Environmental Sustainability Spotlight

Microplastics and Zebrafish

Submitted by the IZFS Environmental Sustainability Committee

Long before making their way to that fateful pet shop in Oregon, zebrafish swam around in the Ganges River basin in South Asia for tens of thousands of years. The slow, temperate waters and predictable weather cycles allowed zebrafish, among hundreds of other fish species, to thrive across the subcontinent [1]. Today, however, it is difficult to look at the Ganges River and imagine anything thriving in its polluted waters. Even if the animals manage to evade the tangled mess of plastic and food waste, the tiny fragments leaching out of them are inescapable.

As plastic products are discarded and exposed to the environment, they slowly release tiny particles called microplastics. This simply refers to any plastic particle smaller than 5 mm but can include a wide range of materials with different properties [2]. They can exist in different shapes and sizes depending on the initial material and environmental conditions, from fragments of plastic bags and bottles to microbeads from beauty products to microfibres from clothes. Factors such as heat, pH, water currents, and wind determine the type of fragments that are released [3]. The one thing they have in common though, is that they are found everywhere: from icy mountain peaks to arid deserts to the depths of the Mariana trench [4, 5]. Living organisms around the world are exposed to them and will often ingest many of these particles on a regular basis. What makes these ubiquitous particles dangerous is that they are not biodegradable. The speed at which we have polluted the world with plastic has meant that evolution has not been allowed the time to adapt.

Fish are not the only species exposed to microplastics. Researchers have looked at the impact of microplastic exposure on various aquatic and terrestrial organisms [6-8]. But we all know how powerful of a model zebrafish can be for an in-depth understanding of various aspects of biology. They benefit from a rare combination of being a prolific model organism and one which is commonly exposed to microplastics in nature. As a result, researchers are beginning to use zebrafish to better understand the biological implications of microplastic pollution [9].

Zebrafish are amenable to high resolution live imaging, allowing researchers to visualize microplastics within the organism. Fluorescently labelled microplastics have been used to study their biodistribution within the body and interactions with different tissues. In embryos and larvae, large particles remain on the chorion and may interfere with hatching, while smaller particles can pass through the pores of the chorion [10, 11]. They then tend to accumulate in the yolk sac and may circulate through the gastrointestinal tract of the developing larva. In adult fish, microplastics can be absorbed through the mouth, gills or skin and circulate through the bloodstream [11, 12]. They have been found in a range of organs including the heart, intestines, muscles, brain, and liver, causing inflammation wherever they accumulate. In particular, researchers have found that exposure of polystyrene microplastics induces oxidative stress in the liver and impaired metabolism of lipids and amino acids, which can cause inflammation of the liver and lipid accumulation [13]. Multiple groups have reported an acute immune response to the foreign particles and severe inflammation that can damage tissues of adult zebrafish [14, 15]. More research will be needed to establish whether the body can clear these tiny poisons and how long-lasting their effects may be.

Decades of research have led to an advanced understanding of the developmental biology of zebrafish, which serves as a strong foundation to study how vertebrate development is affected upon exposure to microplastics. Researchers have found that zebrafish larvae exposed to polyethylene microplastics (the most commonly used type of plastic) developed a range of morphological deformities including larger optic vesicles, enlarged swim bladders and yolk sacs, and abnormal angles between myosepta [10]. Exposure to microplastics also impaired the hatching of larvae, possibly due to physical interference with the chorion, and a lower rate of survival [16]. The few that survive develop morphological defects when older, such as curved spines and pericardial edemas [10]. Similar developmental defects and hatching abnormalities have also been reported for Medaka larvae [17]. These indicate a possible loss of reproductive success for fish species in the wild exposed to water-borne microplastics.

Owing to their small size, microplastics have a high surface area-to-volume ratio, allowing for the adsorption of chemicals and pollutants onto their surface. Researchers have found that various organic compounds commonly found in industrial waste easily attach to microplastic particles due to their hydrophobicity [18]. Their toxicity and potency to cause developmental defects in zebrafish larvae are higher when adsorbed on microplastics than when freely dissolved in water [19]. Additionally, some particles are small enough to interact with individual cells and biomolecules within the body. Adult zebrafish exposed to polystyrene and polyethylene microplastics through their diet exhibited altered transcriptional activity of many genes, often related to inflammation, cell adhesion, and DNA repair [15, 20]. Further research will be needed to understand the specific molecular and sub-cellular interactions, and the zebrafish model will undoubtedly be a key tool.

The most overt evidence of the impact of microplastics is in the way it alters the behaviour of fish. The swimming ability of zebrafish is impaired when exposed to 50 nm sized plastic particles [21]. This has been linked with the capacity of these particles to cross the blood-brain barrier and interfere with neurotransmitters such as acetylcholine, dopamine, melatonin and oxytocin [22]. Such neurological disruptions can impact the feeding and mating behaviour of various aquatic species and impact entire ecosystems. Moreover, this model of neurotoxicity is likely to be of concern for all species exposed to microplastics, including humans.

We are only beginning to understand the impact that our overconsumption of plastic is having on ecosystems around the world. Even as countries sluggishly reduce their plastic waste, the estimated 8.3 billion tons of plastic already present in the environment will continue to poison organisms for decades to come [23]. Zebrafish researchers are leading the way in revealing the impact of microplastics on aquatic life and vertebrates in general. The world needs to keep an eye on the findings from this field as we figure out the best way to conserve ecosystems and limit the adverse health effects of these microscopic toxins.



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  11. Pitt, J.A., Kozal, J.S., Jayasundara, N., Massarsky, A., Trevisan, R., Geitner, N., Wiesner, M., Levin, E.D. and Di Giulio, R.T., 2018. Uptake, tissue distribution, and toxicity of polystyrene nanoparticles in developing zebrafish (Danio rerio). Aquatic Toxicology194, pp.185-194.
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