Fragmentation and depolymerization of conventional and biodegradable microplastics in Eisenia fetida and Eisenia andrei
This study demonstrates that while gut passage through *Eisenia* earthworms significantly fragments and chemically alters polyethylene, polybutylene, and polylactic acid microplastics, the biodegradable polymer (PLA) causes the most severe chemical degradation and earthworm mortality, indicating it is not necessarily the safer option in agricultural soils over short exposure periods.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The soil beneath our feet is a bustling world of tiny creatures that churn, eat, and recycle organic matter, keeping the earth fertile and alive. Among these workers are earthworms, often called the engineers of the ecosystem, because they move vast amounts of soil through their bodies. For decades, scientists have worried about how plastic pollution affects these animals, focusing mostly on how the plastic hurts the worms. But a newer, more complex question has emerged: what happens to the plastic itself when it passes through a worm? Plastics are not just inert chunks of trash; they are long chains of molecules that can break apart. When a worm eats a piece of plastic, the harsh environment inside its gut might chop the plastic into smaller pieces or even chemically alter its structure. This process is critical because if the plastic breaks down into invisible fragments or toxic chemicals, it could become more dangerous to the soil and the creatures living in it, rather than less.
A team of researchers set out to watch this process happen in real time, using two common types of earthworms found in compost: Eisenia fetida and Eisenia andrei. They wanted to see what happens when these worms eat three different kinds of microplastics—tiny particles smaller than a fingernail. Two of these plastics were the traditional, oil-based kinds that are known to be very tough and resistant to breaking down: polyethylene, the material used in many plastic bags, and polybutylene. The third was polylactic acid, a plastic made from plant starch that is marketed as biodegradable, meaning it is supposed to break down naturally in the environment. The scientists placed the worms in containers with compost mixed with these plastics and watched them for 35 days. Afterward, they carefully extracted the worms and retrieved the plastic particles that had passed through their digestive systems to measure exactly how much the plastic had changed.
The results revealed a surprising story about how different plastics behave inside a living creature. First, the worms broke all the plastics into smaller pieces. On average, the particles the worms excreted were about 58 percent smaller than the ones they had eaten. However, the type of plastic mattered greatly. The biodegradable plastic, polylactic acid, was broken down the most, followed by polybutylene, with the traditional polyethylene changing the least. This physical breaking apart was only half the story. When the researchers looked at the chemical structure of the plastics using a special light-based scanner, they found that the biodegradable plastic had also suffered the most chemical damage. Its molecular chains were cleaved or cut apart much more severely than the chains in the other two plastics. In fact, the chemical change was so distinct that the researchers could tell exactly which plastic a worm had eaten just by looking at the chemical signature of the remains.
Perhaps the most striking finding was how this breakdown affected the worms themselves. Contrary to the hope that biodegradable plastics are safer for nature, the worms that ate the biodegradable plastic died at the highest rates. The death rate for worms eating the plant-based plastic was significantly higher than for those eating the oil-based plastics. The researchers found that one species of worm, Eisenia fetida, was generally more sensitive to the plastics than the other, but this difference became much larger when the worms ate the plastics that broke down faster. The more the plastic degraded chemically inside the worm, the more likely the worm was to die. This suggests that the process of the plastic breaking down releases something—perhaps small chemical fragments or byproducts—that is harmful to the worm's health. The traditional, tough plastics caused less chemical change and resulted in fewer deaths, while the "green" plastic, which was supposed to be the safer option, caused the most physical and chemical disruption and the highest mortality.
The study also used computer models to see if they could predict these outcomes based on the data they collected. The computers were very good at predicting the chemical changes in the plastic, confirming that the type of plastic was the main driver of what happened. However, the computers struggled to predict how much the plastic would physically shrink in size. This suggests that while the chemical breakdown is a direct result of the plastic's material, the physical chopping up of the plastic inside the worm is a chaotic process, likely depending on random factors like how the plastic moved through the gut or how hard it was squeezed. This distinction is important because it means that simply knowing a plastic is biodegradable does not guarantee it will be harmless in the short term.
Ultimately, this research challenges the simple idea that biodegradable plastics are automatically better for soil life. Over the 35-day period of the experiment, the biodegradable plastic was not the more benign option; it broke down the fastest and killed the most worms. The study indicates that the intermediate stages of biodegradation, where the plastic is breaking apart but has not yet vanished, might actually be a period of high risk for soil creatures. While these plastics may eventually disappear over long periods, the process of them breaking down in the gut of a soil engineer can release harmful effects that traditional, non-breaking plastics do not cause in the same way. The findings suggest that we need to look beyond just whether a plastic can eventually decompose and consider what happens to the soil and its inhabitants while that decomposition is taking place.
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