Multiomics Provides Insights into the Impacts of Microplastics on Heavy Metal(Loid) Accumulation in Lettuce under Simulated Acid Precipitation

Microplastics (MPs) and heavy metal(loid)s (HMs) are widespread agricultural contaminants. Acid precipitation can create acidic conditions, changing migration of these pollutants in soil-crop systems. However, the impacts of MPs on HMs uptake by crops under acid rain conditions and the underlying mechanisms that govern these processes remain unclear. We conducted pot experiments to assess MP-mediated effects on HM accumulation in lettuce (Lactuca sativa L.) under simulated acid rain. We found that high-dose polyethylene (PE) significantly increased shoot Cd accumulation in lettuce by 51.2% and that the presence of acid rain enhanced this effect, as compared to the single PE treatment. In contrast, coexposure to PE and acid rain decreased the As accumulation by 47.6%. This was primarily due to a decrease in soil pH and a change in HMs availability. In the rhizosphere, PE and acid rain positively influenced the relative abundance of Photobacterium that is the main contributor mediating the Cd/As accumulation. Additionally, acid precipitation enhanced MP stress by regulating metabolic pathways and reducing the biosynthesis of several important metabolites, including signal transduction, amino acid, and carbohydrate. Co-analysis of microbiomics and metagenomics showed the functional genes related to the metabolism of biotin, propanoate, and histidine were altered by MPs and acid precipitation, which may cause increases in the abundance of Photobacterium. These findings underscore complex interactions between MPs and acid rain on HMs transport and availability in soil-crop systems, providing important insights for evaluating their risk in farmland.