A simple method to prepare substrate-coated model particles for in situ enrichment of macromolecule-degrading microorganisms

Large biopolymers such as cellulose, chitin, and proteins constitute a major fraction of particulate organic matter (POM) in soils, sediments, and aquatic systems. Yet their microbial degradation remains difficult to study due to substrate heterogeneity, spatial complexity, and challenges in particle recovery. Here, we present a simple, low-cost, and versatile method to produce millimeter-scale substrate-coated model particles using gellan gum as a stable and biologically inert matrix. Insoluble polymer substrates (e.g. chitin or cellulose) are applied as surface layers or embedded within the gellan matrix, yielding particles that mimic physical and ecological properties of natural POM. The particles can be produced with minimal equipment, tailored in size and composition, and optionally rendered magnetic to facilitate gentle recovery from complex environments such as soils and sediments. When deployed in nylon mesh pouches, the particles permit unrestricted exchange of microorganisms, porewater, and metabolites with surrounding environments while enabling reliable retrieval after incubation. The approach is compatible with a wide range of downstream analyses, including microscopy, flow cytometry, amplicon sequencing, metagenomics, and cultivation-based assays. Such substrate-coated gellan-gum model particles provide a robust platform for investigating microbial colonization, functional succession, and macromolecule degradation across diverse environmental settings.