Different effects and mechanisms of polystyrene micro- and nano-plastics on the uptake of heavy metals (Cu, Zn, Pb and Cd) by lettuce (Lactuca sativa L.)

Heavy metals are widely distributed in soil ecosystems, posing a potential threat to soil biota. Micro- and nano-plastics (MNPs) can impact the accumulation of heavy metals in plants through changing soil microbial community and cause injury to plants. In this work, two concentrations (100 and 1000 mg/kg) polystyrene microplastics (PS-MPs) and nanoplastics (PS-NPs) were adopted to explore the effects and mechanisms of MNPs on the uptake of Cu, Zn, Pb and Cd in lettuce (Lactuca sativa L.). MPs increased the uptake of heavy metals in lettuce by increasing the relative abundance of the key metal-activation bacteria in rhizospheric soil. At the end of experiment, the contents of Cu, Zn, Pb and Cd in NP treatments were significantly (p < 0.05) higher than that of MPs, particularly in 1000 mg/kg of NPs, with concentrations of 52.6, 174, 10.3, and 33.2 mg/kg, respectively. Biomarkers and gene expression reveled that 1000 mg/kg of NPs caused more severe injuries to lettuce plant at the end. Moreover, metabolomic analysis demonstrated that NPs disturbed the metabolism of ATP-binding cassette transporter (ABC transporter) and plant hormone signal transduction of lettuce root, causing increased uptake of heavy metals by lettuce. This work reveals that MPs may increase accumulation of heavy metals by altering the rhizosphere microorganisms, whereas NPs increase accumulation of heavy metals by causing more severe injuries to lettuce plant.