Biological Upcycling of Plastics Waste

Abstract

Plastic wastes accumulate in the environment, impacting wildlife and human health and representing a significant pool of inexpensive waste carbon that could form feedstock for the sustainable production of commodity chemicals, monomers, and specialty chemicals. Current mechanical recycling technologies are not economically attractive due to the lower-quality plastics that are produced in each iteration. Thus, the development of a plastics economy requires a solution that can deconstruct plastics and generate value from the deconstruction products. Biological systems can provide such value by allowing for the processing of mixed plastics waste streams via enzymatic specificity and using engineered metabolic pathways to produce upcycling targets. We focus on the use of biological systems for waste plastics deconstruction and upcycling. We highlight documented and predicted mechanisms through which plastics are biologically deconstructed and assimilated and provide examples of upcycled products from biological systems. Additionally, we detail current challenges in the field, including the discovery and development of microorganisms and enzymes for deconstructing non-polyethylene terephthalate plastics, the selection of appropriate target molecules to incentivize development of a plastic bioeconomy, and the selection of microbial chassis for the valorization of deconstruction products.

Keywords: biological plastic deconstruction; biological plastic upcycling; enzyme discovery; plastics bioeconomy.

Figure 1:. Overview of biological upcycling of plastic waste.

Plastic waste can be biologically deconstructed by microorganisms, microbial communities, or enzymatically. Degradation products are upcycled by microbial chassis into valorized products. Created using Biorender.

Figure 2:. Known and proposed mechanisms of biological plastics deconstruction.

A) Known PET degradation pathway consisting of 1. PETase, 2. MHETase, 3. TPA 1,2-dioxygenase, 4.1,2-dihydroxy-3,5-cyclohexadiene-1,4-dicarboxylate dehydrogenase. B) Proposed PU degradation and chain cleavage by 1. esterases and 2. ureases. C) Proposed PS degradation molecules. Enzymes capable of these chemistries are unknown. Some degradation products would feed into the alkane degradation pathway. D) PVC degradation pathway shown until the point where it intersects with LDPE, HDPE, and PP proposed degradation pathways. Pathway consists of 1. dehalogenases and 2. catalase peroxidases. E) Proposed straight chain alkane degradation pathways. Multiple proposed pathways are viable, starting with terminal oxidation consisting of 1. alkane monooxygenases, 2. alcohol dehydrogenases, 3. aldehyde dehydrogenases, 8. peroxidases and potentially 9. decarboxylases. Non-terminal oxidation allows for more alkane chains to be cleaved off and consists of 1. alkane monooxygenases, 2. alcohol dehydrogenases, 4. Baeyer-Villiger monooxygenases, and 5. esterases. This pathway would feed some molecules back into terminal oxidation. The Finnerty pathway could act as an alternative to terminal oxidation before feeding into the later stages of it. This pathway consists of 7. dioxygenases and 2. alcohol dehydrogenases. Biterminal oxidation is a possibility for small enough fragments of plastics relying on just an ω-fatty acid monooxygenase. Asterisks in the figures denote proposed enzymes.

Figure 3:. Overview of chassis selection and development for plastic upcycling.

A) Model organisms or non-model organisms isolated from plastic degrading communities can be used as upcycling chassis. B) Native metabolism, genetic tractability, and process growth conditions should be considered when selecting a chassis. C) Synthetic biology tools need to be developed for non-model chassis in order to engineer the host for plastic upcycling. Created using Biorender.