Heterogeneous aggregation between microplastics and microalgae: May provide new insights for microplastics removal

Existing studies have shown that microplastics (MPs) as artificial surfaces can be colonized by plankton microorganisms. However, systematic research on exploring the aggregation formation process of MPs and microalgae is still lacking and particularly the influencing factors of aggregation remain to be elucidated. Therefore, this study investigated the heterogeneous aggregation process between various microalgal species (i.e., Chlorella vulgaris, Scenedesmus obliquus, Tetraselmis subcordiformis, Chaetoceros müelleri and Streptococcus westermani) and MPs (i.e., mPS and mPLA) with different sizes (i.e., 74 μm and 613 μm), concentrations (i.e., 0.1 g/L, 1 g/L and 2 g/L) and shapes (i.e., the particle and sheet). The results showed that microalgae can first attach to the holes or protrusions of MPs and highly accumulate in the local region, and then multi-layer aggregation can be formed subsequently. The aggregation degree between MPs and microalgae was closely related to the MPs shape and size, and was less related to the MPs concentration. The aggregation speed of small-sized MPs (e.g., 74 μm) was faster than the large-sized ones (e.g., 613 μm). The MPs in a shape of sheet were more obvious than those in particle on their aggregation with microalgae. The density of aggregates was increased compared with pristine MPs, which is related to the cell density and cell number of attached microalgae. For the same type of MPs, the aggregation degree for the tested microalgae was as follows: Scenedesmus obliquus > C. vulgaris > T. subcordiformis > C. müelleri > S. westermani. Meanwhile, MPs inhibited cell growth of microalgae, particularly under the circumstance of their aggregation, by limiting the gas and mass transfer between microalgal cells and the extracellular environment. The heterogeneous aggregation of MPs and microalgae may provide new ideas for treatment and controlling of MPs in the environment.