Insights into co-occurrence characteristics and interplay of microbial communities and environmental pollutants on biofilm-colonized microplastics in mangrove ecosystems

Microplastics (MPs) function as critical vectors for pollutant transport and microbial colonization in mangrove ecosystems, yet their co-occurrence patterns, interactions and ecological risks remain insufficiently characterized. This in-situ study systematically investigated the co-occurrence characteristics of MP-associated microbial communities, heavy metals (HMs), and persistent organic pollutants (POPs) across mangroves (QMG and TLG), seasons (dry and wet), exposure durations (15, 30, and 60 days), and polymer types (PP, PE, and PET; mean diameter: 3.5 mm) (sample size = 375). Results demonstrated progressive biofilm development on MPs over exposure duration, characterized by increased matrix complexity and microbial richness/diversity during wet season relative to dry season, and at QMG compared to TLG. Core biofilm communities were dominated by Proteobacteria (40.60%-47.68 %), Planctomycetota (11.33%-19.43 %), Actinobacteriota (6.83%-11.81 %), Bacteroidota (6.07%-12.67 %), and Cyanobacteria (0.53%-5.40 %), with geographic location exerting a stronger influence than seasonality over bacterial community composition. Distinct spatio-seasonal biomarkers were identified across taxonomic hierarchies, reflecting habitat-specific adaptations. Pollutant analyses revealed significant time-dependent enrichment of HMs (Mn, 83.70 %; Cr, 11.85 %) and POPs (polycyclic aromatic hydrocarbons, 80.05 %; organochlorine pesticides, 19.58 %) on MPs (p < 0.01), with accumulation intensified during wet season and exhibited marked spatial variation (higher HM levels at QMG, p < 0.01). These spatio-seasonal patterns corresponded with dynamics in microbial diversity and biofilm development, suggesting biofilm may facilitate pollutant adsorption onto MPs. Mantel tests identified seawater temperature (p < 0.01) and pH (p < 0.05) as primary environmental drivers modulating both microbial community composition and pollutant accumulation. Co-occurrence network analysis revealed more frequent associations between bacterial families and HMs than with POPs, indicating pollutant-specific microbial interactions. These findings underscore MP biofilms as dynamic interfaces mediating environmental behaviors of pollutants and potentially exacerbate ecological risks in mangrove ecosystems. Future work should focus on in-situ studies across diverse ecosystems to elucidate MP-biofilm-pollutant interactions for reliable ecological risk assessments.