Natural marine nanocolloids modulate the phytotoxicity of polystyrene nanoplastics on cyanobacterium Synechococcus sp.

Millions of tons of plastics are emitted into the oceans annually and fragmented into nanoplastics through weathering processes, posing significant ecological risks to ocean ecosystems. However, the effect of natural nanocolloids (Nc) on the environmental behavior and ecotoxicity of nanoplastics remains unclear. In this study, we examined the interplay between 250 nm spherical polystyrene nanoplastics (NPS) and Nc, and their combined impact on the globally significant cyanobacterium Synechococcus sp.. Nc drastically suppressed the aggregation of NPS (to <0.02 nm/s), enhancing their stability and bioavailability. While Nc alone did not affect algal growth, Nc significantly altered the cell surface properties (adhesion and modulus), promoting NPS adsorption onto Synechococcus cells. This led to more severe membrane damage, elevated reactive oxygen species levels, and reduced pigment biosynthesis, resulting in decreased photosynthetic efficiency (by 14.4 %) and greater growth inhibition (by 7.2 %) under co-exposure. Metabolomic analysis demonstrated that Nc exacerbated the NPS-induced cellular metabolic response and triggered pronounced adaptive responses in Synechococcus sp. by reshaping energy metabolism under co-exposure. This mechanistic understanding provides critical insights for predicting the fate of nanoplastics and ecological risks in the marine environment, while underscoring the need to incorporate Nc dynamics into environmental risk assessment frameworks for nanoplastic pollution.