The photosynthetic toxicity of nano-polystyrene to Microcystis aeruginosa is influenced by surface modification and light intensity.

It is known that nanoplastics can cause membrane damage and production of reactive oxygen species (ROS) in cyanobacteria, negatively impacting their photosynthetic reactions and growth. However, the synergistic effect of light intensity on nanoplastics' toxicity to cyanobacteria is rarely investigated. Here, we investigated the impact of nano-polystyrene particles (PS) and amino-modified nano-polystyrene particles (PS-NH) on cyanobacterium Microcystis aeruginosa cultivated under two light intensities. We discovered that PS-NH was more toxic to M. aeruginosa compared to PS with more damage of cell membranes by PS-NH. The membrane damage was found by scanning electron microscope and atomic force microscopy. Under low light, PS-NH inhibited the photosynthesis of M. aeruginosa by decreasing the PSII quantum yield, photosynthetic electron transport rate and pigment content, but increasing non-photochemical quenching and Car/chl a ratio to cope with this stress condition. Moreover, high light appeared to increase the toxicity of PS-NH to M. aeruginosa by increasing its in vitro and intracellular ROS content. Specifically, on the one hand, high visible light (without UV) and PS-NH induced more in vitro singlet oxygen, hydroxyl radical and superoxide anion measured by electron paramagnetic resonance spectrometer in vitro, which could be another new toxic mechanism of PS-NH to M. aeruginosa. On the other hand, high light and PS-NH might increase intracellular ROS by inhibiting more photosynthetic electron transfer and accumulating more excess energy and electrons in M. aeruginosa. This research broadens our comprehension of the toxicity mechanisms of nanoplastics to cyanobacteria under varied light conditions and suggests a new toxic mechanism of nanoplastics involving in vitro ROS under visible light, providing vital information for assessing ecotoxicological effects of nanoplastics in the freshwater ecosystem.