Functionalized nanoplastics alter physiology and toxin production in Alexandrium pacificum through surface charge effects

• 1 mg/L nanoplastics promote the growth of Alexandrium pacificum . • Nanoplastics induce the oxidative stress and impair the photosynthesis. • Significant interactions of concentration and functional modification of nanoplasitcs were observed on antioxidative system. • Amino-modified nanoplastics alter the composition of paralytic shellfish toxins. • Amino-modified nanoplastics have greater bioavailability to microalgae compared to carboxyl-modified nanoplastics. Nanoplastics have emerged as a significant threat to marine ecosystems. Those with functional modifications particularly impact biological processes, as their surface potential governs particle-organism interactions and electron transfer. Here, we evaluated the endpoints related to growth, photosynthesis, antioxidant defence, and paralytic shellfish toxins (PSTs) content in Alexandrium pacificum exposed to 100 nm functionally modified nanoplastics (NP, NP-NH 2 and NP-COOH) of 0, 0.1 and 1 mg/L for 21 days. Our findings indicate that: Exposure to 1 mg/L NP increased microalgae growth by 31.4%, while NP-NH 2 (1 mg/L) promoted growth throughout the experiment. Significant nanoplastics accumulation occurred on microalgae surfaces. NP (1 mg/L) induced a significant increase in photosynthetic activity. Toxicity of nanoplastics exhibited a concentration-dependent increase that was independent of functional modification, which, together with concentration, significantly interacted to affect cell membrane permeability. In addition, NP-NH 2 altered the PSTs composition, increasing C1/C2 levels by 121.2% (1 mg/L) and 159.88% (0.1 mg/L) compared to controls. In summary, NP-NH 2 exhibited higher bioavailability than NP-COOH and NP. Our study underscores the critical role of functional groups in mediating the effects of nanoplastics on harmful microalgae physiology, particularly intracellular PSTs dynamics, which may profoundly impact ecosystems through food chain transfer.