Article body copy
The oceans are full of microplastics, yet scientists have a limited understanding of just how these tiny particles impact fish. Part of the issue is that plastic particles in the sea are often covered in microorganisms and chemical pollutants such as oil, and isolating plastic from these contaminants can be difficult.
Now, a systematic review of 46 research projects has assessed the toxicity of pristine plastics on fish, finding that the smallest plastics have the biggest impact, particularly when it comes to behavior and neurological functions.
In the new work, Hugo Jacob and Marc Besson, marine biologists at the International Atomic Energy Agency in Monaco, focused on studies of microplastics and nanoplastics smaller than 0.1 micrometers in size. The scientists analyzed research that examined the impacts of feeding uncontaminated plastic to fish, noting in particular how the plastics affected their biological functions and systems.
They found that the particles adversely impacted a third of the nearly 800 biological outcomes examined, with behavioral, sensory, and neuromuscular functions most severely affected.
Plastic exposure was linked to neurotoxicity and abnormalities in brain development and structure. In studies that looked at larger microparticles, those effects were likely driven by disruptions to the immune system, metabolism, and microbiome. Other studies suggested that plastic particles less than 500 nanometers in size could be small enough to enter the brain and directly initiate such neurological disorders.
Plastic particles smaller than the diameter of a human hair—less than 20 micrometers—were the most toxic to fish, since the minute particles can pass through internal organ tissues and blood vessels.
The review also highlighted major issues with microplastic research. In particular, nearly 75 percent of the studies used spherical plastic particles, known commonly as plastic beads, and 70 percent focused only on polystyrene and polyethylene. Taken together, the studies essentially ignored a suite of plastic compounds of various shapes and sizes that are widely found in the world’s oceans.
Martin Wagner, an environmental toxicologist at the Norwegian University of Science and Technology who was not involved in the research, finds that skew in the literature frustrating. He says the research community is a bit lazy when it comes to buying pre-made plastic beads. “There are many different types of plastic and they are really underrepresented in the research.”
This bias raises questions about how well these findings translate to the real world, Wagner says. He would like to see studies move away from polystyrene and polyethylene and toward other common plastics that pollute the marine environment, like PVC, which is found in everything from pipes to raincoats; PET, which is used in drinking bottles; and polypropylene, which is common in food packaging.
Susanne Brander, an ecotoxicologist at Oregon State University, agrees that too many studies are done with plastic beads, which aren’t “necessarily comparable to what animals will be exposed to in the [wild].” In the last few years, she says, “we’ve seen a big shift away from using just purchased plastics to people actually generating their own plastics in the lab.” Brander and her colleagues now use a specialized mill to grind water bottles and other plastic products into microplastics, which they use in their research.
Brander says the review is also important because it aims to identify which component of plastic causes the poor outcomes in fish. “We are starting to take a step back and say, Okay, maybe it is more about the polymer type and the shape and the size,” she explains, “versus whether there is a little bit of chemical absorbed on it.”