Reducing Gut Dissolution of Zinc Oxide Nanoparticles by Secondary Microplastics with Consequent Impacts on Barnacle Larvae

The environmental impact of sunscreen is a growing concern, yet the combined effects of its components on marine animals are poorly understood. In this study, we investigated the combined effects of sunscreen-extracted zinc oxide nanoparticles (<i>n</i>ZnO) and microplastics (MPs) on the development of barnacle larvae, focusing on the different roles played by primary microplastics (PMPs) and secondary microplastics (SMPs) generated through the phototransformation of PMPs. Our findings revealed that a lower concentration of <i>n</i>ZnO (50 μg/L) enhanced molting and eye development in barnacle larvae, while a higher concentration (500 μg/L) inhibited larval growth. Co-exposure to PMPs had no significant effect on larval development, whereas SMPs mitigated the impact of <i>n</i>ZnO by restricting the in vivo transformation to ionic Zn. Accumulated SMPs reduced gut dissolution of <i>n</i>ZnO by up to 40%, lowering gut acidity by 85% and buffering the in vivo dissolution of <i>n</i>ZnO. We further identified a rough-surfaced Si-5 fragment in SMPs that damaged larval guts, resulting in decreased acidity. Another Si-32 resisted phototransformation and had no discernible effects. Our study presented compelling evidence of the impacts of SMPs on the bioeffect of <i>n</i>ZnO, highlighting the complex interactions between sunscreen components and their combined effects on marine organisms.