Effect of Silver Nanoparticles (AgNPs) on Aquatic and Wetland Plants

Abstract

Among nanomaterials, silver nanoparticles (AgNPs) are cost-effective and exhibit unique physicochemical properties that enable them to become the most used agents for the manufacture of various products known as nano-enabled, including those for personal care, drugs, fabrics, sprays, disinfectants, vacuum cleaners, and air conditioners, with a continuous expansion to different sectors. Industrial discharges, the disposal of wastewater treatment effluents, and indirect runoff from the soil are some factors that are increasing the accumulation of AgNPs in aquatic and wetland ecosystems. Herewith, we critically analyze the progress in the research of the uptake and translocation of AgNPs in aquatic and wetland plants and their phytotoxic effect that depends on the concentration, size, distribution, morphological shape, surface characteristics and chemical composition of the nanoparticles, as well as the plant genotypes, among other factors. Due to biological plasticity, the toxicity level of AgNPs may vary among plant species, which may be further affected by the mode of application, time of exposure, and plant conditions (e.g., agronomic management, growth rate, phenological stage, etc.). Therefore, it is possible to identify and select competent plants for phytoremediation purposes, including superior capabilities for phytoextraction, phytofiltration, and phytostabilization. The review also identifies the main gaps that require attention in future research in order to elucidate a more integrative map aimed to reduce the potential threats to the environment and living organisms including humans.

1. Introduction

Silver nanoparticles (AgNPs) are among the most widely used nanomaterials in a broad ambit of industrial products and processes that take advantage of their distinctive physical and chemical properties, including non-linear optical properties, high thermal and electrical conductivities, surface stimulated Raman scattering, and catalytic activity as well as chemical integrity and potency over time, among others. [1]. These attributes, coupled with the relatively low cost of their manufacture, give them potential value for use in paints, plastics, air and water filters, detergents, textiles, cosmetics, aerosols, and creams. Furthermore, AgNPs are extensively employed in wastewater treatment, the stimulation of seed germination, plant growth and crop yield, preserving flowers, fruits and vegetables, microelectronics, and medicine, the latter discipline being of special interest due to its antimicrobial properties [2].

AgNPs represent more than 50% of the global nanomaterial products used [3]. They are included in the Organization for Economic Cooperation and Development (OECD) list among the eleven manufactured nanomaterials that are currently available for commercial use [4]. Various estimates have been made on the annual production of AgNPs, and in general, the trend is increasing, with the annual production estimated to be more than 800 Mg by 2030 [5].

Though negative effects of AgNPs on humans have not being scientifically proven so far, these nanoparticles may release Ag⁺ ions into the environment that can reach the air and soils. The above can occur throughout the life cycle of the products during their synthesis, manufacturing, distribution, and use or disposal phase [6]. Therefore, it is important to take care of the introduction of new technologies and mitigate possible risks related to environmental impact or health. When aquatic organisms are exposed to AgNPs, silver ions (Ag⁺) or silver salts, they may adjust their metabolism to cope with these compounds at the individual and population level [7]. Besides dose and dosage regimen, other variables including particle size, chemical composition, shape, and surface properties (e.g., coating charge and area) represent critical factors determining the final toxic effects of AgNPs in plants [8].

Aquatic ecosystems are among the main AgNP sinks, and the estimation of their concentration in aquatic environments is based on predictive mathematical models. In Malaysia, the concentrations of AgNPs reached up to 10.16 mg L⁻¹ in rivers and up to 20.02 mg L⁻¹ wastewater [9]. AgNPs can have diverse effects on aquatic organisms including both prokaryotes (i.e., bacteria and archaea) and eukaryotes (i.e., fungi, algae, macrophytes, higher plants and animals) [10,11,12,13,14]. Interestingly, macrophytes have phytoremediation capacity for water and sediments contaminated with metals and are capable of absorbing them in large quantities [15], in addition to being bioindicators of the qualities of ecosystems and their state of conservation [16].

Aquatic and wetland plants have morphological, physiological, and reproductive adaptations that allow them to tolerate excess moisture [17]. These groups of plants encompass a continuum of species with the ability of coping with flooded soils, those that can develop both on land and in water, and plants adapted to live completely submerged long enough to develop anaerobic conditions in the root zone [18]. In addition, depending on the manner in which plants grow in relation to water or water-saturated soils, they are classified as submerged, emergent, floating, or floating leaf. The majority of these plants are angiosperms, with the exception of the gymnosperms Taxodium distichum and Larix laricina and ferns of the Azolla genus. They are found in swamps, marshes, peat bogs, ponds, lake margins, streams, rivers, bays, and estuaries along protected ocean coasts [19].

Aquatic and wetland plants play a variety of roles in the functioning of this ecosystem. As photosynthetic organisms, they capture light energy from sunlight and convert it into chemical energy in the form of sugars, necessary to fuel their metabolism, while supplying oxygen and forming part of the habitat of other species. They efficiently facilitate the mobilization of nutrients from the sediment to the water column, thus greatly affecting the chemical composition of water bodies. Although some morphological and physiological modifications developed by aquatic and wetland plants are also observed in related terrestrial species, a plethora of features are distinctive for these kinds of plants, while those that are shared with terrestrial plants display a high level of specialization in aquatic and wetland plants [20].

Their capacity to improve water quality by absorbing nutrients and non-essential and potentially toxic metals, as well as xenobiotics and other pollutants, has been well studied [21]. Duckweeds and ferns of the genera Azolla and Salvinia can be sensitive to environmental pollution when they perceive it; in sublethal ranges of toxicity caused by metals or organic compounds, and as a consequence of their fast growth rate, they can rapidly develop adaptive mechanisms of resistance against these kinds of pollutants. They have been referred to as invasive plants because they grow profusely in natural or agricultural areas, resulting in profound changes in different processes and the composition of ecosystems including nutrient loading, the replacement of the aboveground biomass of native vegetation, changes in the natural flow of water in rivers or streams, and increased salt contents in water and soil. The success of these invasive species may reflect the vulnerability of the community being altered rather than the aggressiveness of the exotic species introduced [22]. Some of these species, together with rhizosphere microorganisms, are capable of storing or stabilizing metals, metalloids, nanoparticles (NPs), and organic compounds such as herbicides, pesticides, or some petroleum derivatives, making them useful in methodologies for bioremoving contaminants in groundwater, surface water, wastewater, soil, and sediments [23]. In addition, free-floating aquatic plants represent a useful tool for phytoremediation purposes due to their easy accessibility, improved productivity, simplicity of storage, and harvest [24]. They grow in wetlands, the banks of rivers, lakes, and other bodies of water, as well as in marine coastal areas; dealing directly with suspended and dissolved compounds and substances, they have the potential to be exposed to and accumulate particles of different sizes, including nanoparticles [25].

When NPs are in aquatic environments, they undergo physical and chemical modifications. When in contact with water, NPs are diluted and subjected to redox reactions, which may change their size, structure or coatings. Indeed, coatings can be eliminated or substituted by other compounds and substances including humic and fulvic acids, proteins, amino acids and other small organic molecules, which will likely alter the manners, destiny, and potential toxicity of NPs [26]. The dissolution of AgNPs and the following emergence of AgCl, Ag₂S, or Ag⁺ complexes with organic compounds represent some of the most salient AgNP modifications [27].

Environmental factors such as the pH of a solution, ionic strength, and the kind and content of organic matter determine NP aggregation. In freshwater, NPs generally experience minimal aggregation due to low salinity and alkaline pH. However, in saline water with suspended sediments, NP aggregation is greatly stimulated by high ionic strength. In saline water, AgNPs coated with polyvinylpyrrolidone (PVP) show increased homoaggregation and heteroaggregation compared to fresh water [28].

This review presents information regarding the processes of the uptake and translocation of AgNPs in aquatic and wetland plants, as well as their phytotoxic effects, which are dependent on the concentration, size, shape, surface charge, and chemical composition. Furthermore, it shows the potential of higher and lower aquatic and wetland plants for the phytoremediation of environments contaminated with AgNPs. It also identifies the main gaps in knowledge and research in determining the levels of damage caused by these emerging contaminants and which require attention in future research in order to adopt more sustainable nanofabrication to protect humans, plants, animals, microorganisms, and the environment as a whole.

2. AgNP Absorption in Leaves

In terms of crop protection, AgNPs have been extensively used to control fungal and bacterial agents to protect crops from diseases, increase their growth and yield, and even improve seed germination. They have also been used as pesticides to reduce pests in crops, as well as in post-harvest handling for the conservation of agricultural products [29]. The release of AgNPs and their potential negative impact on terrestrial and aquatic ecosystems make organs such as their leaves an optimum model to explore and unveil the mechanism of the foliar absorption of NPs, in addition to the fact that plants absorb contaminants through mechanisms similar to those of nutrient absorption [23].

2.1. Foliar Absorption Routes

In greenhouses and crop fields, NPs are mainly applied as sprays on the surface of leaves, being then absorbed via the stomata and cuticle [30]. As aforementioned, the process of NP uptake is dependent on factors such as shape, size, structure, chemical composition, surface coating, plant species, and stomatal conductance [31]. The cuticle, mainly composed of pectin, cutin and waxes, is virtually found in all the shoot system (i.e., stems, leaves, and the reproductive parts of the plant) and the root system. Indeed, the presence of a cuticle in the root cap covering Arabidopsis thaliana has recently been demonstrated, suggesting that the cuticle may occur not only in the aerial parts of plants [32].

The cuticle acts as a barrier that reduces water loss and gas diffusion, attacks by pathogens and foreign particles, and the entry of NPs into the leaves [30]. Solute transport through the cuticle occurs by two different pathways comprising the lipophilic and hydrophilic ones. The former involves the solution and diffusion of the permeant within the lipophilic cuticular polymer, being an important route for lipophilic chemicals, whereas the latter comprises polar aqueous pores composed of water molecules adsorbed to the hydrophilic components of the cuticle. These aqueous pores exhibit diameters in the range of 0.60 and 4.24 nm [33]. In the majority of aquatic plants that remain submerged, their cuticle and the waxes of their leaves are thinner or even absent in comparison to terrestrial plants in order to enhance water and nutrient uptake [34,35].

Furthermore, the cell wall represents a key element during NP absorption. Being mainly composed of cellulose (40–60%), the cell wall is freely permeable to solutes and small molecules including proteins of 30–60 kDa (9–15 nm) [36]. Since the pores of the cell wall exhibit diameters between 2 and 50 nm [37], NPs larger than 50 nm cannot pass through the cell wall, and those of a smaller size transit virtually freely through the pores, reaching the plasma membrane [38]. AgNPs influence the generation of new pores in the cell wall, which provokes the internalization of large AgNPs through this compartment [39].

On the other hand, the plasmalemma or plasma membrane is a cellular structure that regulates the uptake of different molecules by the cell. AgNPs can enter the cell through ion channels and membrane transporter proteins [20], but they can also enter by endocytosis [40], through the invagination of the plasmalemma and the generation of vesicles that transport AgNPs into the cell [41]. If AgNPs represent solid packages of Ag⁰, Ag⁺, and adsorbed Ag [42], then the dissolution of these packages within the plasma membrane could create microenvironments with high concentrations of Ag⁺ and lead to rapid ion transport. In this case, danger comes from the metal ion and from the AgNPs, which are a way to deliver Ag⁺ ions to the cell and improve their uptake [43]. In addition, Ag⁺ ions can accumulate in organisms because they are compatible with membrane transporters that regulate the uptake of Na⁺ and Cu⁺ [44]. Subsequently, they can reach the plasmodesmata [45].

In addition to the cuticular pathway, stomata are the other route for the absorption of hydrophilic substances. They have a size between 10 and 100 µm, and the diameter of their pores is greater than 40 nm [46], highlighting their importance as structures for the uptake of NPs, including AgNPs, although permeability is extremely variable since the size and stomatal density are different between different plant species (Figure 1) [47].

AgNP uptake via stomata is carried out in water films adsorbed to hydrophilic parts of the cuticle in the guard cells, which can occasionally constitute a hydraulic connection between the inside and the outside components of the leaf [48].

To analyze total foliar absorption, hydathodes and glandular trichomes must also be considered, which are secretory structures that can allow the entry of NPs. Glandular trichomes have a thin cuticle, while hydathodes are smaller openings that communicate directly with the plant’s water conduction system and are located in coleoptiles or tendrils and at the tip (in aquatic plants), on the margin, or on the entire surface of the leaves. These openings are uncovered by the cuticle and allow the guttation of excess water in various plant groups of aquatic plants such as angiosperms, gymnosperms, and ferns [49].

2.2. Factors Affecting Leaf Absorption

2.2.1. Effect of Size and Chemical Composition

When studying the uptake of NPs by plant cells, their size represents a crucial factor affecting the whole process. However, there are few investigations on aquatic plants in which the foliar absorption of metallic NPs has been identified. The absorption of AuNPs increases as a result of the presence of stomata on both sides of the fronds of Azolla caroliniana, so the effects observed in aquatic plants may also be a result of NP uptake by this route [34]. In the aquatic macrophyte Landoltia punctata, AgNPs displaying 19.80 and 27.21 nm in size stimulated the closure of the stomata and decreased in the concentration of photosynthetic pigments and starch, thus reducing the entire photosynthetic process [50].

Although, in some studies, the absorption route has not been specified, it has been found that with an increase in the size of the NPs, their absorption decreases. In some research studies with terrestrial plants, it has been reported that a decrease in the diameter of the stomata in the presence of abscisic acid (ABA) reduces the uptake of AgNPs [51].

Along with size, the chemical composition, such as the surface coating of the NPs, can also affect uptake because it changes the hydrophobicity and the surface charge, in addition to providing steric stability [52]. In the case of aquatic plants such as Lemna minor, AgNPs coated with PVP were more detrimental to the growth rate and the number of fronds per colony compared to AgNPs coated with citrate (Ct), which affected the incidence of chlorosis and the enzymatic activity of glutathione-S-transferase and guaiacol peroxidase more [53].

2.2.2. Effect of Plant Species

Another crucial factor determining the absorption of NPs through leaves is the genotype of the plant, mainly the plant species. In particular, the size, density and distribution of stomatal pores are critical for the uptake of NPs. In the case of aquatic plants, the entry of NPs takes place principally via the stomata of both floating and submerged leaves [54].

The concentration of malondialdehyde (MDA) was higher in the submerged plant Egeria densa than in the emergent plant Juncus effusus [55], clearly indicating that lipid peroxidation and membrane damage was induced by AgNPs, probably due to their greater exposure in the aquatic environment. Indeed, the stomata of submerged leaves represent pivotal organs for the acquisition of nutrients, minerals, and nanopesticides [54]. However, in other submerged aquatic plant species, the stomata may be non-functional [56], as is the case of Isoetes andicola, I. andina, and I. triquetra, which acquire CO₂ from sediments and from their own respiration [57]. Moreover, they exhibit a type of crassulacean acid metabolism (CAM) called aquatic acid metabolism (AAM), although they also maintain a CAM-type metabolism when growing on land. In floating aquatic plants such as the ferns of the Azolla genus, the stomata are functional and are located on the top of the floating lobes and on the top and bottom of the submerged lobes [58] and are of the anomocytic type [59] (Figure 1).

3. Absorption of AgNPs in Roots

Once AgNPs enter the cells of aquatic or wetland plants, they are exposed to different types of biological, physical, and chemical processes including oxidation or reduction, aggregation, agglomeration, dissolution, adsorption on cell surfaces or organic matter, persistence, and sedimentation or deposition [60]. In the case of metallic NPs, they are accessible to aquatic and wetland plants once the ions that compose them become diluted.

The exudation of low-molecular-weight organic acids such as oxalic and malic acid represents a key process that increases the dissolution of NPs in the rhizosphere. Because of their acidic nature, these acids decrease the pH values near the rhizodermis and serve as electron donors, thus reducing the metal ions located on the NPs’ surface [61].

Furthermore, AgNPs are generally unstable in O₂-rich aqueous conditions, and part of them form Ag₂O and subsequently tend to dissolve into Ag⁺ ions. In anaerobic environments, Ag⁺ ions and AgNPs have a high affinity for sulfur to form Ag₂S. In Cl-rich water, AgNPs are initially oxidized and subsequently form AgCl-NPs and AgCl compounds [62]. Additionally, inside plants, organic acids and proteins containing thiols (e.g., cysteine) promote the dissolution of AgNPs, forming Ag-thiol complexes, which together with AgCl-NPs and Ag₂S can have diverse toxicological responses [40].

3.1. Root Uptake Pathways

The cuticle, composed of a lipophilic polymer matrix, the cuticular membrane, and soluble cuticular lipids, protects the root rhizodermis [63]. In order to enable the uptake of water, carbon dioxide and essential nutrients, aquatic and wetland plants have evolved thinner cuticle and waxes [35], so it might not be a physical barrier to NP uptake.

The adsorption of the NPs on the root surface is the first step during the interaction between the nanoparticles and plant root cells. In Azolla filiculoides, AgNPs accumulate on the surface of submerged roots and lobes [64]. Root hairs can synthetize and exudate organic compounds like mucilage or organic acids, which can cause the root surface to become negatively charged. Consequently, the positively charged NPs are easily adsorbed on the root surface [65].

Subsequently, NPs exposed to the rhizodermis come into direct contact with cells, crossing the cell wall and plasmalemma via active and passive mechanisms, and may even follow the endocytic pathway [66]. Metal ions released after the aggregation or dissolution of NPs in aquatic environments can enter the plasmalemma using ion channels and carrier proteins [60] and follow the symplast and apoplast pathways to be translocated to vascular bundles in the root and subsequently to the stem or rhizome and leaves.

Root anatomy in aquatic and wetland plants can vary. In rooted and emergent plants, aerenchyma and parenchyma can be found to be associated with the conduction tissues to allow their flotation, even forming a continuous system from the root to the leaves, while, in others, aerenchyma may be absent in the root but present in leaves and the rhizome [19] (Figure 1). The absorption process of NPs varies among different species of aquatic plants, suggesting that the anatomical, morphological, and physiological characteristics of each genotype are critical for such response [60].

The submerged leaves and roots of aquatic plants can come into direct contact with NPs in the water column during their exposure; despite this, terrestrial plants have more extensive roots than aquatic plants and present a more specialized xylem, the main route for the transport of NPs.

In the aquatic plant Landoltia puctata, when studying the effect of the chemical speciation of Ag (AgNO₃, Ag⁰NPs, or Ag₂SNPs), it was found that after 24 h of treatment with AgNO₃, the Ag⁺ ions could migrate to the meristematic zone of the root apex and were not restricted by the root calyptra, forming biogenic AgNPs in the apical meristem after 60 h. In the case of the root apex exposed to Ag⁰NPs, the primary uptake route was through the binding of these to the calyptra, followed by a dissolution process triggered by the mucilage low pH and the internal incorporation of the dissolved Ag. Sixty hours after treatment application, Ag ions were located in the apical meristem and the early vascular region. The exposure of the root tip to Ag₂SNPs, after 24 and 60 h, showed that the main uptake route of Ag₂SNPs appears to be their binding to the outer layer of roots and the immediate migration of the NPs to different plant tissues [67].

3.2. Factors Affecting Root Uptake

3.2.1. Effect of Size, Chemical Composition

The uptake of NPs by aquatic plants is affected by their physical and chemical characteristics and those found in the aquatic ecosystem. Due to their hydrophobic nature, non-polar coating substances make NPs more likely to bind to plant roots compared to NPs coated with polar substances [40].

In Phragmites australis, up to 41% of the Ag associated with plant biomass was adsorbed on the root surface, while only 19% was absorbed by roots. Surprisingly, only 1% of the total Ag added to the system was translocated to aerial tissues, while more Ag was recorded in old compared to young tissues [68].

Oxidative stress was observed in Spirodela polyrhiza treated with 6 nm gum arabic (GA)-coated AgNPs (GA-AgNPs) and 20 nm PVP-coated AgNPs (PVP-AgNPs); whereas, exposure to micrometer-sized Ag particles (1 µm) did not result in oxidative stress, coupled with the fact that AgNPs interacted weakly with the 10% Hoagland solution, so the solution possibly did not drastically affect the properties of the AgNPs [69].

Furthermore, in Juncus effusus, Phytolacca americana and Scirpus cyperinus, GA-AgNPs were found to reduce germination and growth. Interestingly, the root system was more negatively impacted compared to the shoot system, whereas the exposure to GA-AgNPs caused a stronger inhibition in comparison to PVP-AgNPs [70].

In Lemna gibba, the application of 50 nm AgNPs, suspended in water and culture medium, formed stable agglomerates for 12 h with an average size between 310.5 and 387.1 nm, respectively, negatively affecting the metabolism of the plant [71]. The aggregation of NPs can decrease the availability of the metal and therefore reduce toxicity by decreasing its absorption by plants. Nevertheless, the aggregates deposited in the sediments may gradually set free these NPs in aquatic systems with once favorable conditions [60].

Spherical AgNPs coated with PVP applied in 10 mg L⁻¹ (an average size of 105 ± 46 nm) display greater aggregation in the growth media compared to the application of 1 mg L⁻¹ (average size of 36 ± 15 nm). The concentration of Ag ions was higher in root tissues compared to leaf tissues, indicating that there is an effective uptake of AgNPs by roots where there is translocation to the leaves [72].

3.2.2. Effect of Shape and Charge

The shape and charge of the NPs also influence their absorption by the plant since they have an important effect on aggregation. Spherical AgNPs tend to aggregate when mixed or diluted in the culture medium due to the presence of different ions as already mentioned.

When the aquatic plant L. gibba was exposed to an inorganic culture medium with 50 nm spherical Ag-NPs, the AgNPs began to agglomerate rapidly, mainly due to the pH (6.5) and ionic strength of the medium (4.25 × 10⁻³ M). The average diameter of the agglomerates was 240 nm, and they presented a Z potential of −34.75 ± 2.15 mV. During 24 h of experimentation, a low amount of dissolved Ag (1%) in the medium, released by the agglomerates, was observed. Because the fraction of soluble Ag in the medium was very low, the internal accumulation of Ag came directly from the AgNPs that released Ag ions inside the plant cells, resulting in decreased plant growth derived from the cellular toxicity caused by the NPs applied [73].

The high ionic strength of the culture medium promotes AgNP agglomeration because it decreases the repulsion between particles caused by the reduction in their surface charge. AgNP agglomeration is also influenced by the exposure concentration; however, the effects observed in Spirodela punctata are mainly due to AgNPs since there is an insignificant dissolution of Ag (0.1 mg L⁻¹) when applying 1000 mg L⁻¹ (the highest AgNP concentration tested) [74].

Wetland plants transport O₂ to their roots to promote aerobic respiration and oxidize phytotoxic compounds (Fe₂, Mn₂, and S₂) in the rhizosphere [75], so the general elimination of Ag from the aqueous phase may depend on the activity of the plant. In studies carried out with P. australis using plants with stem and root (active) and plants with a rhizome, without leaves (passive), they found that in aqueous suspension, the AgNPs coated with Ct (spherical, a size of 90.7 ± 7.9 nm, pH 6–8, and Z potential of −50 ± 5 mV) aggregated and precipitated after 24 h. In the active plants, most of the NPs applied were adsorbed on the surface of the roots (41%), and a smaller amount was absorbed (19%) and translocated (1%) to the aerial tissues [68].

3.2.3. Effect of Plant Species

Plant species represent a key factor affecting the absorption of NPs, given their structural, morphological, and anatomical diversity, coupled with the establishment of a symbiotic association in their leaves or roots. For instance, the species of the Azolla genus host a symbiotic community of bacteria in the cavity of their floating lobes, including the cyanobacterium Anabaena azollae [76] or the species of the Fabaceae family such as Neptunia natans, which form nodules with Rhizobium sp. their its roots [77]. In aquatic plants such as Wolffia globosa, with absent roots and without xylem or phloem cells [78], no translocation barriers for NPs to reach the fronds are observed, which makes this species an interesting organism to elucidate the behavior of NPs in plant systems.

Among the families of higher and lower aquatic plants that have been used in toxicity studies with AgNPs is the Lemnaceae family, a group of vascular angiosperms, also called duckweeds, which grow freely, floating on or under the water surface, and are represented by four genera, Lemna, Spirodela, Wolffia, and Wolffiella, as well as the species of the families Poaceae, Hydrocharitaceae, Ricciaceae, Araceae, Salviniaceae, Iridaceae, and Azollaceae. They are characterized by presenting high growth rates and generating a large amount of biomass, which has allowed them to be used in decontamination processes and the regulation of aquatic systems [79]. This group of plants adapts to environmental conditions. Under optimal cultivation conditions, plants respond to environmental contamination when they perceive it; in sublethal ranges of metals or organic compounds, plants can evolve to have resistance mechanisms due to their rapid growth rate.

Rooted emergent and submerged macrophytes have been scarcely studied in toxicity tests, including studies with AgNPs [80]; however, it is necessary to explore these plant species more in detail, given their ecological importance in the nutrient cycle and as an oasis for aquatic life. Moreover, they are constantly exposed to contaminants through their roots and the entire surface of the plant, which deserves further research [81].

4. Translocation and Accumulation of AgNPs

AgNPs internalized by leaves and roots can be translocated to the vascular system via the apoplast or symplast. In the specific case of roots, after the AgNPs cross epidermal and cortex cells through the apoplast, upon reaching the endodermis, they are blocked by the Caspary band, and some of them can cross the plasmalemma and get inside the cytoplasm of the endodermal cells through the symplast [82]. Once AgNPs are inside the endodermis, they are transported via the symplast through plasmodesmata [45]. The mobilization of AgNPs via the symplast means they reach the vascular bundles, the main transport system in higher plants [83]. Once AgNPs have reached the vascular bundles, they are transported from the root system to the shoot system (i.e., stems, leaves, flowers, fruits) via the xylem in a unidirectional flow and also upwards and downwards (from aerial parts to roots via the phloem in a bidirectional flow [84].

5. Phytotoxicity

Among the aquatic plant genera that have been studied for the adverse effects of AgNPs are Lemna, Spirodela, Salvinia, Pistia, Londoltia, Azolla, Wolffia, Egeria, Juncus, Elodea, Najas, Vallisneria, Riccia, and Limnobium, which have been exposed to AgNPs under laboratory and greenhouse conditions, using inorganic culture media [73] and nutrient solutions such as Hoagland [69,74,85] and Yoshida [64], tap water [86], and even systems such as mesocosms [55].

After AgNPs or Ag⁺ ions accumulate in plant cells, they can affect them at the biochemical, physiological, and structural levels. Oxidative stress, starch accumulation in source leaves and structural damage in chloroplasts have mainly been documented [69,73,74,85] (Table 1).

Furthermore, toxic levels of AgNPs may boost a dramatic decrease in the concentration of photosynthetic pigments and photochemical efficiency [78,85,87]. The degradation of photosynthetic pigments has been characterized for chlorosis in the leaves, and the wilting of the aerial part has also been observed, as well as the loss and darkening of the roots [85,88].

Aquatic and wetland plants exposed to toxic AgNPs have been found to exhibit the inhibition of nutrient uptake and growth [72,89]. Whether these plants use leaves or roots to absorb nutrients and water, they bring with them differences in the toxicity of NPs. Submerged aquatic plants such as Hydrilla verticillate with a shorter root system length can absorb nutrients and water through leaves and stems, so the response to dissolved NPs is immediate and precipitated NPs do not affect them; there is even a reduction in their toxicity over time. In contrast, emergent aquatic plants that have an extended root system for nutrient and water absorption such as Phragmites australis are significantly disturbed by agglomerated and precipitated NPs and exhibit phytotoxic symptoms to a greater extent in a time-dependent manner [90].

In S. polyrhiza, the concentration of AgNPs in plant tissue increased significantly as their concentrations increased in the medium, reaching up to 2.81 mg Ag g⁻¹ dry weight at the highest concentration of AgNPs (10 mg L⁻¹) [85]. This accumulation of Ag is possibly the result of the intact internalization of AgNPs or adsorption of NPs in the plant root; however, the presence of the particles inside or outside the plant is not demonstrated with electron microscopy techniques. In L. gibba, concentrations of up to 17.5 mg Ag g⁻¹ dry weight were recorded under treatment with 10 mg AgNPs L⁻¹ [73]. In the roots of Landoltia punctata exposed to AgNPs as silver sulfide (NPs-Ag₂S) and reduced (Ag⁰NPs), similar concentrations of Ag⁺ ions were recorded, 372 and 389 µg g⁻¹ dry weight, respectively [67].

In the aquatic macrophyte Salvinia auriculata exposed to 1.0, 5.0 or 10.0 mg L⁻¹ AgNPs, the highest removal of Ag⁺ by the plant (88%) occurred at 1.0 mg AgNPs L⁻¹, which was coupled with a higher biomass production. Conversely, at 5.0 and 10.0 mg AgNPs L⁻¹, the removal of Ag⁺ and biomass production decreased due to the presence of Ag⁺ that interfered with phosphorus uptake. Nevertheless, this species demonstrated an extraordinary ability to accumulate Ag⁺, which makes it an excellent candidate for AgNP remediation purposes [72].

The application of microscopy techniques for the localization and distribution of NPs and structural damage in aquatic plants is poorly known; particularly for AgNPs, there are few reported studies [64] compared to terrestrial plants [91,92,93]. When exposing the aquatic fern A. filiculoides to 100 mg AgNPs L⁻¹, structural damage in the rhizodermis and the collapse of the vascular cylinder were observed, which demonstrates the dramatic depletion of the functionality of the root system upon NP exposure. Furthermore, AgNPs accumulated outside the rhizodermis cells and to a lesser extent inside the cells of the outer cortex and endodermis, indicating that AgNPs of 26.74 ± 0.52 nm in size do not cross the rhizodermis cells but that the released Ag⁺ ions enter the root and accumulate inside the plants, even forming AgNPs when Ag⁺ ions are reduced as a consequence of acidic organic compounds being present in the cell wall and the cytoplasm [64].

Importantly, AgNP toxicity has been associated with genotoxicity. AgNPs can provoke gene and chromosomal alterations, cell cycle disruption, micronuclei formation, and changes in gene and protein expression. All these alterations ultimately result in disruptions of plant metabolism, growth, and the response to biotic or abiotic stress [94]. Interestingly, these results have been reported in terrestrial plants, while none for aquatic plants have been found in the existing literature so far. Hence, further research is needed in order to delve deeper into this area of knowledge and understand the severity of DNA damage and its repair mechanisms.

Table 1. Impact of different concentrations of AgNPs on aquatic and wetland plants.

Species	Characteristics of the AgNPs	Dose of AgNPs Supplied	Growth, Phenotypic, and Nutrient Effects	Physiological and Biochemical Effects	Ag Concentration in Plant Tissue	References
Spirodela polyrhiza	7.8 nm	0, 0.5, 1, 5 and 10 mg L−1	• Abscission and disintegration of S. polyrhiza colonies observed from 1 mg L−1, while reduction in dry and fresh weight and decrease in nitrate (N-NO3−) content were observed from 5 mg L−1	• From 5 mg L−1, decrease in chl a content, chl a/b ratio, the maximum quantum yield of PSII, and soluble carbohydrate and increase in proline content were observed	At dose of 10 mg L−1, 2.81 mg Ag g−1 dry weight	[85]
	GA-AgNPs
	spherical
Spirodela polyrhiza	20 to 22.9 nm	0 and 10 mg L−1		• Increased SOD and POD activities • Chloroplasts accumulated starch and reduced intergranular thylakoids • Increased in ROS and GSH contents		[69]
	PVP-AgNPs
	spherical
Lemna gibba	20 to 50 nm (spherical) agglomerates (240 nm)	0, 0.01, 0.1, 1 and 10 mg L−1	• From 0.01 mg L−1, reduction in the number of fronds and decrease in growth rate	• From 0.01 mg L−1, reduction in cell viability • From 1 mg L−1, increase in ROS	At dose of 10 mg L−1, 17.5 μg Ag g−1 dry weight	[73]
	Z potential (−34.75 mV)
Spirodela punctuta	40–60 nm	0, 0.01, 0.1, 1, 1000 mg L−1		• Increased free radicals (ROS and RNS) from 0.1 to 1 mg L−1 after 4 days • No plants survived exposure to 1000 mg L−1 over 14 days • At the highest dose, a decrease in total antioxidant capacity		[74]
	spherical
Wolffia globosa	10 nm, hydrodynamic diameter (18.5 nm)	0, 1, 2, 5, 8 and 10 mg L−1 Post- and pre-illuminated conditions		• At dose 10 mg L−1, decrease in photosynthetic pigments and photochemical efficiency in the post-illuminated condition • Increase in the accumulation of carbohydrates from 1 to 10 mg L−1 (23.1–86.8%) • Up 30% of reduction in the Hill reaction at dose 10 mg L−1 in post-illuminated conditions • Increase in MDA and the antioxidant enzyme SOD under pre-illuminated conditions at 10 mg L−1 • Maximal losses in protein content with exposition to 10 mg L−1 in the post- and pre-illuminated conditions		[78]
	Z potential (−21.1 mV)
	spherical
Lemna minor	PVP-AgNPs (91.81 nm) Ct-AgNPs (80.78 nm)	PVP-AgNPs, Ct-AgNPs (0, 0.05, 0.13, 0.32, 0.80 and 2 mg L−1)	• Decrease in growth rate from 0.80 mg PVP-AgNPs L−1 and at 2 mg Ct-AgNPs L−1 • Reduction in the number of fronds per colony with 0.80 mg PVP-AgNPs L−1 after 14 days • Leaf chlorosis with 2 mg Ct-AgNPs L−1	• Increase in GPX from 0.05 mg L−1 with both sources • Rise in GST with 0.80 mg Ct-AgNPs L−1		[53]
	quasi spherical and small-sized agglomerates
Egeria densa and Juncus effusus	P- and C-Ag(0)NPs: 3.9 nm, pH (8.3), Z potential (−46.1 mV), polydispersity (0.344) C-Ag2S-NPs: 24.2 nm pH (6.9), Z potential (−51.88 mV), polydispersity (0.159)	Pulse treatment [P−Ag(0)–NPs]: 450 mg Ag in 609 L Chronic treatment [C−Ag (0)–NPs]: 450 mg Ag year−1		• Increase in MDA and POD after 10 days in both species with P−Ag (0) –NPs • After 1 day increase in SOD with C−Ag2S−NPs in both species	E. densa From 2.38 to 19.50 μg g−1 in the C−Ag(0)−NPs treatment From 2.01 to 6.27 μg g−1 in the C−Ag2S−NPs treatment. From 1.20 to 17.75 μg g−1 in the P−Ag(0)−NPs treatment In J. effussus, all treatments were at or below 2 μg g−1	[55]
	sulfidized AgNPs 24.2 nm, pH (6.9), Z potential (−51.8 mV), polydispersity (0.159)	Weathered AgNP chronic treatment (sulfidized AgNPs) [C−Ag2S−NPs]: 450 mg Ag year−1
Spirodela polyrhiza	20 nm	0, 0.5, 1, 5 and 10 mg L−1		• Decrease in chlorophyll a, chlorophyll b, and carotenoids concentrations from 5 mg L−1 • Reduction in the maximum quantum yield of PSII from 5 mg L−1 • Inhibition of Rubisco activity from 0.5 mg L−1		[87]
	PVP-AgNPs
Elodea canadensis, Najas guadelupensis, Vallisneria spiralis, Riccia fluitans, Limnobium laevigatum, Pistia stratiotes and Salvinia natans	mixture of colloidal solutions of metal nanoparticles (Mn, Cu, Zn, Ag+, Ag2O) (<100 nm)	Mn: 0.75 mg L−1; Cu: 0.37 mg L−1; Zn: 0.44 mg L−1; and Ag+ + Ag2O: 0.75 mg L−1 7 days		• Decrease in chlorophyll a and b and carotenoid concentration in submerged plants N. guadelupensis, E. canadensis, V. spiralis, and R. fluitans		[86]
Lemna gibba	50 nm and agglomerates	0, 0.01, 0.1 and 1 mg L−1	• Decrease in biomass accumulation from 0.1 mg L−1 after 4 days	• After 4 days, decreased chlorophyll content from 0.1 mg L−1 • After 7 days, reduction in the electron transport per reaction centers (ET/RC) from 0.01 mg L−1 and reduction in the maximum quantum yield of PSII with 1 mg L−1		[71]
	spherical
Lemna minor	29.2 (NP1) and 93.52 (NP2) nm	0, 0.005, 0.01, 0.02, 0.04, 0.08 and 0.16 mg L−1	• Reduction in the number of fronds after 7 days with 0.16 mg L−1 of both sizes. After 14 days, reduction in number of fronds with 0.02 (NP1) and 0.08 (NP2) mg L−1 • After 6 days, decrease in growth rate with 0.16 mg L−1 of both sizes			[95]
	Ct-AgNPs
Lemna pausicostata	50 nm	0, 0.1, 1, 2, 10, 20, 40, 50, 100 and 200 mg L−1	• Growth inhibition and decrease in growth rate from 0.1 mg L−1			[96]
Lemna minor	5–20 nm	0.008, 0.016, 0.032 and 0.128 mg L−1	• Decrease in frond growth between 0.008 and 0.032 mg L−1 is greater than the decrease between 0.032 and 0.128 mg L−1		Up to 18.73 mg Ag g−1 dry weight	[89]
	spherical
Azolla filiculoides	26.74 nm	0, 0.1, 1, 5, 10 and 100 mg L−1	• Decrease in the growth rate and accumulation of dry biomass from 1 mg L−1 • Increase in the doubling rate from 1 mg L−1 • Darkening of the roots, mainly young and growing roots with all AgNP treatments		With 10 mg L−1, 13.43 mg Ag g −1 dry weight	[64]
	quasi spherical
Salvinia auriculata	22.32 nm	0, 1. 5 and 10 mg L−1	• Decreased biomass from 1 mg L−1 • Interference with P uptake from 1 mg L−1		After 64 days with 10 mg L−1, 16.03 µg Ag g−1 dry weight of leaves	[72]
	spherical
Pistia stratiotes	SB-AgNPs	0, 0.02 and 2.0 mg L−1	• After 48 h with 2 mg L−1, plants were unhealthy with all leaves completely wilting, along with root loss and darkening		At 2 mg L−1, 127 μg kg−1 root and 2.71 μg kg−1 leaves	[88]
Londoltia punctata	Ag0-NP 6.3 nm Z potential (−11.5 mV) spherical	10 mg L−1			Up to 389 µg Ag g−1 dry weight in Ag0-NPs and 372 µg Ag g−1 dry weight in Ag2S-NPs	[67]
	Ag2S-NP 7.8 nm Z potential (−10.2 mV) spherical

Abbreviations in alphabetical order: chlorophyll (chl), citrate (Ct), copper (Cu), glutathione (GSH), glutathione-S-transferase (GST), guaiacol peroxidase (GPX), gum arabic (GA), malondialdehyde (MDA), manganese (Mn), peroxidase (POD), photosystem II (PSII), polyvinylpyrrolidone (PVP), reactive nitrogen species (RNS), reactive oxygen species (ROS), silver ion (Ag⁺), silver oxide (Ag₂O), silver sulfide (Ag₂S), sodium borohydride (SB), superoxide dismutase (SOD), zinc (Zn).

Table 1. Impact of different concentrations of AgNPs on aquatic and wetland plants.

Species	Characteristics of the AgNPs	Dose of AgNPs Supplied	Growth, Phenotypic, and Nutrient Effects	Physiological and Biochemical Effects	Ag Concentration in Plant Tissue	References
Spirodela polyrhiza	7.8 nm	0, 0.5, 1, 5 and 10 mg L−1	• Abscission and disintegration of S. polyrhiza colonies observed from 1 mg L−1, while reduction in dry and fresh weight and decrease in nitrate (N-NO3−) content were observed from 5 mg L−1	• From 5 mg L−1, decrease in chl a content, chl a/b ratio, the maximum quantum yield of PSII, and soluble carbohydrate and increase in proline content were observed	At dose of 10 mg L−1, 2.81 mg Ag g−1 dry weight	[85]
	GA-AgNPs
	spherical
Spirodela polyrhiza	20 to 22.9 nm	0 and 10 mg L−1		• Increased SOD and POD activities • Chloroplasts accumulated starch and reduced intergranular thylakoids • Increased in ROS and GSH contents		[69]
	PVP-AgNPs
	spherical
Lemna gibba	20 to 50 nm (spherical) agglomerates (240 nm)	0, 0.01, 0.1, 1 and 10 mg L−1	• From 0.01 mg L−1, reduction in the number of fronds and decrease in growth rate	• From 0.01 mg L−1, reduction in cell viability • From 1 mg L−1, increase in ROS	At dose of 10 mg L−1, 17.5 μg Ag g−1 dry weight	[73]
	Z potential (−34.75 mV)
Spirodela punctuta	40–60 nm	0, 0.01, 0.1, 1, 1000 mg L−1		• Increased free radicals (ROS and RNS) from 0.1 to 1 mg L−1 after 4 days • No plants survived exposure to 1000 mg L−1 over 14 days • At the highest dose, a decrease in total antioxidant capacity		[74]
	spherical
Wolffia globosa	10 nm, hydrodynamic diameter (18.5 nm)	0, 1, 2, 5, 8 and 10 mg L−1 Post- and pre-illuminated conditions		• At dose 10 mg L−1, decrease in photosynthetic pigments and photochemical efficiency in the post-illuminated condition • Increase in the accumulation of carbohydrates from 1 to 10 mg L−1 (23.1–86.8%) • Up 30% of reduction in the Hill reaction at dose 10 mg L−1 in post-illuminated conditions • Increase in MDA and the antioxidant enzyme SOD under pre-illuminated conditions at 10 mg L−1 • Maximal losses in protein content with exposition to 10 mg L−1 in the post- and pre-illuminated conditions		[78]
	Z potential (−21.1 mV)
	spherical
Lemna minor	PVP-AgNPs (91.81 nm) Ct-AgNPs (80.78 nm)	PVP-AgNPs, Ct-AgNPs (0, 0.05, 0.13, 0.32, 0.80 and 2 mg L−1)	• Decrease in growth rate from 0.80 mg PVP-AgNPs L−1 and at 2 mg Ct-AgNPs L−1 • Reduction in the number of fronds per colony with 0.80 mg PVP-AgNPs L−1 after 14 days • Leaf chlorosis with 2 mg Ct-AgNPs L−1	• Increase in GPX from 0.05 mg L−1 with both sources • Rise in GST with 0.80 mg Ct-AgNPs L−1		[53]
	quasi spherical and small-sized agglomerates
Egeria densa and Juncus effusus	P- and C-Ag(0)NPs: 3.9 nm, pH (8.3), Z potential (−46.1 mV), polydispersity (0.344) C-Ag2S-NPs: 24.2 nm pH (6.9), Z potential (−51.88 mV), polydispersity (0.159)	Pulse treatment [P−Ag(0)–NPs]: 450 mg Ag in 609 L Chronic treatment [C−Ag (0)–NPs]: 450 mg Ag year−1		• Increase in MDA and POD after 10 days in both species with P−Ag (0) –NPs • After 1 day increase in SOD with C−Ag2S−NPs in both species	E. densa From 2.38 to 19.50 μg g−1 in the C−Ag(0)−NPs treatment From 2.01 to 6.27 μg g−1 in the C−Ag2S−NPs treatment. From 1.20 to 17.75 μg g−1 in the P−Ag(0)−NPs treatment In J. effussus, all treatments were at or below 2 μg g−1	[55]
	sulfidized AgNPs 24.2 nm, pH (6.9), Z potential (−51.8 mV), polydispersity (0.159)	Weathered AgNP chronic treatment (sulfidized AgNPs) [C−Ag2S−NPs]: 450 mg Ag year−1
Spirodela polyrhiza	20 nm	0, 0.5, 1, 5 and 10 mg L−1		• Decrease in chlorophyll a, chlorophyll b, and carotenoids concentrations from 5 mg L−1 • Reduction in the maximum quantum yield of PSII from 5 mg L−1 • Inhibition of Rubisco activity from 0.5 mg L−1		[87]
	PVP-AgNPs
Elodea canadensis, Najas guadelupensis, Vallisneria spiralis, Riccia fluitans, Limnobium laevigatum, Pistia stratiotes and Salvinia natans	mixture of colloidal solutions of metal nanoparticles (Mn, Cu, Zn, Ag+, Ag2O) (<100 nm)	Mn: 0.75 mg L−1; Cu: 0.37 mg L−1; Zn: 0.44 mg L−1; and Ag+ + Ag2O: 0.75 mg L−1 7 days		• Decrease in chlorophyll a and b and carotenoid concentration in submerged plants N. guadelupensis, E. canadensis, V. spiralis, and R. fluitans		[86]
Lemna gibba	50 nm and agglomerates	0, 0.01, 0.1 and 1 mg L−1	• Decrease in biomass accumulation from 0.1 mg L−1 after 4 days	• After 4 days, decreased chlorophyll content from 0.1 mg L−1 • After 7 days, reduction in the electron transport per reaction centers (ET/RC) from 0.01 mg L−1 and reduction in the maximum quantum yield of PSII with 1 mg L−1		[71]
	spherical
Lemna minor	29.2 (NP1) and 93.52 (NP2) nm	0, 0.005, 0.01, 0.02, 0.04, 0.08 and 0.16 mg L−1	• Reduction in the number of fronds after 7 days with 0.16 mg L−1 of both sizes. After 14 days, reduction in number of fronds with 0.02 (NP1) and 0.08 (NP2) mg L−1 • After 6 days, decrease in growth rate with 0.16 mg L−1 of both sizes			[95]
	Ct-AgNPs
Lemna pausicostata	50 nm	0, 0.1, 1, 2, 10, 20, 40, 50, 100 and 200 mg L−1	• Growth inhibition and decrease in growth rate from 0.1 mg L−1			[96]
Lemna minor	5–20 nm	0.008, 0.016, 0.032 and 0.128 mg L−1	• Decrease in frond growth between 0.008 and 0.032 mg L−1 is greater than the decrease between 0.032 and 0.128 mg L−1		Up to 18.73 mg Ag g−1 dry weight	[89]
	spherical
Azolla filiculoides	26.74 nm	0, 0.1, 1, 5, 10 and 100 mg L−1	• Decrease in the growth rate and accumulation of dry biomass from 1 mg L−1 • Increase in the doubling rate from 1 mg L−1 • Darkening of the roots, mainly young and growing roots with all AgNP treatments		With 10 mg L−1, 13.43 mg Ag g −1 dry weight	[64]
	quasi spherical
Salvinia auriculata	22.32 nm	0, 1. 5 and 10 mg L−1	• Decreased biomass from 1 mg L−1 • Interference with P uptake from 1 mg L−1		After 64 days with 10 mg L−1, 16.03 µg Ag g−1 dry weight of leaves	[72]
	spherical
Pistia stratiotes	SB-AgNPs	0, 0.02 and 2.0 mg L−1	• After 48 h with 2 mg L−1, plants were unhealthy with all leaves completely wilting, along with root loss and darkening		At 2 mg L−1, 127 μg kg−1 root and 2.71 μg kg−1 leaves	[88]
Londoltia punctata	Ag0-NP 6.3 nm Z potential (−11.5 mV) spherical	10 mg L−1			Up to 389 µg Ag g−1 dry weight in Ag0-NPs and 372 µg Ag g−1 dry weight in Ag2S-NPs	[67]
	Ag2S-NP 7.8 nm Z potential (−10.2 mV) spherical

Abbreviations in alphabetical order: chlorophyll (chl), citrate (Ct), copper (Cu), glutathione (GSH), glutathione-S-transferase (GST), guaiacol peroxidase (GPX), gum arabic (GA), malondialdehyde (MDA), manganese (Mn), peroxidase (POD), photosystem II (PSII), polyvinylpyrrolidone (PVP), reactive nitrogen species (RNS), reactive oxygen species (ROS), silver ion (Ag⁺), silver oxide (Ag₂O), silver sulfide (Ag₂S), sodium borohydride (SB), superoxide dismutase (SOD), zinc (Zn).

6. Phytoremediation of Water Bodies Affected by AgNPs

The rapid development of nanotechnological products in different production areas has resulted in industrial emissions to the environment, especially in aquatic systems [24].

Conventional physical and chemical methods used to eliminate pollutants such as NPs from contaminated water and soil are considered expensive and can generate side effects [97], so phytoremediation has been found to be a sustainable, low-cost option to treat water and soil with these contaminants [61]. Phytoremediation takes advantage of the capability of some plant species to absorb, translocate, metabolize, stabilize, volatilize, concentrate, sequester and eliminate contaminants from the environment (i.e., industrial streams, sediments, soils, water bodies or sludge) [98].

There are several phytoremediation techniques for the purification of contaminated water, including the following: phytodegradation (phytotransformation), phytofiltration, phytostabilization (phytoimmobilization), phytoextraction (phytoaccumulation, phytoabsorption or phytosequestration), phytovolatilization, and rhizodegradation (phytostimulation) [99]. In phytoextraction, phytofiltration, and phytostabilization, the use of aquatic plants to get rid of AgNPs from wetlands and water bodies has been reported.

In phytoextraction, plants absorb pollutants through their root system and accumulate them either in the roots or in the shoot system. Once absorbed and stored in plant tissues, pollutants may not be compulsorily hydrolyzed, decomposed or degraded but are removed from the habitat where plants grow. During the phytomining process, minerals or metallic pollutants can be recovered for recycling and reuse by burning the whole plant at high temperatures that may range between 350 and 850 °C [98].

During phytofiltration, plants absorb, accumulate and precipitate pollutants, especially radioactive elements, metals and other minerals in solution, using their roots or other submerged vegetative structures. Once the effluents pass through the root system, they are sieved or filtered (rhizofiltration) to take the contaminants apart. Optimal hyperaccumulator species are considered those that develop an extensive root system with a greater biomass, absorption surface, and accumulation capability while tolerating higher concentrations and toxic levels of contaminants [99].

Phytostabilization involves root uptake, adsorption to root surfaces, and the synthesis of organic compounds such as amino acids, sugars and growth factors. These organic compounds are released into the soil or water near the rhizosphere in order to sequester, precipitate, or immobilize the contaminants close to this subsurface living matrix [61].

Currently, over 800 metal hyperaccumulator plants have been recognized worldwide [100]. The floating aquatic plant species with the greatest potential for phytoremediation are common duckweed (Lemna minor), swollen duckweed (Lemna gibba), greater duckweed (Spirodela polyrhiza), dotted duckmeat (Spirodela punctuta), tropical watermeal (Wolffia globosa), water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes), mosquito fern (Azolla filiculoides) and water fern (Salvinia natans), which are part of a list of approximately 50 species of wetland and aquatic plants [61,64,74,78,86,101,102].

It has been established that the investigated aquatic plants have a high removal capacity of metal nanoparticles from an aqueous solution (30–100%), indicating their high phytoremediation potential [86]. Potential aquatic plants for the phytoremediation of AgNPs are presented in Table 2. However, in the case of S Spirodela polyrhiza, there are damages at physiological and biochemical levels that impair nutrient uptake and growth [69,85]. Although it can be used to evaluate the ecotoxicity of AgNPs, it is a more sensitive species compared to other aquatic plants such as Elodea canadensis, Limnobium laevigatum Najas guadelupensis, Pistia stratiotes, Riccia fluitans, Salvinia natans and Vallisneria spiralis [86]. It is suggested to carry out studies on wetland and aquatic plants that resist contamination by metal nanoparticles and at the same time are capable of eliminating them from the aquatic environment. Importantly, there are few studies that refer to the percentage of the removal of AgNPs from an aqueous solution.

Both the species Pistia stratiotes and Eichornia crassipes display phytoextraction properties, allowing them to remove AgNPs from aquatic environments. In an experiment performed with P. stratiotes, plants were able to extract and retain AgNPs and Ag⁺ ions in a period of 12 to 48 h in the roots, while traces of Ag⁺ were detected in the leaves [88]. The species P. stratiotes exhibits a high tolerance capacity against toxic metals and a large and effective root system that is able to store pollutants contained in water. Since it can reproduce both sexually and asexually, it is able to multiply its population in a few weeks, with a concomitant result of lower inputs for its maintenance. In spite of the fact that it is a noxious weed, it is extensively employed with phytoremediation purposes worldwide [103]. Both P. stratiotes and E. crassipes are able to remove 61% of AgNPs in solution, with the root system accumulating higher amounts of NPs compared to the shoot system [104].

In P. australis, the phytofiltration and phytostabilization of NPs have been reported. This species exclusively accumulated AgNPs and Ag⁺ in the root system. Higher amounts of metal accumulation were observed in the presence of AgNPs and in the absence of rhizosediment since the sediments decreased the availability of Ag [105]. In the species P. stratiotes, Salvinia natans, and S. polyrhiza, phytostabilization and phytoextraction [85] have been reported, while in L. gibba, phytoextraction has been observed [73].

It has been found that all the species mentioned in the present review are capable of absorbing AgNPs and removing them from aqueous environments. Nevertheless, when the concentrations of toxic metals to be removed are above the toxicity threshold and the homeostatic mechanisms to regulate their accumulation fail, then diverse types of damages can be observed in the plants, including necrosis and chlorosis; the deformation of trichomes; the disruption of chloroplasts and stomatal cells; a reduction in the contents of chlorophylls, DNA, RNA, amino acids, and proteins; and a general growth impairment. Therefore, the specific resistance level of plants to determined metals needs to be established, while their specific toxicity threshold deserves further research [106,107]. In particular, the physiological status of a plant (i.e., the interaction between the plant genome and the prevailing growth conditions), besides the content of chlorophylls, carotenoids, anthocyanins, phycobilins and other photosynthetic pigments, are critical factors affecting the capability of plants to tolerate toxic AgNP levels.

Table 2. Phytoremediation potential on aquatic and wetland plants.

Species	Characteristic of the AgNPs	Exposure Concentration/ Exposure Duration	Internalization	Remediation Process	Concentration/ Removal	Refe-rences
Spirodela polyrhiza	7.8 nm	10 mg L−1 72 h 10% Hoagland solution	Yes	Phytostabilization and phytoextraction	2.81 mg g−1 dry biomass weight	[85]
	GA-AgNPS
Lemna gibba	20 to 50 nm	0, 0.01, 0.1, 1 and 10 mg L−1 7 days Inorganic growth medium	Yes	Phytoextraction	Ag concentrations at 0.01, 0.1, 1.0, and 10 mg L−1: 7.72 × 10−3, 9.5 × 10−3, 11.3 × 10−3, and 17.5 × 10−3 mg mg−1 dry biomass weight, respectively	[73]
Lemna minor	5 to 20 nm	0.128 mg L−1	Yes	Phytoextraction	Up to 18.73 mg Ag g−1 dry biomass weight	[89]
Pistia stratiotes	15–20 nm	0.02, 0.2, 2 mg L−1 48 h DDIW	Yes	Phytoextraction; accumulation within roots and leaves	At 0.02 mg L−1: 10.4 μg kg−1 in roots and 0.11 μg kg−1 in leaves. At 0.20 mg L−1: 45.2 μg kg−1 in roots and 1.08 μg kg−1 in leaves. At 2.00 mg L−1: 127 μg kg−1 in roots and 2.71 μg kg−1 in leaves. Plant reduced the contamination level below the WHO MCL by 2 h at 0.2 mg L−1	[88]
Elodea canadensis, Najas guadelupensis, Vallisneria spiralis, Riccia fluitans, Limnobium laevigatum, Pistia stratiotes and Salvinia natans	Mixture of colloidal solutions of metal nanoparticles (Mn, Cu, Zn, Ag+ + Ag2O; <100 nm)	Mn: 0.75 mg L−1; Cu: 0.37 mg L−1; Zn: 0.44 mg L−1; and Ag + +Ag2O: −0.75 mg L−1	-	Phytoextraction	Removal of 76% in P. stratiotes and S. natans; 71% in L. laevigatum and E. canadensis; 65% in R. fluitans; and 59% in V. spiralis	[86]
Pistia stratiotes	AgNPs	0.5, 1, 2 and 3 mg L−1 5 days Water	-	Phytoextraction	Removal of 69.88% and 55.61% at 0.5 and 3.0 mg L−1, respectively	[108]
Pistia stratiotes and Eichhornia crassipes	AgNPs	0.007–0.010 mg L−1 96 h Nutrient solution	Yes	Phytoextraction	Concentrations: 13.48 mg kg−1 total dry mass (roots and leaves) of P. stratiotes; 9.45 mg kg−1 total dry mass (roots and leaves) of E. crassipes. Removal of 61% in both species	[104]
Phragmites australis	PVP-AgNPs (<100 nm)	10 mg Ag L−1 (2 mg AgNPs) 7 days 50 g of rhizosediment mixed with 200 mL of estuarine water	Yes	Phytofiltration and phytostabilization; accumulation in roots and rhizome	Concentrations close to 500 μg g−1 in roots and <25 μg g−1 in rhizome	[105]
Egeria densa and Juncus effusus	P-Ag(0)-NPs: (3.9 nm) C-Ag(0)-NPs: (3.9 nm); C-Ag2S-NPs: (24.2 nm)	450 mg Ag year−1 Freshwater emergent wetland mesocosms	Yes	Phytoextraction	C-Ag(0)-NPs: 2.38 to 19.50 μg g−1 in the whole plant; C-Ag2S-NPs: 2.01 to 6.27 μg g−1 in the whole plant; P-Ag(0)-NPs: 1.20 to 17.75 μg g−1 in the whole plant	[55]
Iris pseudacorus	AgNPs 10–50 nm, average hydrodynamic diameter 58.1 nm, polydispersity index 0.280	0.05 and 0.2 mg L−1 450 days Vertical flow constructed wetland	Yes	Phytoextraction	At 0.05 mg L−1: concentrations of 0.75 μg g−1 in stems and leaves; and 4.42 μg g−1 in roots At 2 mg L−1: concentrations of 1.31 μg g−1 in stems and leaves; and 4.91 μg g−1 in roots	[109]
Salvinia auriculata	22.32 nm	1, 5 and 10 mg L−1 64 days Nutrient medium	-	Phytoextraction	Ag in dry plant [Ag in dry root (μg) + Ag in dry leaves (μg)/(dry root (g) + dry leaves(g)]: up to 15.85, 37.11 and 58.05 µg Ag g−1 for 1, 5 and 10 mg L−1, respectively	[72]
Azolla filiculoides	26.74 nm	0.1, 1, 5 and 10 mg L−1 8 days Yoshida nutrient solution	-	Phytoextraction	Up to 0.43, 3.17, 5.52, and 13.43 mg Ag g −1 dry biomass weight for 0.1, 1, 5 and 10 mg L−1, respectively	[64]

Abbreviations in alphabetical order: copper (Cu), chronic exposure with silver nanoparticles (C-Ag), chronic exposure with silver sulfide (C-Ag₂S), doubly deionized water (DDIW), gum arabic (GA), manganese (Mn), maximum contamination limit (MCL), polyvinylpyrrolidone (PVP), silver ion (Ag⁺), silver oxide (Ag₂O), single pulse exposure with silver nanoparticles (P-Ag), World Health Organization (WHO), zinc (Zn).

Table 2. Phytoremediation potential on aquatic and wetland plants.

Species	Characteristic of the AgNPs	Exposure Concentration/ Exposure Duration	Internalization	Remediation Process	Concentration/ Removal	Refe-rences
Spirodela polyrhiza	7.8 nm	10 mg L−1 72 h 10% Hoagland solution	Yes	Phytostabilization and phytoextraction	2.81 mg g−1 dry biomass weight	[85]
	GA-AgNPS
Lemna gibba	20 to 50 nm	0, 0.01, 0.1, 1 and 10 mg L−1 7 days Inorganic growth medium	Yes	Phytoextraction	Ag concentrations at 0.01, 0.1, 1.0, and 10 mg L−1: 7.72 × 10−3, 9.5 × 10−3, 11.3 × 10−3, and 17.5 × 10−3 mg mg−1 dry biomass weight, respectively	[73]
Lemna minor	5 to 20 nm	0.128 mg L−1	Yes	Phytoextraction	Up to 18.73 mg Ag g−1 dry biomass weight	[89]
Pistia stratiotes	15–20 nm	0.02, 0.2, 2 mg L−1 48 h DDIW	Yes	Phytoextraction; accumulation within roots and leaves	At 0.02 mg L−1: 10.4 μg kg−1 in roots and 0.11 μg kg−1 in leaves. At 0.20 mg L−1: 45.2 μg kg−1 in roots and 1.08 μg kg−1 in leaves. At 2.00 mg L−1: 127 μg kg−1 in roots and 2.71 μg kg−1 in leaves. Plant reduced the contamination level below the WHO MCL by 2 h at 0.2 mg L−1	[88]
Elodea canadensis, Najas guadelupensis, Vallisneria spiralis, Riccia fluitans, Limnobium laevigatum, Pistia stratiotes and Salvinia natans	Mixture of colloidal solutions of metal nanoparticles (Mn, Cu, Zn, Ag+ + Ag2O; <100 nm)	Mn: 0.75 mg L−1; Cu: 0.37 mg L−1; Zn: 0.44 mg L−1; and Ag + +Ag2O: −0.75 mg L−1	-	Phytoextraction	Removal of 76% in P. stratiotes and S. natans; 71% in L. laevigatum and E. canadensis; 65% in R. fluitans; and 59% in V. spiralis	[86]
Pistia stratiotes	AgNPs	0.5, 1, 2 and 3 mg L−1 5 days Water	-	Phytoextraction	Removal of 69.88% and 55.61% at 0.5 and 3.0 mg L−1, respectively	[108]
Pistia stratiotes and Eichhornia crassipes	AgNPs	0.007–0.010 mg L−1 96 h Nutrient solution	Yes	Phytoextraction	Concentrations: 13.48 mg kg−1 total dry mass (roots and leaves) of P. stratiotes; 9.45 mg kg−1 total dry mass (roots and leaves) of E. crassipes. Removal of 61% in both species	[104]
Phragmites australis	PVP-AgNPs (<100 nm)	10 mg Ag L−1 (2 mg AgNPs) 7 days 50 g of rhizosediment mixed with 200 mL of estuarine water	Yes	Phytofiltration and phytostabilization; accumulation in roots and rhizome	Concentrations close to 500 μg g−1 in roots and <25 μg g−1 in rhizome	[105]
Egeria densa and Juncus effusus	P-Ag(0)-NPs: (3.9 nm) C-Ag(0)-NPs: (3.9 nm); C-Ag2S-NPs: (24.2 nm)	450 mg Ag year−1 Freshwater emergent wetland mesocosms	Yes	Phytoextraction	C-Ag(0)-NPs: 2.38 to 19.50 μg g−1 in the whole plant; C-Ag2S-NPs: 2.01 to 6.27 μg g−1 in the whole plant; P-Ag(0)-NPs: 1.20 to 17.75 μg g−1 in the whole plant	[55]
Iris pseudacorus	AgNPs 10–50 nm, average hydrodynamic diameter 58.1 nm, polydispersity index 0.280	0.05 and 0.2 mg L−1 450 days Vertical flow constructed wetland	Yes	Phytoextraction	At 0.05 mg L−1: concentrations of 0.75 μg g−1 in stems and leaves; and 4.42 μg g−1 in roots At 2 mg L−1: concentrations of 1.31 μg g−1 in stems and leaves; and 4.91 μg g−1 in roots	[109]
Salvinia auriculata	22.32 nm	1, 5 and 10 mg L−1 64 days Nutrient medium	-	Phytoextraction	Ag in dry plant [Ag in dry root (μg) + Ag in dry leaves (μg)/(dry root (g) + dry leaves(g)]: up to 15.85, 37.11 and 58.05 µg Ag g−1 for 1, 5 and 10 mg L−1, respectively	[72]
Azolla filiculoides	26.74 nm	0.1, 1, 5 and 10 mg L−1 8 days Yoshida nutrient solution	-	Phytoextraction	Up to 0.43, 3.17, 5.52, and 13.43 mg Ag g −1 dry biomass weight for 0.1, 1, 5 and 10 mg L−1, respectively	[64]

Abbreviations in alphabetical order: copper (Cu), chronic exposure with silver nanoparticles (C-Ag), chronic exposure with silver sulfide (C-Ag₂S), doubly deionized water (DDIW), gum arabic (GA), manganese (Mn), maximum contamination limit (MCL), polyvinylpyrrolidone (PVP), silver ion (Ag⁺), silver oxide (Ag₂O), single pulse exposure with silver nanoparticles (P-Ag), World Health Organization (WHO), zinc (Zn).

7. Conclusions and Future Trends

This paper reviewed the absorption, transport, and translocation of AgNPs in aquatic and wetland plants, as well as their impact at the biochemical, physiological, and structural levels. Attention was focused on the toxicity of NPs in terrestrial plants, with few studies in aquatic plants. These can be absorbed by the leaves through the cuticle and stomata and by the roots through ion channels and plasmalemma transport proteins; they can even follow the symplast and apoplast pathways to be directed to different plant tissues through the phloem and xylem.

The physical and chemical properties of NPs, including shape, size, chemical composition, as well as surface charge and coatings, determine how they are absorbed, transported and accumulated in plant tissues. The effects that they produce in aquatic plants are associated with their concentration, environmental conditions, and plant species, even with the symbiotic microorganisms associated with them, a topic in which in-depth studies are needed. Knowledge of the impact induced by AgNPs and Ag⁺ ions on aquatic plants is still limited, as is the case of their effect on genotoxicity. Meanwhile, the use of organic materials for the manufacture of AgNPs has emerged as a possibility to keep them stable, in addition to their biodegradable and edible properties; however, it is an avenue that remains unexplored and where research is also needed. Knowing and understanding the interaction of aquatic and wetland plants with AgNPs is essential to exploit their utility in phytoremediation since they are capable of storing, stabilizing, or degrading organic compounds, metals, metalloids, and even materials on a nanometric scale.

Since the utilization of AgNPs is being continuously expanded, an enhanced understanding of their absorption and destination in plants, as well as their potential to enter trophic webs, is needed to assess the risks to aquatic ecosystems and human health.