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Compostable polyesters carrying degrading enzymes inside the material broke down strongly from coastal water to depths of 5,503 metres. The approach could make some plastics less persistent at sea, but it is not yet a solution for conventional plastic waste.
Compostable plastics are designed to break down under particular conditions, but the ocean is often a poor environment for that process. In the deep sea especially, temperatures are low, pressures are high and microbial activity is limited. A material that degrades in an industrial composting facility can therefore persist for a long time if it ends up in the sea.
A study published in Nature demonstrates a different approach: embedding the enzyme that breaks down the polymer directly into the plastic as the material is manufactured.
The researchers tested four compostable polyesters, PBS, PBSA, PCL and PBAT. They incorporated either the lipase CaLB or the cutinase HiC, enzymes capable of cutting ester bonds in polymer chains, and produced both thin films and small plastic beads.
Rather than only simulating marine conditions in the laboratory, the researchers placed the materials at real marine test sites. Samples were deployed in coastal water at depths of 2–6 metres and at several deep-sea locations ranging from 757 to 5,503 metres.
The difference between materials with and without enzymes was clear. The enzyme-free compostable polyesters lost little or no mass after long periods on the deep-sea floor. Embedding the enzyme substantially accelerated degradation.
For PCL containing three weight percent CaLB at a depth of 757 metres, mass loss increased from 39.7 percent after five months to 99.0 percent after 18 months. The study also reports near-complete degradation of enzyme-embedded polyesters at 757 metres within five months under some of the tested conditions.
Even at 5,503 metres, the researchers observed measurable degradation, although the process was slower under the extreme conditions.
Much less enzyme was needed in coastal water. Films containing only 0.1 weight percent HiC degraded rapidly within one month. PBSA lost 100 percent of its mass and PCL 85.5 percent, while PBS lost 27.4 percent.
The low enzyme loading also had almost no effect on the materials' initial tensile strength. That matters if the principle is to become practical: a plastic designed to degrade after use still has to withstand stress while it is doing its job.
The researchers also measured biological oxygen demand in seawater. The results support a process in which the enzymes first cut the polymers into smaller, water-soluble molecules that can then be further metabolized by microorganisms.
The result does not show that existing plastic in the ocean can suddenly be degraded by adding an enzyme from the outside. The enzyme was integrated into the material during manufacturing, and the study examined specific compostable polyesters.
Common plastics such as polyethylene and polypropylene were not the materials for which this effect was demonstrated. The approach also needs to be assessed for cost, large-scale manufacturing, durability during use and impacts across the full material life cycle.
The important advance is therefore the design principle itself. Instead of relying entirely on the right microbial environment being present wherever the plastic ends up, the material carries part of the machinery that initiates its own breakdown.
The researchers point to fishing lines, fishing nets and other material formats as possible targets for future investigation. If the strategy can be transferred into practical products, some plastic materials could be designed with a much shorter environmental lifetime if they accidentally enter the ocean.
It will not solve plastic pollution by itself. But the study shows that degradability can be engineered more directly into the material, even for environments where nature would otherwise take a very long time to do the job.
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