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Water Today Title October 25, 2021

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By Suzanne Forcese

Scientific inquiry into the ubiquitous and persistent evidence of plastics in our environment is not new but systematic study of the sources, pathways, transformations, impacts and sinks of plastics into the marine environment is an emerging science. Because plastics production continues to grow worldwide there is an urgency to quantify the risks of plastics contamination to solve the pollution crisis.

WaterToday spoke with Aron Stubbins who holds joint appointments in the departments of marine and environmental science, chemistry and chemical biology and civil and environmental engineering at Northeastern University in Boston. His focus is determining what happens to microplastics at sea. “Of all the plastics being released into the environment we are finding that some are being removed by sunlight,” Stubbins told WT.

Plastics contamination in the marine environment was first reported by scientists in the 1970’s, less than 20 years after the rise of commercial plastics production, when less than 5 million metric tons were produced per year. Now it is closer to 8 million metric tons. Plastic debris has been detected worldwide in all major marine habitats in sizes from microns to meters. Concerns about risks to marine wildlife have increased stimulating new research into the extent and consequence of plastic contamination in the marine environment.

“Trillions of plastic fragments are afloat at sea,” Stubbins reports in a joint study with Lixin Zhu, “yet they represent only 1-2% of the plastics entering the ocean annually. The fate of the missing plastic and its impact on marine life remains largely unknown.”

Over time says Stubbins the sun’s powerful ultraviolet rays strike large pieces of plastic floating in the ocean and break them into fragments about the size of a sesame seed or smaller, which scientists called microplastics. As sunlight continues to beat down on these microplastics, it degrades them into dissolved carbon.

Stubbins studied this phenomenon by floating microplastics in seawater under artificial ultraviolet rays. “We irradiated post-consumer microplastics (polyethylene, PE; polypropylene, PP; and expanded polystyrene, EPS), standard PE, and plastic fragments collected from the surface waters of the North Pacific Gyre under a solar simulator.”

In a couple of months, exposure to the light increased the amount of dissolved carbon in the water making the plastics even smaller. “The same energy that causes us to get a tan when exposed to sunlight is degrading certain types of floating plastics.” Anecdotally, Stubbins adds that Styrofoam is not found in the ocean’s gyres but only on coastlines which would suggest that sunlight is responsible for the degradation of this type of plastic. “The potential for sunlight to degrade plastics at sea is clear when it is considered that oceanic plastics accumulate in the subtropical gyres which receive about 55% of the ultraviolet sunlight reaching the earth’s surface and buoyant plastics should spend the vast majority of their time at the sea surface.”

The sun isn’t as effective at degrading all types of plastics. For instance, while a 5 mm piece of Styrofoam would disappear after just a few months of exposure, polyethylene – one of our most abundant plastics – would take “decades to centuries” to break down. “Only plastics that float will dissolve in this manner.

Other plastics will just sink and if they get buried in sediments – either at the bottom of the ocean or in soils, they will stay as big plastics for much longer. They’re much slower to degrade in the dark, so it might be thousands of years before they become microplastics. If plastic ends up in an environment that’s very stable, it essentially lasts forever.”

Stubbins says the better we know the degradation processes involved the more we can understand whether those plastics are likely to build up in the oceans, on beaches and in places where they may have cosmetic, human health or environmental health impacts. Sunlight is not going to solve the problem. We still have to understand what happens to the dissolved carbon.

Stubbins is also investigating how microplastics affect the ocean’s microbe population. “Plastics are made of hydrocarbons that are typically, but not always, derived from fossil fuel feedstocks.” (During the conversion from resin to product, a wide variety of additives – including fillers, plasticizers, flame retardants, UV and thermal stabilizers, and antimicrobial and coloring agents – may be added to the resin.) “Some end up as carbon dioxide and some is dissolved as organic carbon. In our experiments we fed dissolved organic carbon dioxide to marine bacteria. Most bacteria use organic carbon as food.” Stubbins, who grew up near the sea in Wales, says, “Most of my research is in the natural carbon cycle. The ocean acts as a big sponge of carbon dioxide. The real message is we would be in worse shape without the ocean. But the ocean can’t keep up.”

Preliminary evidence suggests that some types of microplastic waste promote the growth of bacteria while other microplastics inhibit that growth. “One sample of PE microplastics released organics or co-leachates that inhibited microbial growth. Thus, although sunlight may remove plastics from the ocean’s surface, leachates formed during plastic photo-degradation may have mixed impacts on ocean microbes and the food webs they support.”

Stubbins adds, “We still need to work out the details. For that matter we aren’t even sure how microplastics directly affect humans. The impact of the processes we can’t see is often greater than the impact of what we can see.” Stubbins will be sharing his findings with school children to inspire the next generation of science ambassadors.

According to a report by Kara Lavender-Law, an associate of Stubbins, “The prevalence of widespread contamination of marine habitats with plastic debris naturally leads to adverse impacts ranging from the subcellular level to populations or community structures that might alter ecosystem functioning….Animals that ingest plastic debris may also be at risk of contamination by chemicals associated with plastics that are incorporated during manufacture or that accumulated from contaminated environmental matrices such as sediment or seawater.”

Plastics are the most abundant material collected in studies of marine debris floating on the ocean surface and collected beach surveys and beach cleanups and they are commonly observed on the seafloor.

“We need to do more research,” Stubbins admits. Because it is a global concern, Stubbins is asking for rigor in producing global data. But more importantly “if it is not degradable it should not be disposable. The best practice would be to capture plastics and recycle.” As scientific attention focuses on smaller and smaller particles it is becoming apparent that plastic debris is everywhere. Stakeholders and policy makers need to understand the ubiquitous nature of the problem, how widespread the harm is and what the best prevention and mitigation strategies are.


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