Porous medium heterogeneity favors chemotaxis to nutrient hotspots in flow

Natural porous environments, such as soils and aquifers, are often highly heterogeneous, exposing microorganisms to variable fluid flow and nutrient landscapes. Chemotactic bacteria can locate nutrient hotspots and thereby accelerate subsurface reactions, yet how chemotaxis is affected by pore-scale flow remains poorly understood. Here, we show that pore-scale heterogeneity enhances the foraging benefits afforded to bacteria by motility and chemotaxis, compared to nonmotile cells advected by the flow. Using a microfluidic porous medium with localized steady nutrient sources, mimicking those found in natural porous media, we systematically test the effects of hydrodynamic heterogeneity, flow magnitude, and bacterial traits on nutrient exposure and pore-scale transport by directly tracking single cells of the soil bacterium Azospirillum brasilense. By introducing an advantage index that quantifies nutrient exposure compared to that of nonmotile cells, we find that the chemotactic gain in nutrient exposure increases with hydrodynamic heterogeneity by 1.5-fold. The underlying mechanism is due to swimming bacteria preferentially occupying low-velocity regions, where they can swim against the flow to climb the chemical gradient. The heterogeneous flow field in porous systems additionally causes chemotaxis to remain advantageous across a wider range of flow rates than in uniform, grain-free systems at the same mean fluid speed. These findings highlight how, due to the strong heterogeneity in the flow field, the quenching effect of flow on chemotaxis is much weaker in porous media than in homogeneous environments, with implications for biogeochemical cycling and predictive modeling of the transport of chemotactic bacteria and contaminants in porous media.