In recent years, all the plastic that ends up in the environment has been increasingly in focus. This ruins the environment and disseminates microplastics into land and marine food chains. However, some organisms appear to be able to break down polyethylene, including the larvae of the greater wax moth that usually eat beeswax and other parts of beehives. Now researchers have come a step closer to understanding how these larvae biodegrade polyethylene. The goal is to be able to recreate these processes so that polyethylene can be recycled rather than incinerated.
Beekeepers curse greater wax moths (Galleria mellonella) from afar because their larvae (waxworm caterpillars) can happily chomp through the honeycombs of their bees, destroying the honey harvest. However, researchers have found that these caterpillars can consume more than honeycombs. They can also degrade certain types of plastic. Exactly how is not yet fully understood, but now researchers have come a step closer to discovering how the caterpillars biodegrade the otherwise nearly nonbiodegradable plastic.
“The big question has been whether it is the waxworm caterpillars themselves or the bacteria inside them that biodegrade the plastic. Our new research shows that the function of the caterpillars is probably to prepare the plastic for biodegrading by increasing the surface area many thousands of times. This enables the plastic to be biodegraded more quickly afterwards, although the bacteria inside the caterpillars carry out the actual biodegradation. If we can learn how to copy this symbiotic biodegradation, we may be able to build facilities that can biodegrade plastic and recycle its components,” explains Jeppe Lund Nielsen, Professor with Specific Responsibilities, Department of Chemistry and Bioscience, Aalborg University.
Very fine plastic strands
Almost 3 years ago, researchers at the University of Cambridge discovered the astonishing properties of these waxworm caterpillars, which normally live on beeswax and other parts of beehives but can also biodegrade plastic bags and food wrappers made of polyethylene, nearly 100 million tonnes of which is produced annually.
“Initially, the suggestion was that these waxworm caterpillars degrade the plastic independently of their gut flora. In our experiments, we wanted to clarify that uncertainty so that we could better understand the role of both the caterpillars and the bacteria. We therefore tried to investigate how the proteins in the caterpillars’ salivary glands were affected by the plastic and whether these proteins could degrade the plastic independently,” says Jeppe Lund Nielsen.
The researchers therefore placed the waxworm caterpillars into small containers and fed them their favourite foods, honey and beeswax – supplemented with small pieces of polyethylene. After 10 days, the researchers removed the caterpillars and examined both the plastic and the salivary glands.
“We found that the proteins in the waxworm caterpillars’ salivary glands did not degrade the polyethylene but rather the mastication by the larvae increased its surface area. So those pieces of plastic had turned into very fine strands instead. The caterpillars had increased the surface area thousands of times. We also noticed some small changes in the chemical groups on the surface,” explains Jeppe Lund Nielsen.
The researchers interpret the results as a sign that the waxworm caterpillars prepare the plastic before it reaches the intestinal microbiota, where these bacteria probably help to carry out the actual degradation of the plastic. Another major question then was whether the caterpillars actually derive energy from eating the plastic.
“We therefore examined the composition of proteins in the salivary glands, and this showed that the waxworm caterpillars alter their energy metabolism when they eat the plastic: for example, the enzymes associated with digesting fatty acids were activated and hunger hormones were secreted. We interpret the results that the caterpillars do not obtain energy from eating the plastic, but instead it stresses their system, and thus it does not seem that they can metabolize it,” says Jeppe Lund Nielsen.
Reusable building blocks
The new results thus suggest that the waxworm caterpillars live in symbiosis with the bacteria, with the caterpillars carrying out the initial processing and the bacteria managing the decisive biodegradation of the material from which energy is obtained from the food, which usually consists of beeswax and other parts of beehives.
“The research came about because our Iranian collaborators were looking for partners to help them investigate and understand the physiology of complex systems: how organisms convert food into energy. We usually characterize microbiomes in eukaryotic systems, such as insects, and work related to microbiology in technical systems, so this system is doubly interesting because we can reveal how nature combines initial processing by insects with biodegradation,” explains Jeppe Lund Nielsen.
Polyethylene adversely affects the entire ecosystem because it is virtually nonbiodegradable, but these waxworm caterpillars have apparently cracked the code, although the natural process is very slow in nature. The researchers therefore hope in the long term to be able to discover and copy the trick of the caterpillars to degrade some of the 100 million tonnes of polyethylene used annually for plastic bags and packaging.
“If we can learn how the waxworm caterpillars use mastication in combination with their enzymes and their microbiome, we might be able to replicate these processes so that instead of incinerating plastic, we can degrade it and recycle the resulting products. These building blocks can then be turned into new plastic or used to produce other substances,” says Jeppe Lund Nielsen.
“Impact of Polyethylene on Salivary Glands Proteome in Galleria Melonella" has been published in Comparative Biochemistry and Physiology. In 2016, the Novo Nordisk Foundation awarded a grant to Jeppe Lund Nielsen for the project Novel Screening Approaches for Identification of Enzymes of Biotechnological Interests Directly in Complex Microbial Consortia.