. There is a growing body of physical literature concerning many aspects surrounding cellular concentration sensing. This includes the effects of receptor cooperativity, cellular shape and memory, and collective effects in multicellular sensing. Notably absent from this list is the role of fluid flow. In this talk, we discuss the problem of a low Reynolds number spherical squirmer directly sensing spatial gradients in concentration. By constructing a renormalization group improved solution of the appropriate advection-diffusion equation, we derive physical limits to the accuracy of spatial gradient sensing by swimming cells. At small Péclet number, advection is a singular perturbation in the problem. As a result, the sensory limits differ qualitatively from the case of pure diffusion which neglects the effects of fluid flow.