Activity induced non local transport in bacterial suspensions: beyond eddy diffusivity

Transport of passive scalars such as nutrients and chemicals is vital in microbial systems, facilitating efficient resource distribution across bacterial populations. We examine the time-averaged transport of such scalars in turbulent flows driven by collective motion in dense active suspensions. Remarkably, the turbulent scalar flux exhibits a nonlocal dependence on the mean scalar gradients—analogous to behavior observed in high Reynolds number turbulence. This nonlocality, which originates purely from the flow field and is independent of the scalar species, is found to be highly sensitive to the activity parameter in the Toner–Tu model of active turbulence. To elucidate the underlying mechanisms, we analyze the scaling of velocity structure functions with activity. Our results suggest that the degree of nonlocality could be experimentally probed via dye dispersion studies in active bacterial suspensions.