Microcystis aeruginosa copes with toxic effects of micro/nano-plastics with varying particle sizes through different self-regulatory mechanisms

Studies have shown that micro/nanoplastics (MNPs) are hazardous to many organisms. However, the underlying toxicity mechanism of MNPs, particularly the impact of particle size, have not yet been clarified. We exposed the freshwater cyanobacterium Microcystis aeruginosa to 5 mg/L of polystyrene (PS) MNPs with varying particle sizes (20 nm, 200 nm, and 2 µm) for 96 h. The results demonstrated that different particle sizes of PS all exerted adverse effects on microalgae, which led to a significant reduction in photosynthesis and an increase in reactive oxygen species (ROS), and integrated biomarker response (IBR) analysis revealed that larger MNPs (2 µm, 200 nm) caused greater physiological stress in M. aeruginosa than 20 nm particles. Laser scanning confocal microscopy (LSCM) images showed that MNPs with different particle sizes coexisted with microalgae in different states, triggering different toxicity mechanisms. Only 20 nm PS could enter algal cells and its intake might cause cell damage or even lysis. 200 nm PS adsorbed to the algal surface, and 2 µm PS aggregated with algae, both producing shading effects. Meanwhile, microalgae self-regulated by enhancing the secretion of various types of extracellular polymeric substance (EPS), increasing CO uptake to produce more energy (72.5-96.3% increase in C/C values) and up-regulating the transcription of different genes (mainly genes associated with ribosomes, photosynthesis, oxidative phosphorylation, and pentose phosphate pathway) to mitigate the negative effects of PS with varying particle sizes to different degrees. Notably, 200 nm and 2 μm PS significantly increased the generation and release of microcystins, raising the freshwater environmental health risk and requiring attention.