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Review
. 2024 Jun 3;15(1):4715.
doi: 10.1038/s41467-024-49146-8.

Bottlenecks in biobased approaches to plastic degradation

Amelia R Bergeson  1 , Ashli J Silvera  1 , Hal S Alper  2   3
Affiliations
Review

Bottlenecks in biobased approaches to plastic degradation

Amelia R Bergeson et al. Nat Commun. .

Abstract

Plastic waste is an environmental challenge, but also presents a biotechnological opportunity as a unique carbon substrate. With modern biotechnological tools, it is possible to enable both recycling and upcycling. To realize a plastics bioeconomy, significant intrinsic barriers must be overcome using a combination of enzyme, strain, and process engineering. This article highlights advances, challenges, and opportunities for a variety of common plastics.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Life cycle of plastic waste.
Possible pathways for plastic waste utilization and repurposing include bio-enabled depolymerization/repolymerization, composting, and upcycling. Depending on the approach used (cell-based or enzyme-based) as well as plastic type, biodegradation may result in either the generation of directly-reusable plastic monomers or oligomers. The biodegradation product generated will then dictate the next phase of the waste plastic’s life cycle and hence entry-point into the biorefinery cycle. Original monomers are more convenient starting points for repolymerization whereas the production of oligomers is more conducive to upcycling.
Fig. 2
Fig. 2. Plastic production by type and mechanical recyclability.
Plastic is not infinitely recyclable through traditional means. The temperature and processing during mechanical recycling can result in degraded polymer and material properties which limits the number of times a plastic can be mechanically recycled–. The number of times a plastic can be mechanically recycled is also drastically impacted by the initial feedstock, the addition of chemical additives, processing temperature, and blending different polymer types together. When these traits are paired together with overall plastic production data (adapted from ref. ), it is clear that new technologies are required to improve the reuse-capability of most of the highly-produced plastics.
Fig. 3
Fig. 3. Proposed enzymatic pathways for conversion of multiple plastic types.
Enzymatic conversion of plastic waste presents opportunities for coupling other treatments, both chemical and further biological, to produce a wide range of products. In some instances, the constituent monomers generated from enzymatic degradation present opportunities for conversion into new product while others pathways can yield the original monomer of interest, thus enabling a full end-to-end infinite biorecycling approach. Hypothesized (dashed arrows) and realized pathways (solid arrows) discussed throughout this article are depicted in the figure along with representative small degradation products. The capability to reuse, upcycle and repurpose these molecules is highlighted through the graphical legend.
Fig. 4
Fig. 4. Bottlenecks to bio-based plastic degradation.
Biological depolymerization of plastic can utilize whole-cell biocatalysts, purified enzymes or a combination of the two. Regardless of this choice, many of the remaining bottlenecks are rooted in the plastic’s material characteristics such as chemical composition/bond types, multilayered nature, additives, crystallinity, and hydrophobicity. Process parameters such as temperature, pH, and downstream products are dependent on the choice of biological approach but can also be leveraged to overcome various material trait bottlenecks.
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