Microplastics are now ubiquitous in marine and inland waters around the world. Scientists have struggled to identify which small plastics persist longest in the environment.
Similarly, a scientific struggle has been to measure their abundance, especially at the smaller end of the size range. That’s where they’re most likely to be consumed by foundational species near the bottom of the food web, like zooplankton.
Researchers from the Bigelow Laboratory for Ocean Sciences and the University of Minnesota-Duluth have now developed an innovative analytical method. It combines different specialized techniques, including flow cytometry, to characterize and count these small varieties of microplastics.
The team tested out the technique using water samples from Lake Superior. It has provided an important step toward future application of the method. The findings were published this month in the journal Limnology and Oceanography Methods.
“With this method, we’re not just counting particles blindly or relying on mathematical models,” said Bigelow Laboratory Senior Research Scientist Nicole Poulton. “We can determine how much plastic is present and what those plastics are.”
What are microplastics?
Microplastics range from a fraction of a hair’s width to the length of a grain of rice. They include all types of plastic compounds. These small particles are often mixed with harmful chemical additives. They can cross sensitive biological barriers in the brain and gut. They can also act as sponges, absorbing and transporting pathogens and pollutants like oil.
The toxicity and environmental impact of these particles are a function of both how much there is and the type of microplastic. This includes its chemical composition, size and shape. Scientists, however, rarely have all that information.
“Risk assessments of microplastics aren’t focusing on the types and sizes of plastics that we think are most prevalent in the environment. This is partly because we’re bad at measuring them,” said Elizabeth Minor, a University of Minnesota Duluth professor.
In their new approach, the researchers first process the water samples to remove organic matter that could be confused for microplastic. They then infuse the samples with a dye called Nile red that stains plastic. They then use a flow cytometer to line up the microscopic particles and hit each with a laser at hundreds of particles per second.
That causes the stained microplastics to light up. It allows the researchers to separate them out from the rest of the sample. Flow cytometry is commonly used in the biomedical field. Bigelow Laboratory’s Center for Aquatic Cytometry has played a critical role in expanding its use in environmental research.
What did this study achieve?
For this study, the researchers used the flow cytometer at Bigelow Laboratory to isolate and measure individual microplastic particles. Those samples were then sent to Minnesota to be further analyzed. Minor’s lab used pyrolysis gas chromatography-mass spectrometry or pyGCMS. This is a popular tool for determining plastic samples’ chemical composition and total weight.
After refining the method in the lab, the researchers tested it with natural samples of surface water from Lake Superior. They found that particles in the five to 45-micrometre size range were more abundant than larger particles. Those larger particles could be easily measured with traditional methods. They also found both polyethylene and polypropylene. These plastics make up countless products, from single-use plastic bags to textiles, an important first step for identifying sources of plastic pollution.
Contamination is a significant challenge at each step, given the prevalence of plastic in everything from lab equipment to clothing. Particles under five microns are still too small for researchers to confidently enumerate. The Nile red dye creates little solid precipitates that can be difficult to distinguish from the smallest bits of plastic.
Despite these limitations, the researchers’ new method provides a more complete picture of both the amount and type of microplastics in a smaller size range than was previously possible. It also highlights the potential benefits of flow cytometry to rapidly count microplastics. This could help improve the real-world applicability of risk assessments.
“While people are concerned about microplastics, we don’t have a good handle on this problem yet,” Minor said. “But we’ve hit this sweet spot with this new approach where we can start looking at things more accurately and in more detail.”
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