Polymer identity shapes freshwater biofilm responses to plastic pollution: implications for bioplastic risk assessment

Plastic particles are widespread contaminants in freshwater ecosystems, yet their effects on microbial communities remain difficult to predict, partly because most studies have focused on petroleum-derived plastics while ignoring biodegradable alternatives. We evaluated whether biofilm responses to plastic particles depend on polymer identity and particle concentration using an experimental exposure with three particle types: high-density polyethylene, polyvinyl chloride, and a corn starch-derived bioplastic, each tested at two environmentally relevant concentrations. Bioplastic particles consistently increased algal biomass (chlorophyll-a, > 70% on average relative to controls) and extracellular polymeric substances (EPS, up to > 150% relative to controls) production, and shifted the community toward autotrophic dominance (<45% lower autotrophic index), whereas PE and PVC produced largely neutral responses across most variables. Total biomass, metabolism (P:R ratio), and algal community composition were not significantly affected by any treatment. Bacterial density showed a concentration-dependent temporal response, increasing at high particle loads regardless of material type. Multivariate analyses confirmed that the bioplastic treatments were driving the primary axis of differentiation while petroleum-derived plastics and control treatments remained overlapping in multivariate space. These results indicate that plastic particles do not uniformly affect freshwater biofilms with biodegradable polymers stimulating more autotrophic growth and matrix production. Our findings highlight that bioplastics should not be assumed environmentally benign, and polymer identity should be incorporated into freshwater plastic pollution assessment, bioplastic regulation, and environmental management strategies.