Humic acid-cation interactions reshape nanoplastic bioaccessibility and mechanistic toxic pathways toward microalgae

Nanoplastics toxicity is strongly shaped by water-chemistry interactions, yet how coexisting humic acid (HA) and cation jointly restructure toxicity pathways remains poorly resolved. Herein, we systematically examined the aggregation behavior, cellular responses, and bioaccessibility of polystyrene nanoplastics (PSNPs) in Chlorella vulgaris under scenarios involving individual HA, individual cations, and their coexistence. Compared with individual cations (i.e., Na⁺ or Ca²⁺), HA-cation coexistence alleviated PSNPs-induced growth inhibition despite promoting PSNPs-algae heteroaggregation and increasing PSNPs bioaccessibility, indicating that bioaccessibility alone did not determine cytotoxicity. This was ascribed to decreased membrane damage (71.5%‒77.5%), accompanied by reduced downstream photosynthetic impairment (17.5%‒86.9%) and apoptosis (49.6%‒62%). In contrast, compared with individual HA, HA-cation coexistence amplified PSNPs toxicity by enhancing particle bioaccessibility (158.7%‒201.9%), elevating oxidative stress (7.9%‒47.5%), and ultimately promoting membrane destabilization (60.2%‒69.7%). Py-GC/MS quantification confirmed that compared with individual HA, HA-cation coexistence increased PSNPs bioaccessibility from 1.14 ± 0.48 to 3.36 ± 0.39 µg/10⁴ cells. Structural equation modeling further revealed that increased bioaccessibility acted as an upstream driver of toxicity only when coupled with oxidative stress and membrane damage, highlighting a conditional bioaccessibility-oxidative stress-membrane damage cascade. Together, these findings demonstrate that HA-cation coexistence induces a mechanistic shift in nanoplastic toxicity that cannot be inferred from single-factor experiments. Therefore, we highlight that realistic water-chemistry interactions involving HA and cations should be explicitly incorporated into nanoplastic risk assessment frameworks to avoid biased predictions of ecological impacts in freshwater systems.