Nixon, Morag
(2025)
Biodegradation of polyethylene using Pseudomonas sp.: developing sustainable processes for a circular economy.
PhD thesis, University of Nottingham.
Abstract
While synthetic polymers are valuable and versatile, their durability has led to an increasing disposal challenge. Current routes for plastic waste are limited to landfill, incineration and mechanical recycling. Biological recycling utilises microorganisms and enzymes to degrade plastic, producing sustainable chemical products. This project focuses specifically on polyethylene (PE), due to the significant contribution (~26 %) PE makes to the global plastic market. Additionally, polyolefin degradation is less understood compared to other plastic degradation.
Environmental strains have been isolated and screened for their ability to utilise PE as a sole carbon source. One isolate, characterised as a bacterium, showed potential and was further investigated. The process was scaled using 2.5 L bioreactors, which allowed for further chemical and genomic characterisation to help identify the key factors and mechanisms responsible for biodegradation. The PE was studied before and after treatment with the bacteria using analysis such as scanning electron microscopy imaging, Fourier-transform infrared spectroscopy, weight loss, differential scanning calorimetry and high temperature gel permeation chromatography. This has provided substantial evidence of chemical modifications and significant weight loss after bacterial treatment. The genome sequence along with multi-omic analysis has highlighted metabolites, genes and proteins upregulated on a feedstock of PE powder. These data provided insights which were able to propose a degradation mechanism and suggest the non-metabolic microbial systems involved in PE biodegradation.
This greater understanding was used to explore the enzyme(s) responsible for degradation. A selection of candidates was chosen for investigation using recombinant expression, during a placement with the industrial collaborator on this project, Johnson Matthey. An enzymatic assay to assess activity on PE powder was developed and preliminary data was obtained using the insoluble fraction of the expressed candidates.
Finally, PE biodegradation is unlike other polymers, during the biodegradation process the chemical monomers are not liberated. For this process a blend of different chain length oxidated hydrocarbons was produced. Therefore, to valorise the process the microorganism was engineered to synthesise a specific precursor to bioplastic. Several experiments were used to confirm the successful engineering. The targeted product could not be detected analytically, possibly due to a competing pathway, but an unidentified, potential intermediate was detected. This also presented an opportunity to scale the process further using 10 L bioreactors.
Overall, this project started by isolating a bacterium from the environment and ended with considerable evidence of a novel bacterium performing PE biodegradation at an unreported scale and efficiency. Detailed analysis was used to gain a greater understanding of the degradation mechanism. These advancements in PE biodegradation have moved research and industry closer to a sustainable, commercial plastic recycling technology.
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