It sounds like a piscine horror story: Tiny pieces of plastic invisible to the human eye penetrate fishes’ brains and turn them into aquatic zombies.
But that scenario is closer to science than fiction. Scientists have now shown that microplastic can be absorbed into fishes’ brains, changing their behavior in potentially harmful ways.
A peer-reviewed paper, published in the September issue of Scientific Reports, suggests that the very smallest pieces of plastic – called nanoplastic – might be disrupting entire ocean ecosystems by changing marine organisms’ natural behaviors. Nanoplastic is defined as bits of plastic that measure 1–100 nanometers in diameter – too small to be seen with the naked eye. An estimated 8 metric tons of plastic finds its way to the ocean every year, where the sun, waves and wind break it down into smaller and smaller pieces.
The researchers reached those conclusions from laboratory experiments where they fed a variety of differently sized pieces of nanoplastic to daphnia, a type of freshwater zooplankton. They found that the daphnia given diets containing higher amounts of the smallest polystyrene nanoparticles had a reduced survival rate. What’s more, when they fed some of the plastic-contaminated daphnia to freshwater carp, the fish began exhibiting unusual behaviors that might make them less competitive in the wild.
“A change in a fish’s hunting behavior may cause reduced growth and a decrease in their ability to avoid predators,” said Karin Mattsson, a postdoc at the University of Gothenberg in Sweden who is an author of the study.
When the researchers used a spectral imaging device, they found bits of polystyrene in the brains of the carp that had been fed with the nanoplastic-contaminated daphnia. The nanoplastic did not appear in the brains of carp given untainted daphnia.
However, the same effects were not seen in daphnia given larger pieces of nanoplastic, according to the scientists. Those fish did not die as quickly as those fed smaller pieces. And carp fed the daphnia that had consumed large pieces of nanoplastic hunted more quickly than any other group of fish tested.
“For me, the most surprising results were that one kind of particle was toxic for daphnia, while the same type of particle but in a larger size was not toxic at all for daphnia during the tested conditions,” said Mattsson, adding that it’s not clear why.
In nature, daphnia and other small aquatic organisms at the bottom of the aquatic food chain consume bits of nanoplastic because they mistake it for algae, yeast or bacteria, which are staples of their diet. Fish in turn eat the daphnia. The scientists hypothesize that the nanoplastic inside the daphnia gets absorbed by the body tissues of predators and crosses into the brain where it could affect their behavior.
Because nanoplastics are so small and have not been well studied, Mattsson said it’s not currently possible to estimate and replicate the nanoplastic concentrations that exist in the wild. So Mattsson’s team studied the effects of one type of nanoplastic at different concentrations and sizes to observe the effects on living organisms.
Plastic that’s been broken up in aquatic ecosystems where pollutants are present tends to absorb flame retardants, perflourinated compounds and other persistent organic pollutants, as well as heavy metals. The known health issues associated with these chemicals include abnormal brain development and functioning, cancer and organ failure. Chemicals added to plastics in the production process, such as polychlorinated biphenyls (PCBs) and phthalates, are known to leach out of plastics that litter the ocean, rivers and lakes.
Neel Aluru, a biologist at Woods Hole Oceanographic Institution, has studied how such PCBs affect behavior in zebra fish. He’s found that fish exposed to PCB-polluted water during development tend to exhibit anxiety-like behaviors later in life. The brains of PCB-exposed fish also show molecular variation, suggesting that PCB exposure has caused changes in their genetic code.
“It’s an interesting idea and there are already studies showing the effects of plastic particles on various stages of fish,” Aluru, who researches how toxins commonly found in plastic affect organisms’ genetic traits, said of the new research. “This study is unique in the sense that they fed plastic to daphnia first, and then daphnia to fish. But the results shown in the paper are unconvincing.”
Aluru said he questioned the large error bars on the scientists’ graphs, which represent variability in data and suggest some uncertainty in their results. Aluru suggested the paper would be more persuasive if it included images of the affected fish brains as well as links to videos of the animals’ feeding behavior.
“We wrote the manuscript for a journal that had a limit regarding number of figures so we decided to show these ones,” said Mattsson. “In supplementary information, we show the spectra from the detection of polystyrene in the brains” in graph form.
She said that while the study had its limits because it focused only on one kind of plastic (polystyrene) in two types of animals (daphnia and carp), it’s possible that different types of nanoplastic are causing different kinds of effects in a wide variety of aquatic organisms. But more research is needed.
Aluru acknowledged that the study is an important first step in determining how nanoplastics affect organisms’ health and behavior. “Based on this study, future research should look at how nanoplastic particles reach the brain,” said Aluru. “Where are they in the brain and how do they affect behavior? I don’t doubt that plastic particles are toxic but it is important to show all the results clearly.”