Molding the Plastic Brain: Nanoplastics in the Age of Climate Change

Written By Alex Kaye

Alex Kaye

Illustrations by Iris Li

By the time you finish reading this sentence, around 100,000 plastic water bottles will have been purchased across the globe [1]. Let’s follow one of these bottles: once used, it is tossed into a trash bin and taken to a landfill, where heavy rainfall carries it off into a nearby stream before ferrying it into the ocean [2]. From here, ocean currents deposit it into an oceanic garbage patch — in this case, the Great Pacific Garbage Patch [3, 4]. While floating in this 1,600,000 cubic kilometer garbage patch, solar radiation and the physical forces of the tides cause the bottle to slowly break apart into millions of microscopic pieces of plastic called microplastics [5]. These pieces continue to degrade down to the nanoscopic scale, where they will appropriately be called nanoplastics. The size of one nanoplastic compared to a plastic bottle is akin to a single grain of sand in an Olympic-sized swimming pool [5]. It is hard to grasp this scale — one bottle cap could break down into one hundred quadrillion (100,000,000,000,000,000) nanoplastics, a quantity similar to the number of pennies you would need to pay off the total global debt [5, 6]. Though plastics degrade into smaller pieces, a reduction in size does not mean a reduction in harm; due to their minuscule size, nanoplastics have unique properties that pose an active threat to organisms [7, 8, 9]. While the health effects of nanoplastics are still being studied, the landscape of research over the past few years has painted a stark image: nanoplastics are filling the stomachs, brains, and cells of countless animals, causing tissue damage, abnormal behaviors, and a reduced ability to adjust to rapidly changing environmental conditions [10, 11, 12].

Meet the Plastics: Nanoplastics in the Food Chain

So, what happens to these plastics? Plastic is often referred to as a ‘forever chemical’ because it takes hundreds of years to chemically transform into naturally occurring molecules, such as carbon dioxide [7]. Across oceanic garbage patches, most plastics float at the surface of the water. As they break apart, they often sink to the bottom of the ocean like a noxious snowfall [13]. Microscopic organisms called zooplankton mistake these nanoplastics for food and consume them, and because most animals cannot digest nanoplastics, plastic slowly accumulates inside their bodies [13, 14, 15]. When fish and other predators consume nanoplastic-filled zooplankton, they ingest a highly concentrated reservoir of plastic; nanoplastics are funneled into the fish, accumulating in their cells, tissues, and bodily fluids [13, 14, 16]. As these fish are eaten by even larger fish, the concentration of nanoplastics inside the larger fish increases exponentially [13, 17, 18]. Plastic pollution turns food chains into a plastic pyramid scheme, with those at the top of the pyramid carrying a disproportionately large share of plastics [13]. It is worth noting that humans are at the top of this pyramid — on average, we consume up to one credit card worth of plastic every week [19, 20]. Not even someone with a vegetarian diet and a strong water filter can avoid nanoplastic ingestion; nanoplastics are able to infiltrate plants, ensuring that all organisms are doomed to a life full of plastic [21, 22, 23].

“Toxic” ft. Britney Spears & Nanoplastics: Inflammatory Effects of Nanoplastics

Once inside the body, nanoplastics exhibit a litany of toxic effects: they damage cell membranes, provoke inflammation, and cause genetic damage [10, 24, 25]. All cells are surrounded by a protective membrane that allows vital nutrients to enter and keeps harmful substances out [24]. Nanoplastics can stick to these membranes, physically deforming them and encouraging the entry of nanoplastics into the cell [24]. Nanoplastics may even be able to embed themselves inside cell membranes, blocking the passage of important molecules into and out of the cell [25]. The deformation and blockage of cell membranes prevents the cell from carrying out essential functions — such as the movement of materials across membranes — decreasing the cell’s ability to survive and potentially causing cell death [25]. Furthermore, the physical structure of nanoplastics allows them to slip through the cell membrane with ease, granting them access to critical parts of the cell, such as mitochondria, the structures that provide the cell with energy [26, 27]. Nanoplastics have a propensity to accumulate inside mitochondria, where they cause damage and stress, disrupt energy production, and produce molecules called reactive oxygen species [28, 29, 30]. Reactive oxygen species (ROS) can cause further damage to the cell by initiating harmful chemical reactions [26, 31].

The immune system is swift, decisive, and bold, providing the body with strong defenses that protect it from infections and harmful substances. Such boldness typically serves the body well, but in some cases, activation of the immune system can cause more harm than good [32, 33, 34]. The body’s immune cells are capable of recognizing ROS and can respond aggressively, resulting in a cascade of immune responses that causes destruction and widespread health effects [26, 35, 36]. ROS production causes inflammation, which — when coupled with chemical energy reserves that have been depleted due to the damaged mitochondria — results in increased numbers of senescent cells [37]. Senescent cells are cells that have reached the end of their life span but do not actually die. Instead, senescent cells stick around, becoming dysfunctional and continuously secreting molecules that further provoke the immune system and lead to a vicious cycle of inflammatory damage [37, 38, 39].

One Fish, Two Fish, Dead Fish, Rude Fish: Neuroplastic Adaptation in a Changing World

The term ‘plastic’ refers to something highly malleable and easily shaped. Much like the plastic straws and water bottles we use in everyday life, the brain is also easily remolded. Neuroplasticity — the ability of the brain and body to dynamically adapt to the world around it through the rewiring of neural connections — is key to an organism’s resilience [40, 41, 42]. Think of a pool party on a warm summer day: though seemingly innocuous, you may need to fan yourself off to prevent overheating or dodge unruly partygoers to avoid being thrown into the pool. You were not born with these pool party survival skills, but instead learned them at some point in your life. If this ability to learn from the environment had been inhibited, the summer heat would be a lot more brutal, and pool parties a lot less fun. [43, 44, 45]. Nanoplastics cause inflammation and cellular damage that may interfere with our ability to learn and respond to the outside world. Because of their small size, nanoplastics can easily infiltrate the brain, where they damage neurons and increase inflammation [8, 11, 46]. This type of damage frequently occurs in the hippocampus, a structure of the brain involved in memory consolidation and learning [47]. Nanoplastics can also disrupt cell growth and cause mitochondrial dysfunction, impairing the production of new neurons in the hippocampus [26, 48, 49]. Hippocampal function is a critical component of neuroplasticity, making damage to this part of the brain particularly disruptive to animal behavior [48, 50].

Nanoplastics have also been shown to modify the production and recognition of molecules related to learning and memory, which are both critical to an organism’s ability to adjust to changing environments [51, 52, 53]. Nanoplastics can inhibit molecules that break down acetylcholine (ACh) — a neurotransmitter important in learning, memory, and muscle movement — and their presence is correlated with an excess of acetylcholine levels in muscles and the brain [11, 54, 55, 56, 57]. Altered ACh levels may explain some of the behavioral and locomotor abnormalities observed in organisms exposed to nanoplastics, such as impaired swimming in fish and hyperactivity in roundworms [8, 9, 58]. Moreover, the molecule that breaks down ACh is itself important for the brain’s ability to rewire itself — this molecule has been shown to promote the formation of connections between neurons [57, 59, 60]. However, ACh is just one small piece of the puzzle. Nanoplastics have been shown to impact levels of a variety of other neurotransmitters central to learning and memory [52]. Untangling the causes of these alterations is complex; alterations of neurotransmitter levels may stem from neuronal damage, genetic damage, direct interactions between nanoplastics and neurotransmitter-degrading molecules, or some combination thereof. However, one thing is clear: nanoplastics exert a range of toxic effects on the nervous system which ultimately impair neuroplasticity [8, 9, 52].

The impact of nanoplastics on neuroplasticity and the brain is illustrated by the abnormal behaviors seen in animals with high concentrations of nanoplastics: this accumulation has been correlated with aggression, impaired predator avoidance, and disrupted circadian rhythms in zebrafish [8, 61]. The importance of neuroplasticity is particularly salient in the context of the changing world imparted by climate change [44, 62]. These new landscapes present a host of new challenges, including changing temperatures, upended food chains, and altered resource availability [44, 49, 63]. Neuroplasticity permits organisms to adapt to these changes, whether by adjusting to new temperatures or learning to consume a new source of food [44, 64]. Impaired neuroplasticity stifles this sort of short-term adaptation, accelerating the ecological effects of climate change and the rate of species extinction [44, 65, 66].

Life in Plastic, It’s (Not) Fantastic: Addressing the Impact of Plastic Pollution

While the effects of nanoplastics on organisms and ecosystems are already concerning, the dangers of plastic pollution stand out even more in the context of climate change. Nanoplastics elevate animals’ sensitivity to shifts in the environment, and unfortunately for life on Earth, such environmental shifts in the twenty-first century are widespread [61, 67, 68, 69]. Urbanization, for example, rapidly removes natural habitats from animals and forces them to either adjust to a new environment, move to a different location, or die [70, 71, 72]. In effect, impaired neuroplasticity means that a vulnerable species may fade into history, unable to keep up with the changing world [44, 65, 66]. This slide into history can be combated via the active removal of nanoplastics from waterways, firm restrictions on plastic production, and a transition toward the use of biodegradable plastics [73, 74]. Bioplastics — which can be made from plant fibers, fungi, and starches — are a promising alternative to the enduring, highly toxic plastics we rely on today. [74, 75]. Promising new innovations aim to use bacteria to ‘upcycle’ plastics into biodegradable forms, which would transform plastic pollution into environmentally friendly forms of plastic [76, 77, 78]. Regardless of the path forward, without prompt action to stop the manufacturing of plastics, plastic pollution will only continue to rise globally, and the associated health effects of climate change and nanoplastics will only continue to compound [79, 80, 81]. Because nanoplastics impair animals’ ability to adjust to climate change, this trend threatens to accelerate the rate of species’ extinction worldwide [12, 66, 79]. Curbing plastic manufacturing and opting for environmentally friendly alternatives are thus necessary steps to combat the degradation of our brains and ecosystems globally [82, 83]. So, the next time you find yourself in the midst of a record-breaking heat wave, perhaps opt for a reusable water bottle over the hundred-quadrillion nanoplastics waiting to be unleashed from a single disposable plastic bottle.

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