Time-resolved colonization patterns of bacteria and fungi on polystyrene microplastics in floodplain soils

Microplastic (MP) contamination in soil ecosystems is a growing concern because MPs can accumulate, interact with soil biota, and have negative impact on soil functioning. Microbial biofilms forming on MP surfaces modify their physicochemical properties, potentially influence the transport and bioavailability of MPs, and possibly contribute to microbial degradation of the particles. While many studies have examined how MPs affect soil organisms, less attention has been given to how soil biota, particularly microbial biofilms, influence MPs in soils. This study investigates bacterial and fungal colonization on polystyrene MPs incubated in two parallel experiments in floodplain soil in the field as well as under controlled laboratory conditions over 4, 8, and 16 weeks. Using scanning electron microscopy and biofilm biomass assays, we observed progressive biofilm formation. We found higher biomass on MPs under laboratory conditions compared to natural incubation after 16 weeks. Metabarcoding analysis (16S rRNA genes for bacteria and ITS genes for fungi) showed that bacterial communities on MPs exhibited distinct dynamics under laboratory and natural conditions, with Acidobacteriota and Proteobacteria dominating and indicating temporal succession in natural conditions. In contrast, fungal communities, dominated by Ascomycota and Basidiomycota, remained more stable in composition across both conditions over time. Genera with known PS degradation potential, such as Pseudomonas, Bacillus, and Penicillium, were also detected, suggesting potential microbial involvement in MP breakdown. Our findings underscore the significance of natural incubations in elucidating MP-microbe interactions in soils, with a particular focus on bacterial and fungal communities. This study also calls for longer-term, polymer-diverse studies to better assess MP fate in soil ecosystems.