The role of biofouling and microbial colonization in shaping macroplastic fate in freshwaters

Abstract Freshwater ecosystems in densely populated areas are susceptible to plastic pollution, often reaching concentrations higher than those in marine systems. The ‘plastisphere,’ a specialized microbial community on plastic surfaces, plays a crucial role in determining macroplastic fate and ecological impact. Here, in a 12-week mesocosm experiment simulating urban freshwater systems exposed to macroplastic, we investigated factors affecting polymer-specific biofilm formation, microbial community dynamics and macroplastic settling behaviour. Biofilm growth was significantly influenced by time and plastic type, with rough surfaces (for example, PS (polysytrene) lids, HDPE (high-density polyethylene) bags) supporting higher cell densities. Biofilm development altered buoyancy, increasing sinking rates of dense (PS lids, PLA (polylactic acid (plant-based biopolymer)) cups) and those with thick biofilms (HDPE and PE (polyethylene) bags). Algae accounted for ~99% of the estimated biofilm thickness. Microbial communities in the biofilm exhibited clear temporal succession with peak abundances of bacteria, algae and cyanobacteria over time. However, no differences were detected between biodegradable and conventional plastics. Early dominance of known biofilm-forming and potential plastic-degrading bacteria declined over time, indicating limited biodegradation potential. Overall, water quality primarily shaped microbial community composition, while plastic properties governed biofilm development and settling. Most tested plastics (six of eight) sank during the experiment, highlighting freshwater systems’ vulnerability to plastic accumulation and associated water quality degradation.