Size-dependent cellular internalization and effects of polystyrene microplastics in microalgae P. helgolandica var. tsingtaoensis and S. quadricauda

Microplastics (MPs) are persistent contaminants in aquatic environments. Microalgae, as the main phytoplankton and primary producers, usually co-exist with MPs. Despite previous studies that have proved the interaction of MPs and microalgae, it is largely unknown whether MPs can be uptake into cells of microalgae. In this study, both marine P. helgolandica var. tsingtaoensis and freshwater microalgae S. quadricauda were respectively exposed to 10 mg/L polystyrene microbeads with five diameter sizes: 1.0, 2.0, 3.0, 4.0, and 5.0 μm. Confocal laser scanning and 3D image analysis showed that mean 24.0 % or 11.3 % cells of P. helgolandica var. tsingtaoensis contained 1.0 μm or 2.0 μm MPs after 72 h exposure. While mean 43.3 % or 15.3 % of S. quadricauda individuals engulfed 1.0 μm or 2.0 μm MPs within cells. But, none of 3.0-5.0 μm MPs were observed within algal cells. These results demonstrate the size-dependent cellular internalization of MPs in microalgae. Exposure to 1.0-2.0 μm PS MPs caused a significant reduction in the density of microalgae and influenced photosynthesis, which suggests cellular internalization of MPs can influence algal fertility and growth. This discovery first confirms cellular internalization of MPs in phytoplankton, of significance for the fate and eco-toxicity of MPs in the aquatic ecosystem.