Nanoplastics – plastic particles that are a thousand times smaller than the width of a human hair – are in the marine food chain. In the ocean, larger plastics like bottles and bags break down over time from physical and chemical weathering processes including sunlight penetration, wave action, and sand abrasion. This turns the plastics into microplastics – fragments that are less than five millimeters in size – and eventually into submicron-sized nanoplastics that drift imperceptibly around the ocean.
That is, until fish find them.
Nanoplastics may be invisible, but that doesn’t stop fish from eating them. But when they do, where do the particles go? Do they hang around the gastrointestinal tract, or do they branch out to other organs?
These were questions MIT-WHOI joint program student Jordan Pitt tackled as an undergraduate at one of Duke University’s summer programs.
“We studied larval zebrafish in the lab to understand where the particles were going, and what types of physiological impacts, if any, they were having on the animals,” said Pitt. “The fish were less than a week old when we ran the experiments, since this is when some of their organs first start to be functional.”
In the lab, Pitt and her team incubated zebrafish embryos in small glass vials filled with seawater and droplets of fluorescent nanoplastics from a pre-made solution. The fish were exposed to the plastics for several days to determine where in their popcorn-sized bodies the particles would end up, and if there were any apparent health impacts.
“Our first question was whether the nanoplastics could penetrate the thick outer walls protecting the embryo,” said Pitt. “Typically, this outer membrane tends to naturally sort out what’s allowed in, and previous studies have reported that other types of nanoparticles couldn’t pass through. So we assumed it would be the same for nanoplastics.”
To Pitt’s surprise, the nanoplastics had no trouble getting through the protective membrane. As Pitt peered into a microscope, green fluorescence from the plastic beads could be seen in the yolk sacs of each embryo. From there, the nano-sized particles migrated to the gastrointestinal track, gallbladder, liver, pancreas, heart, and brain.
The presence of plastics, according to Pitt, did not deform or kill the fish despite exposure during a critical life stage when “a lot of things are happening for the fish’s survival.” There was, however, evidence of lower heart rates and decreased fitness/swimming ability among some of the samples.
“We saw these effects in the lab,” she said, “but whether they would be significant in the wild is another question.”
She explained that a range of nanoplastic concentrations were used to see when and where responses were occurring, but it’s hard to know if the concentrations were representative of real-world ocean conditions. “There isn’t much data out there on concentrations of these particles in the ocean, so we don’t know how environmentally-relevant the levels used in our experiments were.”
Regardless, the work helps confirm reports from previous studies suggesting nanoplastics can migrate to different organs within fish. But for Pitt, it’s just the beginning. Working with toxicologist Mark Hahn and other scientists at WHOI, she’ll be building off her zebrafish research to shed light on other questions, such as how different plastics like polystyrene, polypropylene, and polyethylene may spread differently throughout fish, and if there are particular sizes of nanoplastics that don’t travel through organisms.
According to Hahn, Pitt’s work intersects with one of the big science questions at WHOI: under what circumstances do nanoplastics cross cellular membranes and the blood-brain barrier.
“Microplastics tend to stay in the intestines of organisms and then get flushed out,” he said, “but nanoplastics may be small enough to get into the circulation and cross the tight protective barrier that blood vessels form around the brain. If that’s happening, we’d then need to understand the effect it may have on how the brain functions.”
Ultimately, the goal is to better understand the concentration of nanoplastics in fish and other marine life, and be able to predict where these particles travel throughout an organism’s body. Given that many people eat seafood, there’s an important human health component to the research as well.
“We’re ingesting nanoplastics at unknown concentrations,” said Pitt, “so it’s important that we determine what the plastics do after we consume them.”