Size-dependent effect of microplastics on toxicity and fate of diclofenac in two algae

Microplastics (MPs) are frequently detected in natural waters and usually acted as vectors for other pollutants, leading to possible threats to aquatic organisms. This study investigated the impact of polystyrene MPs (PS MPs) with different diameters on two algae Phaeodactylum tricornutum and Euglena sp., and the combined toxicity of PS MPs and diclofenac (DCF) in two algae was also studied. Significant inhibition of P. tricornutum was observed after 1 d exposure of 0.03 µm MPs at 1 mg L^-1, whereas the decreased growth rate of Euglena sp. was recovered after 2 d exposure. However, their toxicity decreased in the presence of MPs with larger diameters. The oxidative stress contributed a major for the size-dependent toxicity of PS MPs in P. tricornutum, while in Euglena sp. the toxicity was mainly caused by a combination of oxidative damage and hetero-aggregation. Also, PS MPs alleviated the toxicity of DCF in P. tricornutum and the DCF toxicity continually decreased as their diameter increased, whereas the DCF at environmentally concentration could weaken the toxicity of MPs in Euglena sp. Moreover, the Euglena sp. revealed a higher removal for DCF, especially in the presence of MPs, but the higher accumulation and bioaccumulation factors (BCFs) indicated a possible ecological risk in natural waters. The present study explored discrepancy on the size-dependent toxicity and removal of MPs associated with DCF in two algae, providing valuable data for risk assessment and pollution control of MPs associated with DCF.