Transfer of microplastics (MPs) from the environment to the food chain might have serious impacts on human health. Most MPs are emitted to the terrestrial environment and also accumulate in much larger amounts in our soils but most attention is focused on the oceans and freshwater ecosystems. Increasing amounts of smaller plastic particles emitted directly to the environment and secondary particles formed by the degradation of plastics represent a substantial potential threat to the terrestrial ecosystem (Figure 1). It is very important to evaluate the uptake of MPs by crop plants because of the ubiquity of small-size MPs in wastewaters and the role of wastewaters as important sources of water for agricultural irrigation worldwide.
Figure 1 Leafy green vegetables cultivated in greenhouse systems which may contain microplastics originating from plastic mulching and sewage sludge application, organic fertilization, wastewater irrigation, and other sources such as atmospheric particulate deposition. (Photos source by Li Jian, Yu Hongli and Li Lianzhen)
In our recent study, first of all we assessed and validated the effective uptake of submicrometer- (0.2 μm) polystyrene (PS) beads in a controlled nutrient solution culture. In addition to the well-known mode of apoplastic uptake of nanometer-sized materials at root apex, our results revealed evidence in support of microplastic penetrating the stele via cracks in the epidermis that initiate lateral root formation (Figure 2). Our study also showed exciting evidences even the micrometer-sized (2.0 μm) PS beads can enter the xylem vessels at the lateral root emergence sites.
Figure 2 Schematic illustration of adherence, uptake, transport, and penetration of two sizes (0.2 μm and 2.0 μm) of polystyrene beads by two edible plants wheat and lettuce.
The use of treated wastewater for agricultural irrigation is increasing worldwide and is not limited to arid zones1. Considering the ubiquity of microplastics in treated wastewater2, it is very important to evaluate their uptake by crop plants under conditions that are as similar as possible to those of agricultural farming. Polymethylmethacrylate (PMMA) is among the most commonly used plastic type as microbeads in cosmetics in Europe and in other countries3,4. PMMA beads are likely to be transported to wastewater treatment plants, and it is anticipated that a substantial proportion will pass through filtration systems due to their small size5. Consequently, they will enter the environment. Further studies using spiked wastewater confirmed that both PS and PMMA particles are also taken up by the plants from real wastewater.
We finally found that both submicrometer- (0.2 μm) and micrometer-sized (2.0 μm) PS beads enter the xylem vessels at the lateral root emergence sites in sand culture and these particles were also present in the xylem sap of wheat and lettuce. Accumulation of 0.2 μm fluorescently labeled PS beads was observed in roots of wheat grown for 20 days in a sandy soil. We think that this is proof of principle that will most likely yield the same results in natural field soils.
These results shed a new light on the possibility of food chain transfer of microplastics and have important implications for crops grown on fields contaminated with either wastewater treatment discharge or sewage sludge.
- Sato et al., Agric. Water Manage. 130, 1-13 (2013).
- Kalčíková et al., Chemosphere 188, 25-31(2017).
- Gouin et al., SOFW-J. 3, 40-46 (2015).
- UNEP (2015) Plastic in Cosmetics, ISBN: 978-92-807-3466-9 pp33.
- S. Fendall, M.A. Sewell, Mar. Pollut. Bull. 58, 1225-1228 (2009).