Drag acting on suspended sediment increased by microbial colonization

The dynamic interplay between microbial communities and sediment transport shapes continental landscapes and influences particulate matter fluxes across the Earth’s surface. Microbial colonization transforms individual sediment grains into aggregates with intricate and varied morphologies, complicating sediment transport. However, current models often simplify this morphological complexity, assuming that aggregates experience fluid drag equal to that of smooth spheres or idealized shapes. Here we apply an X-ray micro-computed tomography method combined with computational fluid dynamics simulations to analyse aggregate morphology at high spatial resolution and determine the relationship with drag. Instead of aggregate size or gross shape being the primary controls on drag, we find that microbial colonization alters the fine-scale aggregate morphology and increases drag by factors of 1–3 compared with smooth surfaces. We propose a morphology-corrected drag law that accounts for this complexity, reconciling the differences in drag across diverse aggregates. Our findings suggest that a shift from focusing on gross scale variabilities (size or gross shape) to fine-scale morphologies could enable greater accuracy in transport predictions, and improve understanding of microbially colonized aggregates in fluvial, coastal and oceanic systems.