Polystyrene nanoparticles stimulate the growth and microcystin synthesis of Microcystis aeruginosa

Nanoplastics are ubiquitous in freshwater ecosystems and pose a threat to environmental and human health. They inhibit the growth of Microcystis aeruginosa and increase its microcystin content, yet the underlying mechanisms remain elusive. Here we investigated the effects of nano-polystyrene on the growth, photosynthesis, microcystin content and transcriptome of M. aeruginosa. The results showed that 10 mg/L nano-polystyrene slightly inhibited the growth during the first two days of exposure (toxicity phase), followed by a rapid recovery and enhanced growth of M. aeruginosa (promotion phase). During the toxicity phase, 10 mg/L nano-polystyrene decreased the content of phycocyanin, ATP and NADPH, and increased transcriptional levels of psbA, cpcA, grxC, trxA, gst, gshB and sod2 key genes in the studied species. During the promotion phase, the growth rate of M. aeruginosa increased with increased phycocyanin, ATP and NADPH production, and a series of photosynthetic parameters increased, which were reflected by the up-regulation of photosynthesis-related genes (e.g., psbH, psb28-1, petJ) to increase the photosynthetic electron transport rate, and the corA gene to absorb more necessary metals to support growth. Additionally, M. aeruginosa increased the microcystin content during both the toxicity and promotion phases, which might suggest it could potentially play a role in promoting photosynthesis and growth. The results indicated that nano-polystyrene could increase the growth and the microcystin content of M. aeruginosa, which could amplify the adverse and harmful effects of nanoplastics on freshwater ecosystems and human health. This study provides novel insights into the restoration and remediation of aquatic ecosystems.