Microplastics (MPs) are now ubiquitous in global ecosystem, therefore all biota is at risk of exposure and potential toxicity. In this study, we presented an overview of information based on literature concerning exposure to MPs and the toxicity of such exposure. Currently, four major routes of exposure have been identified including entanglement, contact, ingestion and inhalation. Humans maybe the most exposed organism because they are at the peak of the food chain. Toxicology effect to marine and freshwater organisms are classified based on exposure dosage as either high (mortality, decreased reproductive output, organ damage) or low (changes in behavior with time). On plants, reports have shown that MPs exposure can affect negatively the growth and depending on exposure concentration and types of MPs and oxidative activities. However, effects on plants maybe short-term and transient. Although, toxicity studies regarding human are still ongoing as per reports, plants and animals are still scantly studied. Animal toxicity studies have widely used
Plastic products with tremendous consumption are ubiquitous in our daily lives and the annual production of plastics is drastically increasing [
Succinctly, plastic products are made up of mixtures of polymers, fillers, and multiple additives to improve its usability. Also, there are other chemicals including unreacted monomers, starting substances and non-intentionally added substances (NIAS; impurities, side or breakdown products) that are also present in plastic. However, most of these chemicals are not covalently bound to the polymer, so they can be released at all stages of the plastics’ life cycle via migration to liquids or solids or via volatilization [
According to the recent studies around toxicities of plastics and their chemical releases, more analyses of the dangers of MP pollution to terrestrial biodiversity are required because the abundance, composition and physicochemical surface properties of particulate material follow typical patterns in terrestrial and continental environments [
MPs are ubiquitous in the environment since they have been found in all components of the ecosystems [
Plants are exposed to MPs through contact (from MPs fallout or through contaminated water or irrigation water) and ingestion by uptake through the rhizosphere in the plastic-soil matrix. Contact exposure was reported for aquatic plant e.g. Duckweed (
Recent studies have also suggested that plants which are exposed to MPs in the soil matrix have the potential to uptake them [
The exposure route of animals, including marine and soil organisms, to MPs is by entanglement and ingestion [
The number of marine species reported to have interacted with plastic debris has increased over time with current counts over 1,000, while about 800 of these have shown interaction with MPs [
Land or terrestrial animals are mainly herbivorous, e.g. goat, cow, etc., and they may be exposed indirectly by consuming MP-contaminated plant and/or directly from consuming contaminated animal feed or water. Carnivores, e.g. lion, tiger, etc., may then be exposed through food-chain process or trophic transfer by consuming or feeding on herbivores tissues and bio-system. The food chain exposure processes are also often exhibited in the aquatic ecosystem, for example, whale or sharks feeding on smaller marine animals may have ingested MPs or amphipod
The routes of exposure of humans to MPs are diverse. The summary and mechanism of exposure are presented in
Humans may also be directly exposed to MPs through the actual ingestion of these particles from drinking water, honey, beers and table salt [
MPs can also be ingested indirectly through personal care products such as toothpaste, face wash, scrubs and soap [
The food chain exposure process is based on human consuming MPs contaminated aquatic organisms, animals or plants, which in turn may have consumed the plastic through MP loaded water or the feeding from other organisms. There are many studies which reported the presence MPs in various organisms from the lowest levels of food chain such as zooplanktonic organisms to the highest levels in both invertebrates (Crustacea, mollusks) and vertebrates (fish) [
Dermal exposure may occur when humans interact with water or soil contaminated with MPs or from contact with particulate MPs. Contact (dermal) exposure will be through skin pores penetration. Exposure by this means is based on individual susceptibility as human skin pores vary by individual. Very small synthetic fibers (<25 μm) can penetrate skin pores which is as small as 40–80 μm, but they will bypassed the striatum corneum [
Humans may also be exposed to MPs through inhalation. This is only possible when the MPs become airborne [
Chemicals can be adsorbed to MPs surfaces and may pose threat biota post-ingestion. The precise mechanisms by which MPs adsorb toxic chemicals in the environment is yet to be understood fully, although the plausible mechanism involves: hydrophobic adsorption, biofilm and plastic additives [
The effects of MPs on soil-plant and water-plant systems have started gaining increasing attention recently. MPs in soil matrix can affect the soil properties including soil aggregation, bulk density and water holding capacity and may also affects water properties such as total dissolved and suspended solids, electrical conductivity, pH and dissolved oxygen [
Although using animal as biomarker are commonly used for neuro- and genotoxicity, this review focused more on the studies that related toxicity to sorption of toxic chemical by MPs [
Outcome comparison of single and combined effects of MPs approach has been commonly adopted across studies. For example, Rehse et al. using
For other chemicals, such as glyphosate, 17a-ethinylestradiol and Ni, toxicity and MPs type (e.g. PVC), reports have suggested that MP-presence may decrease or increase contaminant toxicity depending on treatment methods. Zhang et al. observed an antagonistic effect on the growth of the algae (
For terrestrial organisms, soil invertebrates, such as nematodes, collembolan, oligochaeta (e.g. earthworms) and isopods have been studied [
Currently, the toxicity of MPs to humans is still speculative as there are no studies yet to confirm the toxic effect. Previous reviews have explained in detail the potential effects of MPs to human from different environmental compartments [
Some recent
Currently MPs are a threat to ecosystems. Studies on exposure akin toxicity in tandem with toxic chemical is very important to understanding MPs dynamics and developing a mitigation strategy. Humans are perhaps the most exposed organism mainly because they are at the top of the food chain. Exposure and toxicology effect to marine and freshwater organisms are classified based on exposure dosage as either high - involving decreased reproductive output, damages to organ and death, or low - involving changes in behavior with time. Although toxicity studies regarding humans are still ongoing as per reports, plants and animals are still scantly studied. It is recommended that if a study adopted a co-exposure of MPs and chemical contaminants treatment testing, the experimental design should allow some kind of comparison between single and combined toxicities [
Few plants have only been studied for MPs exposure and effects on vegetative and reproductive growth. Meanwhile, there are more than 200,000 plant species, so it is clear that there is a need for more studies on more different plant species to better understand their response to MPs exposure.
There is a need for study to determine if plants can accumulate toxic chemicals and MPs together from the soil. Also determining if the combined exposure effect on plants is synergistic, antagonistic or additive. This is a basis for precise risk assessment.
As pointed by van Gestel and Selonen, studies focusing on a full dose-response be conducted to address the precise toxicity effects of MPs on organisms is required [
There is a clear need for more scientific studies on the effects of MPs on human health.
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The authors declare no conflict of interest.
Summary of exposure route to environmental microplastics (MPs)
Summary of toxicological studies of microplastics (MPs) exposure on plants
Matrix studied | MPs type | Treatment | Plant studied | Plant part | Result | Conclusion | References |
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Soil | Bio and LDPE | 1 % with and without earthworm | wheat ( |
Above- ground | Bio-affected above ground biomass negatively reducing plant height, number of tillers, fruits and causing thinner stem during the growth process while LDPE showed no clear effect in comparison with control. | Earthworm had a positive effect on the wheat growth and chiefly alleviated the impairments made by plastic residues increasing total biomass by 26.2% after 4 months. However, both Bio and LDPE significantly affected the total biomass. | [ |
Below-ground | Significant decrease in below-ground biomass | ||||||
Synthetic fibre, Bio-PLA and HDPE | 0.001% w/w, 0.1 % w/w and 0.1 % w/w respectively. | perennial ryegrass ( |
Above-ground | Shoot height was reduced especially for synthetic fibre and Bio-PLA. Increased chlorophyll a and b contents compared to control. | The study provided evidence that MPs manufactured of HDPE. PLA, and synthetic fibers can affect the development of L. perenne health | [ | |
Below-ground | Seed germination was reduced especially for synthetic fibre and Bio-PLA. Increased root biomass compared to control except bio-PLA | ||||||
LDPE, PP and PS | 0.5 % w/w (LDPE, PP, PS respectively), 1 % w/w (for LDPE+PP, LDPE+PS, PP+PS respectively) and 1.5 % w/w (LDPE+PP+PS) | Juvenile Lime Tree ( |
Above ground | Showed more negative effects compared to control by reducing height, number of branches, leaf number and area. Highest negative effect shown by LDPE while least by LDPE+PP+PS | C.aurantium generally showed high tolerance (>70%) to the different treatment groups. Despite the observed toxicity symptoms as the decrease in total plant biomass, there was no unfavorable impact on the relative growth rate (RGR) of the tree. | [ | |
Green fluorescent Microspheres | 103 to 107 Particles/mL | vascular plant ( |
Below-ground | Reduced germination after 8 hours of exposure with no significant differences after 24 hours from the control | Increasing exposure concentration, size and time reduces germinating rate and chlorophyll contents. | [ | |
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Sediment | PS | 10 % dry weight | Above-ground | Shoot length reduced with increasing concentration | All effects occurred at higher than environmentally realistic concentrations, suggesting no immediate implications for ecological risks. | [ | |
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Water | PE Microbeads | 0, 10, 50, and 100 mg/L | duckweed ( |
Root | Significant decrease in root length with increasing concentration | Overall, results showed that specific leaf growth rate and content of photosynthetic pigments in duckweed leaves are not negatively affected by polyethylene microbeads. However, investigated particles significantly affected the root growth | [ |
Leaves | After seven days treatment caused less than 10% inhibition in comparison to control (<8%) with no significant effect. Furthermore, photosynthetic pigment concentration (chlorophyll a and b) was not significantly affected compared to controls | ||||||
PE | 50,000 MPs/mL | duckweed ( |
Root | Root length increased with time from 24 to 168 hours with no significant differences from the control | Over seven days PE MPs did not affect photosynthetic efficiency and plant growth. However, 30-day chronic exposure showed some negative effect on root and leaves. | [ | |
Leaves | Photosynthetic pigment concentration (chlorophyll a and b) was not significantly affected compared to controls | ||||||
PS-MPs | 5 μm with 10, 50 and 100 mg/L | Vicia faba | Whole plant | Reduced growth with varying effect on enzyme activities such as catalase (CAT) (decreased significantly), superoxide dismutase (SOD) and peroxidase (POD) (both increased significantly). | Showed mild genotoxic effects on the plant and most probably block cell connections or cell wall pores for transport of nutrients. | [ |
Bio: Biodegradable; PLA: polylactic acid, HDPE: high density polyethylene, PP: polypropylene, PS: polystyrene, LDPE: low density polyethylene.