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.

Xu G ，Lin X ，Yu Y 《-》

[Effects of Polystyrene Microplastics Combined with Cadmium Contamination on Soil Physicochemical Properties and Physiological Ecology of Lactuca sativa].

Niu JR ，Zou YJ ，Jian MF ，Huang CH ，Li JY ，Mu T ，Liu SL ... -  《-》

Microplastics can affect soil properties and chemical speciation of metals in yellow-brown soil.

Although the influence of microplastics (MPs) in different soil environments has been investigated, their effects on the physiochemical properties and chemical speciation of heavy metals in yellow-brown soil remains unknown. This study aimed to determine the effects of various concentrations of linear low-density polyethylene (LLDPE), polyamide (PA), polyurethane (PU), polystyrene (PS), and low-density polyethylene (LDPE) MPs on the yellow-brown soil environment and chemical speciation of the heavy metals cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn). MPs influenced the physicochemical properties and chemical speciation of heavy metals in yellow-brown soil. The physicochemical properties of yellow-brown soil can be altered by changing the concentrations of LDPE MP. The relationship between changes in field capacity (FC) and LDPE concentrations was approximately linear. The physiochemical properties of yellow-brown soil containing added PA, PU, and LDPE MPs were substantially improved (control vs. MPs): FC, 39 % vs. 42.50 % for PU, cation exchange capacity (CEC) 45.77, 56.65, and 57.44 cmol.kg-1 for PA, PU, and LDPE respectively, and organic matter (OM) content, 40.16 vs. 51.68 g.kg-1 for PA. The LLDPE and PU MPs also simultaneously affected the chemical speciation of heavy metals in yellow-brown soil. The LLDPE MPs increased the acid-soluble (45.17-54.67 % (Cd-F1), 7.24-11.30 % (Cu-F1), 4.20-7.23 % (Pb-F1), 21.21-31.47 % (Zn-F1)) and reducible (24.02-29.41 % (Cd-F2), 25.69-34.95 % (Cu-F2), 74.29-81.07 % (Pb-F2), 28.77-34.19 % (Zn-F2)) fractions of heavy metals, which increased their bioavailability. However, PU MPs reduced the ecological risk of heavy metals in yellow-brown soil by increasing the content of the residual fraction (26.11-40.21 % (Cd-F4), 47.63-59.67 % (Cu-F4), 17.25-26.76 % (Pb-F4), 32.63-50.46 % (Zn-F4)). Changes in the properties of yellow-brown soil and the impact of MPs on heavy metals, might change the chemical speciation of heavy metals. The impact of MPs on heavy metals in yellow-brown soil requires further investigation.

Wen X ，Yin L ，Zhou Z ，Kang Z ，Sun Q ，Zhang Y ，Long Y ，Nie X ，Wu Z ，Jiang C ... -  《-》

Polyethylene microplastics increase cadmium uptake in lettuce (Lactuca sativa L.) by altering the soil microenvironment.

Little research has focused on the combined pollution of microplastics (MPs) and heavy metals in soil, especially the mechanism of their interaction. We conducted a 45-day microcosm experiment to test the hypothesis that polyethylene (PE) MPs and cadmium (Cd) had a joint toxicity to lettuce fitness. The effects of MPs at different addition ratios on Cd bioavailability and soil properties were also investigated in the microenvironment of three levels of Cd-contaminated soils. The results showed that the 10% MPs had an adverse impact on the plant biomass and significantly decreased soil pH and cation exchange capacity (CEC), but significantly increased soil dissolved organic carbon (DOC). The presence of MPs increased the soil Cd bioavailability and plant Cd concentrations and accumulations across all three levels of Cd-contaminated soils, which potentially aggregated the combined toxicity. The amounts of the bacterial 16SRNA and the fungal ITSRNA genes displayed a hormesis effect in response to the MP addition ratios while the abundance of Cd resistance genes cadA and czcA increased across all three Cd levels. The regression path analysis indicated that MPs affected shoot Cd concentrations by altering soil properties, which directly and indirectly contributed to the alteration mechanism, while the soil pH, DOC, and Cd bioavailability played core roles. The results suggest that the co-exposure of PE MPs in heavy metal-contaminated soil may therefore increase the toxicity, uptake, accumulation, and bioavailability of heavy metals by altering the properties of the soil microenvironment, which deserves further research.

Wang F ，Wang X ，Song N 《-》

Effects and mechanisms of polystyrene micro- and nano-plastics on the spread of antibiotic resistance genes from soil to lettuce.

Xu G ，Li Y ，Lin X ，Yu Y ... -  《-》