Beyond growth: comparative impact of plastics and natural particles on EPS dynamics and aggregation in Rhodomonas Salina

Understanding the ecological impact of micro- and nanoplastics (MNPs) requires going beyond traditional growth inhibition endpoints. The authors have investigated the comparative effects of plastics, namely virgin polyethylene terephthalate (vPET), weathered PET (wPET), and the natural particle kaolin on the algae-bacteria consortium of the microalga Rhodomonas salina, with analysis of growth dynamics, extracellular polymeric substance (EPS) production, and aggregation behaviour in terms of number, size and composition. Maximum growth inhibition values between 53.32% and 67.05% were observed. Both wPET and kaolin caused stronger and earlier growth inhibition than vPET, and their effects were significantly correlated with increased EPS production indicating a coordinated stress response. The protein fraction of the bound EPS was affected the most, with observed increases between 165.6% and 422.2%. Furthermore, particle exposure increased the number, size and composition of aggregates formed, while highlighting the role of bacteria in microbial aggregation processes. The similar biological responses induce by wPET and kaolin suggest that particle properties, rather than material identity, predominantly drive algal stress and aggregation patterns. The observed responses reflect not merely individual algal stress, but highlight the emergent properties of the microbial consortium, with EPS acting as a central mediator with ecosystem-scale implications. This study takes a first step in exploring the impact of ecologically relevant MNPs on an algae-bacteria consortium, addressing a well-recognized gap in the existing literature.