Bacteria drive many of the processes that regulate soil fertility, carbon cycling, and groundwater quality, yet how they locate nutrients in complex subsurface environments remains poorly understood. Researchers from ETH Zurich and Eawag show that the physical heterogeneity of porous soils enhances bacterial chemotaxis, helping microorganisms reach nutrient hotspots even under fluid flow. The findings open new perspectives for predicting and managing subsurface ecosystem processes.
Microorganisms regulate key processes in soils and aquifers, including nutrient cycling, carbon turnover, and contaminant degradation. Many bacteria use chemotaxis, the ability to sense and swim towards chemical signals, to locate nutrient-rich hotspots. However, natural subsurface environments are highly heterogeneous, containing complex pore networks that create strong variations in fluid flow and nutrient availability. How these physical conditions influence bacterial foraging has remained poorly understood.
Researchers from ETH Zurich, Eawag, and IDAEA-CSIC developed a novel microfluidic platform that reproduces realistic subsurface conditions while allowing direct observation of individual bacterial cells. The system creates controlled nutrient hotspots that mimic those found around soil aggregates, organic matter, contaminants, or plant roots. Using this platform, the team tracked the movements of single cells of the soil bacterium Azospirillum brasilense under different flow conditions and pore-space structures.
Heterogeneity favors bacterial motility and chemotaxis
The study revealed that physical heterogeneity enhances the benefits of bacterial motility and chemotaxis. Contrary to the common assumption that fluid flow suppresses bacterial navigation, the researchers found that complex porous structures create low-velocity regions where bacteria can remain longer and effectively swim towards nutrient sources. As a result, chemotactic bacteria experienced substantially greater nutrient exposure than non-motile cells transported passively by the flow. The advantage of chemotaxis increased by about 1.5-fold in highly heterogeneous porous media compared with uniform flow environments.
The researchers also showed that pore-scale heterogeneity allows chemotaxis to remain advantageous across a broader range of flow velocities. In porous media, local flow variations enable bacteria to continue accessing nutrient hotspots even under conditions where chemotaxis becomes ineffective in homogeneous environments.
These findings demonstrate that microscale physical heterogeneity is a key factor shaping microbial behaviour in the subsurface and provide new insights for understanding soil respiration, nutrient cycling, groundwater remediation, and environmental management.
Reference
Stoll, M.F., Dentz, M., Stocker, R., Jimenez-Martinez, J.
external page Porous medium heterogeneity favors chemotaxis to nutrient hotspots in flow.
PNAS, 2026. doi: 10.1073/pnas.2616336123
