Can Microplastic Pollution Change Important Aquatic Bacterial Communities?

Scientists have discovered that microplastics are polluting many environments worldwide, including our oceans and coastlines. Some of these plastics will make their way into a particularly important environment—coastal sediments, or the layer of mud below the water. This sediment is home to diverse bacterial life, which plays a key role in nutrient cycles of the ecosystem. These bacteria are critical for healthy environments, but are also easily affected by environmental pollution. Unfortunately, little is known about how the bacteria respond to microplastic pollution. We studied the effects of different microplastics on bacteria living in marine sediments, as well as the subsequent impacts on nutrient cycling. We found, for the first time, that different microplastics can significantly alter these bacterial communities and the nitrogen cycle, which should be studied further to understand lasting impacts on our natural environments.


MICROPLASTIC POLLUTION
You may have heard of microplastics, but you may not know what they MICROPLASTIC A plastic particle that is < mm in the longest direction, often formed by the breakdown of larger pieces of plastic in the environment. are or why everyone seems to be talking about them. Microplastics are simple-they are small bits of plastic, generally 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 [ ]! While this sounds simple, microplastics in the environment are extremely complex. This is because there are many di erent kinds of plastics. The building blocks or "backbone" of any plastic are called polymers. Common plastics are most often named after their polymer, POLYMER Large chemicals that form the building blocks of plastics, often called the plastic "backbone." 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 [ ].
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 [ ]! In aquatic environments, microplastics accumulate in sediment-the layers of mud and dirt that settle to the bottom,

SEDIMENT
The layers of mud and sand that settle below the water. below the water. Many di erent animals live in the sediment and may interact with the microplastic pollution. Scientists have been working to understand the e ects that diverse microplastics might have on living things [ ].

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

BACTERIAL COMMUNITY
A group of di erent species of bacteria that live in the same environment.
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.

Figure
The bacteria in sediments and those in your gut serve similar roles-helping to process nutrients! The left side of the figure illustrates how this works in the aquatic environment, where organic matter (generally dead organisms and wastes, but more recently, plastic) is processed by bacteria in the sediments. The right side compares this to your gut, where bacteria help process the organic matter (i.e., food) that you eat every day! (Created with BioRender.com).
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 ). 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 a ects 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 NITROGEN An essential element that is used to build many of the compounds that living things need to survive.
[ ]. The nitrogen cycle is complex, so we will only focus on a few of its features. In sediments and water, many di erent chemicals contain nitrogen, including ammonium (NH + ) and nitrate (NO − ). Di erent bacteria can transform these compounds from one form to another. Two very important transformations in sediments are nitrification and NITRIFICATION A process that converts one form of nitrogen, ammonium (NH + ), to another form, nitrate (NO − ), which is made possible by microbes.
denitrification. Both processes can work in combination to convert The changes to nitrification (blue) and denitrification (red) that we saw in our experiment are shown above the microcosms containing each type of plastic added to sediments. Both polylactic acid and polyurethane foam microplastics increased nitrification and denitrification; polyvinyl chloride decreased both processes; polyethylene did not significantly change either process compared to the control with no added microplastics (Created with BioRender.com).
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 su cient 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 di erent steps of nitrogen cycling have a di erent 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 di erent microplastics in sediments change the bacterial community composition? And if so, does this also a ect 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 di erent types of microplastics to the sediments in the di erent 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 di erent purposes, so they contain di erent polymers and additives that could a ect the bacterial community di erently. We also made one container with no added microplastics, to represent a normal kids.frontiersin.org May | Volume | Article | situation. This is called a control. The containers were monitored over 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 di erent microcosms, to see if the microplastics changed the bacterial community or nitrogen cycling.
We discovered that the communities containing added microplastics were indeed di erent from the control, and the polymer types resulted in di erent changes! We also found that the microplastics a ected 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 a ect the sediment bacterial community as much as the other plastics ( Figure B). This experiment told us that microplastic pollution in the environment may a ect 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 a ect the sediment bacterial community, and therefore a ect 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 e ects? Is that harm caused by the polymer or the additives? And how much plastic must be in those sediments to a ect the bacterial community? When we have this knowledge, scientists can work with policy makers to help protect coastal zones from the harmful e ects of microplastic pollution.

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.
COPYRIGHT © Seeley, Song and Hale. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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MEREDITH EVANS SEELEY
Meredith Evans Seeley is a marine scientist interested in the fate and e ects of pollutants, particularly microplastics. Currently, she is a Ph.D. student at the Virginia Institute of Marine Science. She previously studied oil spills as a Master's student at the University of Texas Marine Science Institute. She is interested in using her research to inform policy that can protect and preserve our oceanic ecosystems in the face of increasing human influence. In her free time, she loves traveling and exploring the outdoors. *meseeley@vims.edu He has been studying a broad range of research topics, including microbiome responses to natural and human disturbances, the roles of microbiomes in marine organismal health, novel microbes and processes in the biogeochemical nitrogen cycle, microbial regulation of greenhouse gas emission in soils, and microplastic contamination in natural and engineered environments. Over the last years of research, most of his emphasis has been on aquatic environments, but his interests include terrestrial and human systems, as all are interconnected. Rob's work often melds chemistry, biology, and toxicology, and uses chemical analysis techniques to track pollutants, which include historic and emerging contaminants, such as microplastics and polymer additives.