Microplastics on the toxicity of Ag+ and Cu2+ to Chlamydomonas reinhardtii: Exposure modes and modulation mechanisms.

This study aimed to investigate the effects of polystyrene microplastics (MPs) on the toxicity of Ag+ and Cu2+ to Chlamydomonas reinhardtii (C. reinhardtii). Acute toxicity tests (96 h) were conducted under individual, mixed, and loaded exposure modes. Physiological responses, including growth inhibition, photosynthetic activity, cell permeability, and extracellular polymeric substances (EPS) secretion were assessed. The distribution and bioavailability of Ag+ and Cu2+ were also evaluated. Experimental results showed that MPs significantly reduced the toxicity of Ag+ and Cu2+ through competitive adsorption, thereby decreasing the internalized fraction. The loaded exposure groups exhibited lower toxicity, which was attributed to reduced internalization of Ag+ and Cu2+ caused by strong adsorption and low desorption of heavy metals by MPs. MPs also alleviated oxidative stress in C. reinhardtii while promoting EPS secretion to some degree. Additionally, the loaded group showed reduced cell permeability damage and less inhibition of the photosynthetic system compared to the mixed group, which was partially associated with the physical shielding effect of MPs. Overall, although MPs mitigate the toxicity of Ag+ and Cu2+ to C. reinhardtii by reducing bioavailability via competitive adsorption, they also exacerbate indirect stress through physical interactions. These findings highlight the dual role of MPs as both "vectors" and "mitigators" of heavy metal toxicity in aquatic environments.