Transformations, interactions, and acute biological responses of nanoplastics on mixotrophic microalgae Poterioochromonas malhamensis†

Nanoplastics show a high potential risk to aquatic organisms due to their small size, high mobility and large surface area. However, our knowledge about their behavior in different water conditions and their potential effect on freshwater microalgae as well as the influential toxicity factor remains elusive. Here, we study the transformation and toxicity of polystyrene nanoplastics (PS-NPLs) of 20 and 100 nm to a widespread mixotrophic phytoflagellate, Poterioochromonas malhamensis, using Lake Geneva water and cell culture medium. The colloidal stability of PS-NPLs, cellular burden, variety of physiological responses, and the potential of producing diverse high-value biocompounds upon exposure to PS-NPLs under two exposure conditions were evaluated. Significant effects are observed after 24 h of exposure for cell viability and PS-NPLs associated with/taken up by cells at the lowest tested concentration of 0.1 mg L⁻¹. It is also found that in both Lake Geneva water and culture medium, the biological effects of 20 nm PS-NPLs are 1.7–11.5 times higher than those of 100 nm PS-NPLs. PS-NPLs exhibit 2.6–20.5 times higher biological effect in Lake Geneva water compared to the culture medium, demonstrating that the effect of NPLs is concentration-, size- and medium-dependent. Moreover, PS-NPLs induced changes in protein and carbohydrate content within microalgae cells. Principal component analysis of 12 key parameters in both Lake Geneva water and culture medium indicates that aggregation behavior is the most influential factor controlling the biological effects of NPLs. These results highlight a clear mechanistic link between NPLs' intrinsic and water properties in the evaluation of NPL biological responses.