Imaging the interaction of microalgae cultures with PET microplastic particles: Implications for wastewater treatment applications

Microplastics are emerging contaminants of concern in wastewater systems, with limited understanding of their interactions during biological treatment processes. In this study, the interactions between microalgae cultures ( Scenedesmus rubescens , Scenedesmus obliquus and Chlorella vulgaris ) and polyethylene terephthalate (PET) microplastic particles (< 300 µm) are examined to assess the potential fate and effects of these microplastics when a microalgae cultivation step is integrated into domestic wastewater treatment. Comparative cultivation runs in defined medium were carried out, in the presence and absence of microplastic, with statistically equal results for growth and nutrient consumption. Using both optical microscopy coupled to a fluorescence module and nuclear microscopy, it was possible to confirm the formation of algae-microplastic aggregates, varying with the microalga strain. Cultivation of C. vulgaris in microplastic laden medium lead to the formation of aggregates including microplastic particles covering the entire range of sizes in the added samples. S. obliquus cultures also resulted in algae-microplastic aggregates, but mainly with the smaller sized particles (< 50 µm). In contrast, S. rubescens cells showed little interaction with the PET particles. Also, the joint cultivation of the three microalgae strains resulted in interaction behaviours different than those observed in monoculture. These results indicate that microalgae-based systems may contribute to microplastic removal during biomass harvesting, improving final water quality, although this behaviour is species-dependent and requires careful considerations for practical applications.