The Ecological Effects of Micro(nano)plastics in the Water Environment

1. Introduction

Aquatic systems worldwide are subject to severe and diverse anthropogenic pressures, almost always acting simultaneously and inevitably challenging the resilience of whole ecosystems [1,2,3]. One of the main drivers of change is pollution, due to the accumulation and/or constant input of pollutants. However, there also exists a major internal source of so-called secondary pollutants that originated from existing ones as they degraded (e.g., drug metabolites or fragmented pieces). A striking example of an emerging secondary pollutant is fragments of primary plastic products discarded into the environment, leading to the creation of micro- and nanoplastics. Secondary microplastics (e.g., fragments of disintegrated fabric or Styrofoam) may compose 20–100% of plastic items found on beaches [4]. In fact, fibers are the dominant type and shape of microplastics (>90%) found in the environment [5,6]. Interestingly, the most abundant primary plastic type found in aquatic systems has been identified as plastic of industrial origin, e.g., beads found in personal care products (particles < 1 mm) [7,8].

Plastic products can break into smaller pieces due to, among others, mechanical damage [9], interaction with the environment [10], biological action [11], and UV irradiation [12]. It is worth mentioning that, until now, the environmental concentration of microplastics is largely unknown, as complete extraction and identification of plastic material from an environmental matrix (water, sediment, and soil) is practically impossible. Even less is known about nanoplastic concentrations in the environment. Due to methodological challenges [13], environmental compartments are not sampled for nanoplastics.

Nevertheless, laboratory-based (eco)toxicological studies employing a wide range of micro- and nanoplastic particle concentrations provide ample evidence of particle ingestion and short-term (e.g., a few hours) effects on the molecular and cellular level in various tested organisms, such as differential gene expression, oxidative stress, and immunological response. For example, significant stimulation of the immune response in the form of phenoloxidase activation occurred in the midge Chironomus riparius larvae exposed to 5 and 20 g kg⁻¹ of polyethylene microplastic within 48 h [14]. In the blue mussel Mytilus spp., as low as 0.008 and 10 µg L⁻¹ of plastic particle mixture (polyethylene and polypropylene pieces from commercially available products) caused a significant increase in superoxide dismutase and catalase activities (an antioxidant response initiated in response to oxidative stress) [15]. However, some responses can be transitory. For example, oxidative stress caused by plastic ingestion in various aquatic organisms did not lead to oxidative damage [16].

Long-term effects on, e.g., the organismal and multi-generational levels are not so prominent, although they were studied much less than the short-term ones, and there is a need for more studies in this direction. For example, in C. riparius, exposed to polyamide microplastic (1 g kg⁻¹) for seven generations, allele frequency changes occurred (indicating strong selection pressure exerted by polyamide), and enrichment for oxidative stress pathways was revealed. Midges of the first generation showed reduced emergence, but they completely recovered in the third generation, suggesting an adaptive response of this insect to polyamide contamination [17]. On the contrary, in the fly Drosophila melanogaster exposed to polystyrene nanoplastic (100 nm; 1, 10, 50, and 100 mg L⁻¹) over five generations, only at the highest concentrations (50 and 100 mg L⁻¹) and only at the peak spawning period (4/20 days) did egg production decrease starting at the third generation and egg eclosion rate reduce starting from the fourth generation. However, genes related to reproductive system functioning were affected, indicating the potential of polystyrene nanoparticles to induce reproductive toxicity in the flies [18]. Discrepancies between genetic and molecular markers are common and can be caused, for example, by sampling time when transcription has already occurred but is not yet reflected in the enzyme activity [19]. Recovery in gene expression may also occur under changing environmental conditions [20].

This Editorial highlights the findings of the Special Issue collection. Editorial comments following each work provide an outlook beyond these results and put them into a broader context of available, often contradictory, knowledge on the micro- and nanoplastics impacts on aquatic species.

2. Main Messages of the Special Issue

Six articles were published in this collection, presenting recent advances in related topics.

Bottari et al. (Contribution 1) analyzed the impact of plastic litter on marine biodiversity using data from public posts on Google, Instagram, and Facebook from 2009–2023. Moreover, 61% of the reports were related to entanglement (defined as plastic near or within a part of the body), and 39% were ingestion cases (defined as plastic in the digestive tract). Marine turtles were the most affected animals (78%), while teleosts and invertebrates amounted to nearly 2% each. Moreover, 61% of entanglements were caused by abandoned fishing nets, and 24% by fishing lines. According to the authors, entanglement reports can be useful when developing marine conservation measures. Indeed, adjustments by the fishing industry towards plastic-less practices may greatly reduce the presence and impact of plastic litter in the seas (e.g., removal of already abandoned/lost nets and the obligation to dump worn-out fishing nets in appropriate places). Further, having a detailed, verified database of entanglement cases may indicate the distribution of plastic litter by source and size and may also help identify potentially vulnerable parts of the seas for the marine wildlife to which biodiversity conservation efforts may be directed.

Blinova et al. (Contribution 2) investigated the toxicity of a plastic additive, 1-hydroxycyclohexyl phenyl ketone (1-HCHPK), to freshwater animals. Although the potential health hazards of 1-HCHPK, such as stimulation of breast tumors in animals, have been known, there is scarce information about their environmental hazards, especially when most of the available toxicity data have been derived from acute toxicity tests. The acute toxicity to pelagic (Daphnia magna and Thamnocephalus platyurus) and benthic (Heterocypris incongruens) freshwater crustaceans ranged from 27–55 mg L⁻¹. However, in the course of three consecutive generations, the concentration of 1-HCHPK < 1 mg L⁻¹ was not shown to be toxic to D. magna. Based on the European Environment Agency data [21] on the ready biodegradability of 1-HCHPK in water, the authors concluded that the environmental concentration of 1-HCHPK is unlikely to exceed 1 mg L⁻¹ and that 1-HCHPK is not hazardous below this concentration. Nevertheless, further research on the long-term toxic potential of 1-HCHPK involving other pelagic and benthic organisms is needed to confirm the safety of 1-HCHPK for a wide range of aquatic inhabitants. This may also include gene expression studies to exclude any initiation of the action of this pollutant at this concentration level.

Aguirre-Martinez et al. (Contribution 3) addressed the combined effect of microplastic (polyethylene 1–5, 27–32, 45–53, and 212–250 µm, different concentrations from 16–16,000 particles mL⁻¹) and organic micropollutants (phenanthrene and chlorpyrifos) on aquatic and benthic crustaceans D. magna and podocopid ostracods. The ingestion of pure microplastic particles or their adherence to appendages did not induce toxic effects in the organisms, except for particles of the smallest size (53% mortality in juvenile daphnids). However, the exposure to polluted microplastics (after sorption of organics) resulted in 50–100% mortality in juvenile daphnids and 33–37% mortality in the ostracods within 24 or 48 h. Indeed, evidence of the role of microplastics as vectors of various aquatic pollutants is accumulating. One of our increasing concerns in the near future will be the presence of small-sized secondary plastics, which will have a larger surface area and hence a high sorption capacity for environmental pollutants. This may represent a greater hazard for aquatic wildlife in the future.

Khosrovyan et al. (Contribution 4) addressed the toxic potential of conventional (polyamide, 0–180 µm) and compostable (a soft shopping bag ~5 mm and a relatively hard PLA-based cup ≤ 5 mm) plastic particles for the goldfish Carassius auratus (96 h exposure, concentration 30 mg L⁻¹). In addition, two environmental states of all the plastic materials were considered, i.e., as it is (virgin) and after weathering by UV-irradiation. The fish were noted to pick up and immediately reject the compostable plastic particles. Nevertheless, one of such plastics (PLA-based particles) caused lipid peroxidation (oxidative damage) in the brain or gills of the fish. UV-weathering did not induce additional toxicity compared to non-weathered (virgin) ones. Indeed, various studies conducted in laboratory conditions reported that plastic particles are rejected by fish (e.g., [22]). However, particles also come into contact with organisms through skin interaction, lodging in gills, attachment to other body parts, and tissue damage. For example, they could cause irritation or tissue damage when retained in the oral cavity before rejection or when attached to the gills before being washed out. This could be one of the reasons for the observed oxidative damage, even during a short exposure time. However, this effect can be transitory and not necessarily yield long-term harm. Nevertheless, bioplastics or biodegradable plastics have often been reported to produce similar toxic effects as conventional ones (e.g., [23,24]). This increases our concern about whether the alternatives to conventional plastic are safe for the environment.

Ganzha et al. (Contribution 5) demonstrated that the climbing perch Anabas testudineus also avoided plastic particles (expanded polystyrene pellets 2.5–3.5 mm). While the fish grasped and retained the particles in the oral cavity, they were all finally rejected, likely due to effective taste and tactile systems. Indeed, selective feeding of fish depends on fish and prey traits and can be affected by factors such as prey size, drift behavior, feeding type, and abundance [25]. There is considerable evidence of plastic ingestion by wild fish [26,27,28]. In fact, floating or drifting plastic particles can be caught occasionally (e.g., confused with food particles). The higher the concentration of plastic particles in the surrounding water, the greater the chance of their ingestion.

Morgalev et al. (Contribution 6) suggested a method for determining the spread and concentration of micro- and nanoplastic particles. It is based on the phototrophic behavioral response of saltwater crustaceans to light stimuli. The application of two successive light stimuli of different intensities to the plastic-contaminated medium (0.15 mg dm⁻³ of polypropylene bag fibers and 0.3 mg dm⁻³ of polystyrene spheres, 100 nm) showed a decrease in the concentration of crustaceans Artemia salina and Moina salina over 3 h. This effect of micro- and nanoplastics on altering the behavior of crustaceans demonstrates the possibility of using this method for the detection of plastic contamination in water bodies. According to the authors, it can also serve as a tool for monitoring the state of mesoplankton communities. However, on the other hand, the results of the study may suggest that micro- and nanoplastics may not be dangerous for these organisms as they avoid contaminated areas, thus greatly reducing the chance of their ingestion. In addition, ingested plastic particles can be successfully removed from the body, and if they do not damage or obstruct the gut, they may not pose harm to organisms [29]. Nevertheless, a sharp increase in the amount of secondary plastic in the environment will inevitably increase its bioavailability to wildlife.

3. Conclusions

In spite of all the global evidence of the negative impacts of plastic pollution on aquatic systems, the results are often contradictory, and there are no systematic studies starting from the molecular action of micro- and nanoplastics on the long-term effects on aquatic species. The research in this field should be raised to a higher systemic level, and the outcomes should be tied to the observation/analysis of field-collected organisms. By doing so, we can extend our current knowledge to the population level and prepare the groundwork for the standardization of testing methods for risk assessment of plastic contaminants in aquatic systems.