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Abstract Microbes are known to shape topographies; however, mechanisms of biofilm‐sediment interactions and the dynamic evolution of biofilm‐covered bedforms remain poorly understood. Here, we explore the effects of synthetic biofilms on the geometry and temporal evolution of underwater bedforms through flume experiments. Our results demonstrate that synthetic biofilms can produce sedimentary structures similar to those formed by natural microbes, including wrinkles, pits, flip‐overs, roll‐ups, mat chips, and erosional edges. We observed the formation of wrinkles, a common geological feature, due to the accumulation of sand grains on the biofilms. Furthermore, we demonstrated that biofilms can reduce bed roughness by an order of magnitude in the low flow regime. However, the subsequent biofilm‐sediment interactions can increase local bedform size, forming multi‐scale geometries of bedforms. Our study improves the fundamental understanding of the landscape dynamics of bedforms covered by natural biofilms. 

Abstract Acoustic Backscatter Technique has been used to detect underwater targets and characterize seabed and river floor sediment properties. Microbial biofilms, consisting of microorganisms enclosed in Extracellular Polymeric Substances (EPS), have the potential to modify acoustic backscatter by altering the sediment topography, permeability, and porosity; however, these effects remain uncharacterized. Here, we performed laboratory experiments to explore the effects of microbial biofilms and EPS within sediments on acoustic backscatter. Our results show that the biofilm mat‐like structure formed by microorganisms above the sediment bed can reduce acoustic backscatter by up to 6 dB, while EPS within sediments can increase backscatter strength by up to 6 dB, with more significant changes at lower incident angles. This study highlights the critical role of biofilms in acoustic backscatter and paves the way for detecting targets and characterizing the properties of biofilm‐covered sediment beds.