Marine microorganisms live and die in a fluid environment dominated by viscosity, which controls fundamental processes such as molecular diffusion and particle motion. Crucially, marine organisms can engineer their own immediate environment by releasing or degrading viscous organic matter, and in the process influence large-scale ocean food webs and elemental cycling. This session focuses on microbial generation of and response to physical microenvironments that are characterized by viscoelastic exopolymeric substances (EPS), colloids, gels, and aggregates. We welcome in-situ, laboratory, numerical, and theoretical studies that investigate microbial ecology (e.g. chemotaxis, nutrient acquisition, chemical communication, predation, symbioses) from a biophysical perspective (e.g. motility, diffusion, aggregation, fragmentation, sedimentation, rheology, fluid mechanics). Topics of interest include, but are not limited to, physical interactions of microorganisms with marine snow, biofilms, phycospheres, sea surface microlayers, slimes, gelatinous zooplankton, kelp, coral, fish mucus membranes, and environmental contaminants such as oil and nano- and microplastics.