A worm’s saliva to solve the plastic waste crisis?

Of the 8.3 billion tons of plastic produced through 2015, 6.3 billion went to plastic waste. Among them, only 9% have been recycled, according to a study published in The progress of science in 2017. Most of this waste accumulates in landfills, is buried or ends up in the wild. Polyethylene (PE), which is particularly robust, is one of the most common types of plastic … and the hardest to destroy! However, a small insect has the secret behind its degradation.

Enzymes that break down plastic waste

This is the false wax moth, also called “gallery”, a species of Lepidoptera common in Europe. In 2017, an international team of researchers discovered that the larva of this insect (called the “wax worm”), when brought into direct contact with PE, was able to rapidly degrade it. The researchers then reported in the journal Current biology an average degradation rate of 0.23 mg per cm² and per. hour. However, this rate was clearly higher than the rate of biodegradation of PET observed until then – however, the structure of PET (or polyethylene terephthalate) was “easier” to degrade.

PE consists of a long linear skeleton of carbon atoms, which is particularly resistant to degradation. Scientists had not yet determined at that time what allowed the wax worm to break these chemical bonds. Federica Bertocchini, who had contributed to this discovery, returns today to the subject with a new study that solves this mystery. It turns out that saliva from this larva contains enzymes that are able to trigger the breakdown of PE at room temperature.

Cumulative production and disposal of plastic waste (in millions of metric tons). Drawn lines show historical data from 1950 to 2015; dashed lines show projections of historical trends up to 2050. Credits: R. Geyer et al., Sciences Advances (2017)

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Together with polypropylene and polystyrene, PE accounts for 70% of total plastic production. It is now urgent to find solutions to solve the problem of generated plastic waste. One of the most promising areas of research is the biodegradation of plastics, which involves microorganisms such as bacteria and fungi.

But to date, only a handful of microorganisms are known to be able to degrade tough plastic polymers such as polyethylene. Also, in most cases, aggressive pretreatment is required to ensure that the material oxidizes and thus allows microorganisms to exert a certain effect on the plastic. However, the process is extremely slow.

The end of a bottleneck

In fact, oxygen must enter the polymer chain in order for PE to degrade (to cause the CC bonds to break down and form smaller molecules). This is the first phase of oxidation, and it is usually due to exposure to sunlight or high temperatures – which refers to the processes of photodegradation and thermal degradation, respectively. ” This is why, under normal environmental conditions, plastic takes months or even years to decompose. », Explains Federica Bertocchini. It is therefore this phase that is the most problematic: specialists consider it to be a “bottleneck” that slows down the degradation of plastic.

When they discovered the potential in the larvae of Galleria mellonella, the researchers first thought of the microorganisms that live in the digestive system of these worms. They assumed that the latter fed on plastic; its degradation would thus be due to their metabolic activity and their digestive processes. Eventually, the team quickly turned their search toward the worm’s oral cavity. Among more than 200 proteins, they succeeded in isolating two spy enzymes, dubbed Demetra and Ceres.

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According to the researcher, these enzymes support the oxidation step and thus speed up the degradation process. These two enzymes, from the phenol oxidase family, are capable of reproducing the oxidation produced by saliva as a whole alone. They are the first and only known enzymes capable of degrading polyethylene without the need for pretreatment. This discovery “opens up a highway of possibilities for solving the problem of plastic waste pollution”, the researchers write.

New perspectives for recycling and production

The researchers also noted that each of these enzymes had a different effect on the polyethylene. The effect of the Demetra enzyme was visible to the naked eye: Small craters appeared on the surface of the plastic when it decomposed. In addition, this enzyme caused the formation of degradation products (including ketones of different lengths). The oxidation due to the action of the Ceres enzyme is much more discreet: it leaves no visible traces and no by-product has been detected.

Why do these enzymes attack polyethylene? And how did the wax worms get this ability? It turns out that phenols are molecules used by plants to defend themselves against potential predators, such as insect larvae, the researchers explain. Insects could therefore produce phenol oxidase enzymes to oxidize plant phenols and neutralize them – so they can feed themselves safely. However, phenols are also present in many plastic additives, making them the primary target for wax worms.

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This ability to oxidize phenols may be due to an evolutionary process, the team believes. Waxworms actually feed on beeswax (hence their name) and pollen from a wide variety of plant species; since the wax is partly composed of phenols, phenol oxidase enzymes are therefore particularly useful to them.

Other experiments will be needed to study the mechanisms of action of the enzymes in depth and confirm these different hypotheses, the researchers emphasize. In the meantime, this research opens up new perspectives in the form of plastic recycling, but also for the future production of this material – new formulations that facilitate enzymatic degradation were conceivable.

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