Plastic pollution in freshwater ecosystems is an increasing environmental concern, prompting the search for biodegradable polymer (BP) alternatives. However, their degradation in natural aquatic environments remains poorly investigated and understood. This four-month in situ study compared the degradation in a lentic freshwater ecosystem of two compostable items, Mater-Bi^® shopping bag and disposable dish, with their respective pure polymer matrices, poly(butylene adipate-co-terephthalate) (PBAT) and polylactic acid (PLA). Additionally, biodegradable poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and oil-based polypropylene (PP) were also tested. Changes in morphology, chemical composition and thermal and mechanical properties, as well as microbial colonization, were analyzed over time. A validated cleaning protocol was employed to ensure accurate surface analysis. Results showed detectable but limited degradation of pure polymers and their matrices in commercial products after 120 days of immersion with variations observed among polymer materials. Compostable materials exhibited significant leaching of fillers (starch, inorganic particles), leading to morphological changes and fragmentation. PHBV showed the fastest degradation among tested polyesters. PP exhibited only minor surface changes. Microbial colonization varied with polymer structure and degradability, but long-term degradation was limited by polymer properties and the gradual development of the plastisphere. This study highlights that standard laboratory tests may overestimate the environmental degradability of BPs and emphasizes the importance of in situ assessments, careful cleaning procedures and property characterizations to accurately assess polymer degradation in freshwater systems.

Microplastic-associated biofilms (plastisphere) have been proposed as potential reservoirs of antibiotic resistance genes (ARGs), potential pathogens, and opportunistic microorganisms. However, information on freshwater ecosystems remains limited. In this study, the occurrence and distribution of selected ARGs, predicted resistance-associated functional profiles, and potentially pathogenic bacterial taxa were investigated in the plastisphere and planktonic communities of four Italian lakes. Quantitative PCR was used to quantify four ARGs (blaCTX-M, tetA, sul2 and ermB), while 16S rRNA gene datasets were analyzed to identify potentially pathogenic bacterial genera and reconstruct predicted resistance-associated functional profiles. ARGs were detected more frequently and at higher relative abundance in the plastisphere than in surrounding waters, although their distribution was highly heterogeneous among lakes and sampling sites. In particular, blaCTX-M was frequently detected on microplastics (MPs) but only rarely in planktonic communities. Predicted resistance-associated functional profiles showed a clear separation between plastisphere and planktonic communities, with MP-associated biofilms characterized by a higher contribution of resistance- and stress-associated functions. No consistent enrichment of potentially pathogenic bacterial genera was observed on MPs. Overall, freshwater plastisphere communities may represent distinct microbial microhabitats in which ARGs and resistance-associated functions accumulate more frequently than in surrounding waters, although local ecological conditions appear to strongly influence their distribution.

Plastic debris in river ecosystems provides persistent surfaces for microbial colonization, forming complex plastic-associated biofilm communities known as the riverine plastisphere. Despite the growing number of studies on plastic-associated microorganisms, current knowledge remains strongly biased toward marine environments, whereas riverine plastisphere communities are still comparatively understudied. This mini-review critically examines the current state of knowledge on the riverine plastisphere, focusing on microbial community composition, ecological drivers, biofilm development, ecological significance, and microbiological aspects related to pathogens and antibiotic resistance genes (ARGs). Current evidence indicates that riverine plastisphere communities are influenced by a complex interplay of environmental conditions, anthropogenic pressures, and substrate-related factors. The available literature suggests that these plastic-associated biofilms may support distinct microbial assemblages and microbial traits linked to nutrient cycling, pollutant transformation, pathogen occurrence, and ARG persistence. However, current knowledge remains largely focused on community composition and occurrence patterns, whereas evidence linking plastisphere communities to ecosystem-level processes remains limited. Recent studies have also highlighted the importance of considering the plastisphere as a biofilm system developing at the interface between plastic substrates and the surrounding environment, where biological, chemical, and physical processes interact. Information on ecological relevance, microbial viability, virulence, and gene mobility remains comparatively scarce. The mini-review also highlights the challenges associated with methodological heterogeneity, limited comparability among studies, and the absence of harmonized indicators capable of supporting robust ecological and public health risk assessments. Overall, the riverine plastisphere represents a rapidly developing research field that offers new opportunities to investigate the interactions among plastic pollution, microbial ecology, and lotic ecosystem processes. Future progress will depend on methodological harmonization and on the development of approaches capable of linking microbial occurrence to ecosystem processes and translating occurrence data into ecologically meaningful information and comparable risk assessments.