The impacts of nanoplastic toxicity on the accumulation, hormonal regulation and tolerance mechanisms in a potential hyperaccumulator - Lemna minor L.
Introduction
The fact that it is durable, affordable, and versatile has made plastic one of the irreplaceable materials in today's industry. Global plastic production reached 367 million tons in 2020 (Plastics Europe, 2021). However, plastic waste is difficult to manage and dispose of due to the same characteristics. As a result, plastic residues accumulate in the environment after brake down into small fragments by natural processes such as ultraviolet radiation, biological degradation or mechanical forces (Zhang et al., 2021). Concerns about plastic pollution and its effects on the environment are increasing with the inclusion of particles called microplastics (MP; <5 mm) and nanoplastics (NP; <1 µm) in aquatic and terrestrial ecosystems (Mitrano et al., 2021). The distribution and transport of these plastic fragments in the environment, especially in nano-sizes, are accelerating significantly. For example, Wastewater Treatment Plants (WWTPs) appears to be an important source of NP intrusion into freshwaters, as current practices in WWTPs are insufficient to retain NP (Keller et al., 2020, Mintenig et al., 2017).
The ecotoxicity of plastic particles varies according to polymer type, size, surface area, hydrophobicity, interaction with other contaminants and surface charge (Banerjee and Shelver, 2021, Saavedra et al., 2019). Polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyurethane (PU) are widely used non-degradable plastic polymer types (Wayman and Niemann, 2021). Computational studies have found a negative correlation between the size of plastic particles and their capacity to adsorb and transport other contaminants, which increases the NP-induced hazard in ecosystems (Mo et al., 2021). Many studies have revealed that NPs alter soil structure and have a negative impact on the soil microbiota by inhibiting enzyme activities involved in nutrient cycles (Allouzi et al., 2021, Awet et al., 2018). Similarly, the presence of NP inhibits growth by limiting root development, water and nutrient uptake in plants (Giorgetti et al., 2020). In addition, recent studies have shown that sub-micrometer plastic particles can be taken up by plant roots and translocated to above-ground tissues (Matthews et al., 2021, Sun et al., 2020). While internalized NP cause membrane damage, excessive reactive oxygen species (ROS) production, inhibition of photosynthesis reactions and DNA damage in plant cells, their accumulation in edible parts of crop plants raises concerns (Lian et al., 2021, Sun et al., 2020, Zhou et al., 2021). The entry of NP into the food chain and water sources also endangers human health by triggering apoptosis, genotoxicity, oxidative stress and inflammation (Banerjee and Shelver, 2021, Matthews et al., 2021, Xu et al., 2019).
The removal of plastic pollution and other contaminants from endangered freshwater sources is on the agenda of environmental studies, and both experimental and computational studies are ongoing to reveal the remediation and adsorption mechanisms of plastic particles (Lu et al., 2021, Pari et al., 2017). One promising technique for removing water pollutants is phytoremediation, which is a sustainable, feasible, and adaptable approach based on the adsorption and absorption of environmental pollutants by plants. However, phytoremediation of MP and NP contaminations is a relatively new and complicated approach (Bhatt et al., 2021). Lemna species have been proposed for use in toxicity tests for ecologically relevant analysis of water pollutants (Fekete-Kertész et al., 2015, Juhel et al., 2011). Lemna minor also stands out in phytoremediation studies with its rapid growth, high bioaccumulation capacity and resistance to contaminated water and has been used in wastewater treatments for a long time (Ekperusi et al., 2019, Khan et al., 2020). Rozman et al. (2022) suggested that duckweed can be used in remediation by the adhesion of MP particles to the plant body in the aquatic environment. Mateos-Cardenas et al. (2019) found no adverse effects of exposure to MP on growth and photosynthesis-related parameters in L. minor plants. Kalcikova et al. (2017) also showed that growth rate and chlorophyll content were not affected in L. minor leaves exposed to PE microbeads. Although the data on MP is promising for the use of duckweed in plastic remediation, nano-sized particles can penetrate roots and cause cellular toxicities (Juhel et al., 2011). In studies on the phytoremediation of internalized contaminants, it is essential to reveal the physiological responses, photosynthesis efficiency and antioxidant system activity of potential plants (Baran and Ekmekci, 2022). To the best of our knowledge, no studies are conducted on the accumulation, tolerance mechanism, phytohormonal regulation and stress responses of L. minor when exposed to NP.
We hypothesized that L. minor plants with high resistance to water contaminants could tolerate NP toxicity and be particularly effective in remediation of NP pollution in wastewater. Therefore, this study was designed to (i) elucidate the absorption and toxicity of submicrometer PS plastics in duckweed plants, (ii) investigate the effects of different doses of NP exposure (100 and 200 mg L-1 PS NP) on growth parameters and stress response, (iii) determine the effects of PS stress on photosynthesis efficiency and chlorophyll a fluorescence parameters of L. minor leaves, (iv) demonstrate the antioxidant system and hormonal signaling in Lemna under PS treatment in detail and (v) reveal the potential of L. minor to be used in wastewater treatment containing NP pollution.
Section snippets
Experimental design and treatments
Duckweed (Lemna minor L.) cultures were hydroponically grown in Hoagland solution under controlled conditions (16/8 h light/dark regime at 24 °C, 70 % relative humidity and 350 μmol m−2 s−1 photosynthetic photon flux density) and the solutions were refreshed every three days. In this study, two high doses with observed effects (P-1, 100 mg L-1; P-2, 200 mg L-1 PS NP) were used to elucidate the NP toxicity of duckweed and reveal the phytoremediation potential (Rozman et al., 2022). Although we
PS NP characterization
The SEM image of PS NP was shown in Fig. 1A. PS NP with uniform size were successfully prepared via emulsion polymerization process. The size of PS NP was ranging between 230 and 260 nm. The SEM images of dried plant samples before PS loading and after batch adsorption experiment were represented in Fig. 1B-D. It can be seen that surface characteristics of Lemna minor leaves completely changed when they were exposed to different levels of PS NP contamination. PS NP was shown with red squares
Discussion
The primary effect of plastic pollution on plants is to impair water transport by clogging cell wall pores and limiting root growth (Allouzi et al., 2021). Mateos-Cardenas et al. (2021) reported that NP change root and soil interactions, disrupt water and nutrient uptake, and reduce plant growth and elongation. Taylor et al. (2020a) showed strong adhesion of NP in the root tip and surfaces of maize plants. Recent studies showed that PS NP can penetrate the roots and be transported to the shoots
Conclusion
Interestingly, the two concentrations of PS stimulated different responses on defense pathways and photosynthetic capacity in Lemna minor. Photosynthesis efficiency and photochemical reactions were preserved in L. minor plants under 100 mg L-1 PS treatment. On the other hand, the 200 mg L-1 PS-mediated disturbances on the photosystems and chlorophyll a fluorescence were more obvious and reduced in the quantum yield and electron transport of PSII. ROS accumulation was inhibited under the lowest
Environmental implication
Nanoplastic residues are becoming more prevalent in terrestrial and aquatic ecosystems by the day. However, most previous research has focused on plastic pollution in the marine environment. Also, studies on terrestrial ecosystems and water sources mainly cover micro-sized plastic particles. This report presents the effects of polystyrene nanoplastic exposure on growth, photosynthesis, antioxidant system, and hormonal changes in freshwater macrophyte Lemna minor. Lemna species also stand out
CRediT authorship contribution statement
E.Y. and C.O.K. designed experiments; B.A., F.N.A., E.Y. and C.O.K. carried out data analysis; H.C., PS characterization; M.T., PS content; B.A., C.O.K. and E.Y. interpreted the results and wrote up the first draft of the manuscript; C.O.K. and E.Y. critically edited the whole manuscript. All authors read and approved the final manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References (80)
- et al.
Micro (nano) plastic pollution: the ecological influence on soil-plant system and human health
Sci. Total Environ.
(2021) - et al.
Micro- and nanoplastic induced cellular toxicity in mammals: a review
Sci. Total Environ.
(2021) - et al.
Superoxide dismutase: improved assays and an assay applicable to acrylamide gels
Anal. Biochem.
(1971) - et al.
NADPH oxidase-dependent H2O2 production is required for salt-induced antioxidant defense in Arabidopsis thaliana
J. Plant Physiol.
(2015) - et al.
Microplastic contaminants in the aqueous environment, fate, toxicity consequences, and remediation strategies
Environ. Res.
(2021) A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding
Analytical Biochemistry
(1976)- et al.
Application of common duckweed (Lemna minor) in phytoremediation of chemicals in the environment: State and future perspective
Chemosphere
(2019) - et al.
Exploring the interaction between polystyrene nanoplastics and Allium cepa during germination: internalization in root cells, induction of toxicity and oxidative stress
Plant Physiol. Biochem.
(2020) - et al.
Alumina nanoparticles enhance growth of Lemna minor
Aquat. Toxicol.
(2011) - et al.
Impact of polyethylene microbeads on the floating freshwater plant duckweed Lemna minor
Environ. Pollut.
(2017)