Size-dependent toxicity of nano- and microplastics with zinc oxide nanoparticles in the marine rotifer Brachionus koreanus

This study of the combined toxic effects of zinc oxide (ZnO) and nano- and microplastics (NMPs) on the marine rotifer Brachionus koreanus demonstrates that co-exposure leads to significant physiological disruptions. The presence of NMPs increased the acute toxicity of ZnO compared to ZnO-only exposure, particularly in NP rather than MP. Combination exposure reduced both reproduction and population compared to ZnO-only exposure. We observed an increased ingestion of fluorescent MNPs in combined exposures. The ZnO + NP group showed a higher bioaccumulation of Zn compared to the ZnO and ZnO + MP groups, which resulted in increased toxicity. Also, ZnO mitigated the oxidative stress, antioxidant activity, and gene expression levels caused by NMPs. Overall, the combined exposures had more complex effects than individual exposures. ZnO altered acetylcholine esterase activity with and without NMPs, suggesting an adverse neurotoxic impact. Only the ZnO + NP group showed enhanced ERK protein level, a mitogen-activated protein kinase, suggesting a modulated cellular stress mechanism. Molecular analyses supported these in vivo findings, revealing that the combined effects of ZnO and NMPs vary by particle size, resulting in distinct toxicity pathways for NMPs. These findings highlight the need for comprehensive environmental assessments considering multiple pollutant interactions to understand their full ecological impact.