Dynamic multicellular group assembly in patchy marine systems

Marine microbes drive global carbon remineralization, yet the physical rules that assemble microbial communities in the ocean's patchy landscape remain poorly understood. Molecules of organic matter in the water column exist in a range of sizes that span orders of magnitude, and accumulate in blooms. Within such patches, heterotrophic bacterial cell densities surge, forcing microbes into a dynamic, non-equilibrium regime where cells swim, grow, collide, adhere, compete, and disperse. These processes can generate multicellular aggregates whose size, clonality, and packing architecture feed back onto microbial fitness, community diversity, and carbon turnover. Here, we combine experiments with Vibrio splendidus, a marine heterotroph that forms 20–50 µm transient aggregates, with biophysical modeling to uncover the principles of group assembly in patchy environments. We adapt coagulation theory from aerosol and droplet physics to quantify how collision rates, adhesin-mediated sticking, and growth jointly shape aggregate size distributions. We identify physical regimes where blooms are dominated by clonal microcolonies versus genetically mixed consortia formed through coagulation. Crucially, we seek to link group architecture to ecological function: how cell packing shifts carbon consumption, may alter competitive outcomes, and could restructure diversity within patches.