Abstract

Navigating fluid flow is a fundamental challenge for microbial life across diverse aquatic environments. While rheotaxis in swimming microorganisms has been extensively studied, it remains unresolved whether near-bed shear merely perturbs gliding motility or instead provides directional cues for active navigation on surfaces. Here we show that the benthic diatom Navicula cryptocephala utilises a purely mechanical strategy to achieve downstream rheotaxis and anisotropic spreading on submerged surfaces. Single-cell ellipsoidal tracking reveals a direction-dependent angular response that reorients gliding cells towards the downstream direction. Using interference reflection microscopy, we further reveal that shear induces rolling of obliquely gliding cells, laterally shifting the cell–substrate contact site. This shift renders raphe-based propulsion non-collinear with substrate friction, generating a downstream-restoring yaw torque. Crucially, our results rule out alternative explanations based on longitudinal shifts of the raphe contact site or direct hydrodynamic yaw torque. A minimal stochastic model confirms that this mechanical reorientation alone is sufficient to reproduce the observed drift and diffusion patterns, without invoking either orientation-dependent switching between motility states or orientation-dependent dwell times of those states. Our findings uncover a mechanism by which ambient shear is converted into directional guidance for active surface motility, providing new insights into microbial transport, retention, and resilience on submerged surfaces. ### Competing Interest Statement The authors have declared no competing interest. National Natural Science Foundation of China, 52394233, 12502482