The relative size of microalgal cells and microplastics determines the toxicity of microplastics to microalgae

Microplastics (MPs), as an emergent pollutant, are causing global attention in terms of water pollution, which poses a great detriment to aquatic organisms, like algae, fish and shellfish. This study aimed to investigate the toxicity of MPs of different particle sizes to microalgae with different cell diameters. Here, we conducted a microcosm batch experiment to investigate the toxicity of four particle sizes (0.2 µm, 0.5 µm, 1 µm and 5 µm) of polystyrene MPs (PS-MPs) to 12 species of microalgae. Our results found that PS-MPs of 0.5 and 1 µm had the highest inhibition rate on the microalgae. By classifying 12 species of microalgae into small, medium and large particles sizes, the inhibition of PS-MPs on algal growth gradually increased with larger cells, indicating that they were more susceptible to inhibition. The inhibition rate of PS-MPs on three phyla was Chlorophyta < Cyanobacteria < Bacillariophyta. The contents of reactive oxygen species and malondialdehyde in microalgae cells increased under PS-MPs exposure, suggesting that PS-MPs caused damage to the antioxidant system of microalgae. Together, these findings identified that the toxicity of MPs to microalgae was correlated with the relative size of each other, contributing to evaluate the risk of MPs for aquatic ecosystems.