Microplastic Pollution

You may have heard of microplastics, but you may not know what they are or why everyone seems to be talking about them. Microplastics are simple—they are small bits of plastic, generally 5 mm wide (about the size of a pencil eraser) or smaller. Plastics can be manufactured to be microplastic size, such as small beads that are added to skin care products or toothpaste. Most often, however, microplastics come from the breakdown of larger pieces of plastic. This is especially true in the ocean environment, where a lot of plastic trash accumulates over time and is exposed to wind, waves, and sunlight, eventually creating microplastics [1]!

While this sounds simple, microplastics in the environment are extremely complex. This is because there are many different kinds of plastics. The building blocks or “backbone” of any plastic are called polymers. Common plastics are most often named after their polymer, for example, polyethylene or polyvinyl chloride. In addition, other chemicals are often added to help a plastic product serve its purpose. These so-called additives vary, but can include color dyes, for example. As such, no two microplastics are identical to each other [2].

In recent years, scientists have discovered that microplastic pollution is widespread, and is growing due to population rise and increasing plastic usage. Microplastics have been found in oceans, lakes, rivers, and soils, on remote mountain tops, inside glacier ice, and suspended in the air [1]! In aquatic environments, microplastics accumulate in sediment—the layers of mud and dirt that settle to the bottom, below the water. Many different animals live in the sediment and may interact with the microplastic pollution. Scientists have been working to understand the effects that diverse microplastics might have on living things [1].

The Sediment Bacterial Community

If you explore sediments in streams, lakes, and rivers, you will discover many living organisms—worms, clams, and crabs, to name a few. But if you explore sediment with a microscope, you will see that there is also a rich community of microbes (i.e., organisms that cannot be seen with the naked eye)! Bacteria are a type of microbe that are very abundant in sediments. Thousands of species of bacteria co-exist, forming a bacterial community. Scientists describe bacterial sediment communities by the kinds of species and how many of each species are present. These bacterial communities perform very important jobs for the entire ecosystem.

One of the roles of sediment bacterial communities is transforming nutrients. Nutrients are needed by plants and animals (including humans) to build essential biomolecules, including proteins and DNA. You get nutrients from food, with the help of bacteria inside your gut. When you eat food, bacteria in your digestive tract help process the food to release the nutrients that your body needs. Sediment bacteria do the same thing for organisms in the water. When organic material, such as dead organisms or feces, sink to the sediments, most nutrients are still trapped inside and not available for other living organisms to use. The bacteria in sediments break down organic material, releasing nutrients for themselves and for other organisms (Figure 1). At the same time, they also make sure there is not too much of any one nutrient, which may cause harm. Thus, the bacterial community affects the types and health of higher organisms that can live in that environment.

The Sediment Nitrogen Cycle

An important nutrient that sediment bacteria help regulate is nitrogen [3]. The nitrogen cycle is complex, so we will only focus on a few of its features. In sediments and water, many different chemicals contain nitrogen, including ammonium (NH₄⁺) and nitrate (NO₃⁻). Different bacteria can transform these compounds from one form to another. Two very important transformations in sediments are nitrification and denitrification. Both processes can work in combination to convert ammonium to nitrate via nitrification, and nitrate to dinitrogen gas (N₂) via denitrification (Figure 2A).

Having the right bacteria in the right amounts ensures that the correct level of ammonium and nitrate are present in an ecosystem. If the balance is wrong, the ecosystem can be changed or harmed. Specifically, there should be sufficient bacteria that complete nitrification and denitrification to remove excess ammonium (which can harm the environment), but not too much to completely remove ammonium (which, in the right amount, is an important nutrient for other organisms). All bacterial species that are important for different steps of nitrogen cycling have a different genetic code. Scientists can read this code to see which bacterial species are present and what roles in the nutrient cycle they perform. We decided to use this tool to ask two important questions.

Our Experiment

Our two questions were: Do different microplastics in sediments change the bacterial community composition? And if so, does this also affect nitrogen-transforming activities? To answer these, we set up an experiment! We collected sediment from a local marsh, distributed it into containers, and added water to create smaller replicas of the natural ecosystem, called microcosms. We added four different types of microplastics to the sediments in the different containers: polyethylene, polyvinyl chloride, polyurethane foam, and polylactic acid. These microplastics were made by grinding larger pieces of plastic in a specialized grinder. These microplastics are used for different purposes, so they contain different polymers and additives that could affect the bacterial community differently. We also made one container with no added microplastics, to represent a normal situation. This is called a control. The containers were monitored over 16 days.

We analyzed the bacterial communities in the sediments before, during, and after the experiment by reading the genetic codes of the bacteria present in each microcosm. First, we used this information to characterize the bacterial community—what species are present and how abundant are they? Then, we specifically looked for the parts of the genetic code responsible for nitrification and denitrification. We compared our findings between the different microcosms, to see if the microplastics changed the bacterial community or nitrogen cycling.

We discovered that the communities containing added microplastics were indeed different from the control, and the polymer types resulted in different changes! We also found that the microplastics affected nitrogen cycling activities; i.e., they altered the abundance of species capable of performing nitrification and denitrification. We found that polyvinyl chloride-treated sediments had the biggest change in the bacterial community at the end of the experiment. In sediments with polyvinyl chloride microplastics, nitrification and denitrification were significantly reduced. In the microcosms with polylactic acid and polyurethane foam microplastics, however, nitrification and denitrification were elevated. The microcosms with polyethylene microplastics were similar to our control, suggesting this plastic did not affect the sediment bacterial community as much as the other plastics (Figure 2B). This experiment told us that microplastic pollution in the environment may affect key bacterial communities and the nitrogen cycle.

Why Does This Matter?

Scientists have long known that sediments play an important role in nutrient cycles, driven by the bacteria. Nutrient cycles are critical to organisms living in the sediments, but also those in the overlying water and beyond. Scientists have recently learned that large amounts of microplastics are polluting aquatic sediments globally, and our research is the first report that microplastics can affect the sediment bacterial community, and therefore affect their nitrogen-cycling activities. The balance of the nitrogen cycle in sediments keeps the nutrients at the right levels for the health of the animals present, including worms, fish, phytoplankton, and aquatic grasses! With this knowledge, we can follow up with important research questions: Which types of plastic cause the most harmful effects? Is that harm caused by the polymer or the additives? And how much plastic must be in those sediments to affect the bacterial community? When we have this knowledge, scientists can work with policy makers to help protect coastal zones from the harmful effects of microplastic pollution.

Most microplastics are formed when bigger pieces of plastic break down in the environment. So, if we want to stop microplastic pollution, we must stop plastic pollution! Plastic pollution does not only happen when we litter, though. Humans produce a lot of plastic trash and, sometimes, it escapes into the environment before it reaches the landfill. This can happen because of an overflowing trash can, a trash bag being lost off a truck, or trash that is swept away in a storm and reaches a local river, lake, or ocean. If we reduce our plastic waste, there will be less that can leak into the environment. You may think of some ways that you can reduce the plastic trash you generate every day, or places where plastic debris leaking into the environment could be captured. Both are critical steps toward reducing microplastic pollution!

Glossary

Microplastic: ↑ A plastic particle that is <5 mm in the longest direction, often formed by the breakdown of larger pieces of plastic in the environment.

Polymer: ↑ Large chemicals that form the building blocks of plastics, often called the plastic “backbone.”

Sediment: ↑ The layers of mud and sand that settle below the water.

Bacterial Community: ↑ A group of different species of bacteria that live in the same environment.

Nitrogen: ↑ An essential element that is used to build many of the compounds that living things need to survive.

Nitrification: ↑ A process that converts one form of nitrogen, ammonium (NH₄⁺), to another form, nitrate (NO₃⁻), which is made possible by microbes.

Denitrification: ↑ A process that converts one form of nitrogen, nitrate (NO₃⁻), to another form, nitrogen gas (N₂), which is made possible by microbes.

Microcosm: ↑ A small replica of a natural environment, used by scientists to answer research questions about how different things may change that environment.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We thank the Freeman Family Foundation for support, via the Virginia Institute of Marine Science (VIMS) Freeman Family Fellowship. We also appreciate a Plumeri Faculty Excellence Award from William & Mary and VIMS Academic Studies department. This is a contribution 3965 of the Virginia Institute of Marine Science, William & Mary. Images were partially created with biorender.com.