How Microplastics Are Reshaping Aquatic Life

A close-up view of microplastics on a finger.

Plastic has become an almost essential material in our lives. Invented in 1869 by John Wesley Hyatt, the first partially synthetic plastic was created to imitate natural resources like ivory, tortoiseshell, and linen. The introduction of plastic to modern society was groundbreaking because it offered a durable alternative to animal materials, sparing countless animals from slaughter. So, if plastic was made to withstand the test of time, why are we seeing it wear down into miniscule pollutants?

The United Nations Environment Programme (UNEP) supports defining microplastics as “any fragment of plastic that is between one nanometer and five millimetres wide.” For reference, one nanometer is about the length of five atoms placed end to end and five millimeters is around the size of a pencil eraser. There are two types of microplastics: primary and secondary, which we encounter more regularly. Primary microplastics are those that are intentionally manufactured from scratch, with common culprits including microbeads in skin care, glitter, pellets, and microfibers. Meanwhile, microplastics deriving from the breakdown of larger plastic debris, like bottles, bags, tires, are classified as secondary. Most conventional plastics consist of covalent carbon-carbon (C-C) bonds, which are incredibly strong and thus make biodegradation impossible and breakdown a drawn-out process.

The key forces of plastic breakdown are mechanical stress from waves, sand abrasion, and heat combined with ultraviolet (UV) radiation. UV rays from the sun break down plastic through a chemical process called photodegradation. Over time, the intense energy from the sun weakens the carbon bonds in plastic, eroding the material and resulting in fragmentation. These forces prove most disastrous when it comes to aquatic life, since the very waters they feed from harbor hundreds of millions of tons of plastic. Microscopic animals called zooplankton form the base of the aquatic food chain, and it becomes difficult for them to distinguish their usual food (microscopic algae) from microplastic, resulting in the ingestion of the material. Fish and oysters then consume zooplankton, passing microplastics up the food chain. According to the Plastic Pollution Coalition, more than 914 marine species have been observed ingesting plastics. Among them are seabirds, which have developed plasticosis—scarring and inflammation of the digestive tract due to plastic ingestion; coral reefs, which show signs of gut blockages and reduced food intake; and crucial marine organisms like whales, crabs, and as previously mentioned, zooplankton.

Similarly to zooplankton, many sea turtles, fish, and marine mammals mistake plastic for food. When these animals consume large amounts of microplastics or even macroplastics, they feel a false sense of fullness. As their stomachs fill with non-nutritive debris, malnutrition occurs and starvation follows. Although plastic is largely indigestible, it doesn’t just remain passively in the digestive tract. Instead, the microplastics seep into the tissues and organs of various marine species. A study on zebrafish found that microplastic particles accumulate largely in liver and gut tissues, triggering oxidative stress, a state where free radical molecules outnumber antioxidants. This causes damage to proteins, lipids, and DNA, and it also contributes to diseases such as cancer and diabetes. This oxidative imbalance activates inflammatory responses, altering gut microbiome composition and impairing nutrient absorption.

The Israeli Journal of Aquaculture reports that microplastics disrupt endocrine function in various sea animals like loggerhead turtles and Japanese rice fish (medaka). According to the study, these disruptions can alter levels of key reproductive hormones and affect the normal development of gonadal tissues, potentially reducing fertility. This study and other similar ones provide great insight into how microplastics affect bodily systems, but it should be noted that they are conducted with higher concentrations of microplastic particles than those that naturally occur.

A diagram showing how carbon dioxide from the atmosphere leads to ocean acidification, thus impeding calcification, a process by which marine organisms build skeletons and shells.

Furthermore, plastics themselves directly affect the chemical processes that occur in oceans and freshwater reservoirs. Plastics emit potent greenhouse gases such as methane, carbon dioxide, and ethylene as they degrade. When water contains a high concentration of carbon dioxide (CO2), it produces carbonic acid (H2CO3), marked by a reduced pH and higher acidity. This is deadly to shellfish because the acidity dissolves their calcium carbonate shells and prevents them from building new ones. Outside of plastics releasing carbon dioxide into the ocean, CO2 is a known pollutant of our air above the water’s surface as well. Luckily for land animals (including humans), the ocean is a massive carbon sink, absorbing 30% of CO2 emitted into the atmosphere. Unfortunately for sea life, this carbon raises the temperature of surface water, resulting in decreased oxygen solubility and less overall oxygen.

While the ingestion of microplastics largely affects aquatic life, the food chain doesn’t stop at the continental shelf. The seafood we eat has been found to contain filtered microplastics from seawater and primary/secondary consumers. Microplastic particles have been detected in human organs, including, but not limited to, the heart, placenta, and brain. Though research is still ongoing, studies indicate that disruptions in endocrine function and reproductive abnormalities can be caused by plastic particles in the tissues.

Though it is impossible to remove 100% of all microplastics from the Earth, there are ways to reduce their prevalence in human lives. Many water treatment plants add coagulants (iron/aluminum salts) to water, which trap destabilized microplastics. They are then removed through filtration or sedimentation. Using filtered water like this and avoiding plastic containers will reduce the daily ingestion of microplastics. While we humans can slightly limit the amount of microplastics we consume, our aquatic friends have no such option, making it imperative for us to reduce, reuse, and recycle.