Biological cohesion as the architect of bed movement under wave action

Cohesive extracellular polymeric substances (EPS) generated by microorganisms abundant on Earth are regarded as bed “stabilizers” increasing the erosion threshold in sedimentary systems. However, most observations of this phenomenon have been taken under steady flow conditions. In contrast, we present how EPS affect the bed movement under wave action, showing a destabilization of the system. We demonstrate a complex behavior of the bio-sedimentary deposits, which encompasses liquefaction, mass motion, varying bed formations and erosion, depending on the amount of EPS present. Small quantities of EPS induce higher mobility of the sediments, liquefying an otherwise stable bed. Bed with larger quantities of EPS undergoes a synchronized mechanical oscillation. Our analysis clarifies how biological cohesion can potentially put coastal wetlands at risk by increasing their vulnerability to waves. Our findings lead to a revised understanding of the different roles played by microbial life, and their importance as mediators of seabed mobility.