Unravelling the toxicity mechanisms of polystyrene nanoplastics on physiological and transcriptomic responses of the marine dinoflagellate Alexandrium minutum

Dinoflagellates are a group of marine phytoplankton, and many species are considered to be responsible for harmful algal blooms (HABs) due to their toxin production. Nanoplastics (NP) have hazardous effects on algae, but physiological response, toxin production and underlying regulation mechanisms that they induce in dinoflagellates are poorly understood. In this study, a Chinese Alexandrium minutum strain was exposed to various concentrations (0.1, 0.5, 1.0, 5.0, 10.0 and 50.0 mg/L) of polystyrene nanoplastics (PSNP, 50 nm), and its growth, photosynthesis, oxidative stress response, saxitoxin (STX) production, and transcriptomic response were explored in these conditions. The growth and photosynthesis efficiency were inhibited by relative higher concentrations (10.0 and 50.0 mg/L) of PSNP exposure. The chlorophyll a, malondialdehyde (MDA) contents, antioxidant enzyme activities and STX levels gradually increased at low PSNP concentrations and short time exposures, and subsequently decreased at high PSNP concentrations. Most photosynthesis-related genes have similar decreasing trends with physiological changes, while the antioxidant system genes were upregulated to resist the oxidative stress caused under PSNP. The genes involved in N metabolism were upregulated to maintain normal biological activity and synthesize substrates for STX production. In addition, several STX biosynthesis genes were differentially expressed under PSNP treatment. These findings provide a comprehensive understanding of the response mechanisms and STX synthesis in toxin-producing dinoflagellates exposed to PSNP.