Biofilm formation on polyethylene and polylactic acid microplastics in freshwater : influence of environmental factors

To overcome the environmental challenges posed by conventional plastics, new alternative plastic materials are being developed and increasingly used. However, their fate in the natural environment is still poorly understood. This study compared biofilm formation on polyethylene (PE) and polylactic acid (PLA) microplastics (MPs) under different conditions i.e. water regimes (static and moving conditions), nutrient levels (excess of nitrogen and phosphorous), light exposure (light and dark) and pollution levels (presence and absence of wastewater). Although PE and PLA differ in their composition and physicochemical properties, the processes of aging and biofilm formation on both materials exhibited many similarities. Light availability had the strongest influence, as the formation of biofilm on both PE and PLA was suppressed in the dark, as shown by reduced biomass, chlorophyll $a$, and extracellular polymeric substances (EPS). The availability of nutrients strongly influenced the composition of the biofilm. High N:P ratios favoured photosynthetic microorganisms with increased chlorophyll $a$ and EPS content, while phosphorus enrichment reduced their presence. Pollution, simulated by adding sterilized municipal wastewater, slightly reduced total biofilm mass but supported the growth of photosynthetic microorganisms. Static conditions led to the formation of elongated microbial structures. Biofilm growth altered the density of MPs (increasing for PE, decreasing for PLA), reaching ~1.1 g/cm$^3$ for both, suggesting biofilm-induced sedimentation. Crystallinity of both PE and PLA decreased by 9–30% due to the presence of biofilm, which has amorphous structure. The multivariate analysis confirmed environmental factors as primary drivers of biofilm development over polymer type.