Upcycling of hydrolyzed PET by microbial conversion to a fatty acid derivative

Biobased de novo synthesis, upcycling, and recycling - the heartbeat toward a green and sustainable polyethylene terephthalate industry

Polyethylene terephthalate (PET) has revolutionized the industrial sector because of its versatility, with its predominant uses in the textiles and packaging materials industries. Despite the various advantages of this polymer, its synthesis is, unfavorably, tightly intertwined with nonrenewable fossil resources. Additionally, given its widespread use, accumulating PET waste poses a significant environmental challenge. As a result, current research in the areas of biological recycling, upcycling, and de novo synthesis is intensifying. Biological recycling involves the use of micro-organisms or enzymes to breakdown PET into monomers, offering a sustainable alternative to traditional recycling. Upcycling transforms PET waste into value-added products, expanding its potential application range and promoting a circular economy. Moreover, studies of cascading biological and chemical processes driven by microbial cell factories have explored generating PET using renewable, biobased feedstocks such as lignin. These avenues of research promise to mitigate the environmental footprint of PET, underlining the importance of sustainable innovations in the industry.

Recent advances in the biological depolymerization and upcycling of polyethylene terephthalate

Polyethylene terephthalate (PET) is favored for its exceptional properties and widespread daily use. This review highlights recent advancements that enable the development of biological tools for PET decomposition, transforming PET into valuable platform chemicals and materials in upcycling processes. Enhancing PET hydrolases' catalytic activity and efficiency through protein engineering strategies is a priority, facilitating more effective PET waste management. Efforts to create novel PET hydrolases for large-scale PET depolymerization continue, but cost-effectiveness remains challenging. Hydrolyzed monomers must add additional value to make PET recycling economically attractive. Valorization of hydrolysis products through the upcycling process is expected to produce new compounds with different values and qualities from the initial polymer, making the decomposed monomers more appealing. Advances in synthetic biology and enzyme engineering hold promise for PET upcycling. While biological depolymerization offers environmental benefits, further research is needed to make PET upcycling sustainable and economically feasible.

Engineering Comamonas testosteroni for the production of 2-pyrone-4,6-dicarboxylic acid as a promising building block

BACKGROUND

Plastics are an indispensable part of our daily life. However, mismanagement at their end-of-life results in severe environmental consequences. The microbial conversion of these polymers into new value-added products offers a promising alternative. In this study, we engineered the soil-bacterium Comamonas testosteroni KF-1, a natural degrader of terephthalic acid, for the conversion of the latter to the high-value product 2-pyrone-4,6-dicarboxylic acid.

RESULTS

In order to convert terephthalic acid to 2-pyrone-4,6-dicarboxylic acid, we deleted the native PDC hydrolase and observed only a limited amount of product formation. To test whether this was the result of an inhibition of terephthalic acid uptake by the carbon source for growth (i.e. glycolic acid), the consumption of both carbon sources was monitored in the wild-type strain. Both carbon sources were consumed at the same time, indicating that catabolite repression was not the case. Next, we investigated if the activity of pathway enzymes remained the same in the wild-type and mutant strain. Here again, no statistical differences could be observed. Finally, we hypothesized that the presence of a pmdK variant in the degradation operon could be responsible for the observed phenotype and created a double deletion mutant strain. This newly created strain accumulated PDC to a larger extent and again consumed both carbon sources. The double deletion strain was then used in a bioreactor experiment, leading to the accumulation of 6.5 g/L of product in 24 h with an overall productivity of 0.27 g/L/h.

CONCLUSIONS

This study shows the production of the chemical building block 2-pyrone-4,6-dicarboxylic acid from terephthalic acid through an engineered C. testosteroni KF-1 strain. It was observed that both a deletion of the native PDC hydrolase as well as a pmdK variant is needed to achieve high conversion yields. A product titer of 6.5 g/L in 24 h with an overall productivity of 0.27 g/L/h was achieved.

Bio-upcycling of multilayer materials and blends: closing the plastics loop

The urge to discover and develop new technologies for closing the plastic carbon cycle is motivating industries, governments, and academia to work closely together to find suitable solutions in a timely manner. In this review article, a combination of uprising breakthrough technologies is presented highlighting their potential and complementarity to be integrated one with the other, therefore providing a potential solution to efficiently solve the plastics problem. First, modern approaches for bio-exploration and engineering of polymer-active enzymes are presented to degrade polymers into valuable building blocks. Special focus is placed on the recovery of components from multilayered materials since these complex materials can only be recycled insufficiently or not at all by existing technologies. Then, the potential of microbes and enzymes for resynthesis of polymers and reuse of building blocks is summarized and discussed. Finally, examples for improvement of the bio-based content and enzymatic degradability and future perspectives are given.

Recent Advances in the Chemobiological Upcycling of Polyethylene Terephthalate (PET) into Value-Added Chemicals

Polyethylene terephthalate (PET) is a plastic material commonly applied to beverage packaging used in everyday life. Owing to PET's versatility and ease of use, its consumption has continuously increased, resulting in considerable waste generation. Several physical and chemical recycling processes have been developed to address this problem. Recently, biological upcycling is being actively studied and has come to be regarded as a powerful technology for overcoming the economic issues associated with conventional recycling methods. For upcycling, PET should be degraded into small molecules, such as terephthalic acid and ethylene glycol, which are utilized as substrates for bioconversion, through various degradation processes, including gasification, pyrolysis, and chemical/biological depolymerization. Furthermore, biological upcycling methods have been applied to biosynthesize value-added chemicals, such as adipic acid, muconic acid, catechol, vanillin, and glycolic acid. In this review, we introduce and discuss various degradation methods that yield substrates for bioconversion and biological upcycling processes to produce value-added biochemicals. These technologies encourage a circular economy, which reduces the amount of waste released into the environment.

Chemo-Biological Upcycling of Poly(ethylene terephthalate) to Multifunctional Coating Materials

Chemo-biological upcycling of poly(ethylene terephthalate) (PET) developed in this study includes the following key steps: chemo-enzymatic PET depolymerization, biotransformation of terephthalic acid (TPA) into catechol, and its application as a coating agent. Monomeric units were first produced through PET glycolysis into bis(2-hydroxyethyl) terephthalate (BHET), mono(2-hydroxyethyl) terephthalate (MHET), and PET oligomers, and enzymatic hydrolysis of these glycolyzed products using Bacillus subtilis esterase (Bs2Est). Bs2Est efficiently hydrolyzed glycolyzed products into TPA as a key enzyme for chemo-enzymatic depolymerization. Furthermore, catechol solution produced from TPA via a whole-cell biotransformation (Escherichia coli) could be directly used for functional coating on various substrates after simple cell removal from the culture medium without further purification and water-evaporation. This work demonstrates a proof-of-concept of a PET upcycling strategy via a combination of chemo-biological conversion of PET waste into multifunctional coating materials.

Strategic Approach Towards Plastic Waste Valorization: Challenges and Promising Chemical Upcycling Possibilities

Plastic waste, which is one of the major sources of pollution in the landfills and oceans, has raised global concern, primarily due to the huge production rate, high durability, and the lack of utilization of the available waste management techniques. Recycling methods are preferable to reduce the impact of plastic pollution to some extent. However, most of the recycling techniques are associated with different drawbacks, high cost and downgrading of product quality being among the notable ones. The sustainable option here is to upcycle the plastic waste to create high-value materials to compensate for the cost of production. Several upcycling techniques are constantly being investigated and explored, which is currently the only economical option to resolve the plastic waste issue. This Review provides a comprehensive insight on the promising chemical routes available for upcycling of the most widely used plastic and mixed plastic wastes. The challenges inherent to these processes, the recent advances, and the significant role of the science and research community in resolving these issues are further emphasized.

MIXed plastics biodegradation and UPcycling using microbial communities: EU Horizon 2020 project MIX-UP started January 2020

This article introduces the EU Horizon 2020 research project MIX-UP, "Mixed plastics biodegradation and upcycling using microbial communities". The project focuses on changing the traditional linear value chain of plastics to a sustainable, biodegradable based one. Plastic mixtures contain five of the top six fossil-based recalcitrant plastics [polyethylene (PE), polyurethane (PUR), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS)], along with upcoming bioplastics polyhydroxyalkanoate (PHA) and polylactate (PLA) will be used as feedstock for microbial transformations. Consecutive controlled enzymatic and microbial degradation of mechanically pre-treated plastics wastes combined with subsequent microbial conversion to polymers and value-added chemicals by mixed cultures. Known plastic-degrading enzymes will be optimised by integrated protein engineering to achieve high specific binding capacities, stability, and catalytic efficacy towards a broad spectrum of plastic polymers under high salt and temperature conditions. Another focus lies in the search and isolation of novel enzymes active on recalcitrant polymers. MIX-UP will formulate enzyme cocktails tailored to specific waste streams and strives to enhance enzyme production significantly. In vivo and in vitro application of these cocktails enable stable, self-sustaining microbiomes to convert the released plastic monomers selectively into value-added products, key building blocks, and biomass. Any remaining material recalcitrant to the enzymatic activities will be recirculated into the process by physicochemical treatment. The Chinese-European MIX-UP consortium is multidisciplinary and industry-participating to address the market need for novel sustainable routes to valorise plastic waste streams. The project's new workflow realises a circular (bio)plastic economy and adds value to present poorly recycled plastic wastes where mechanical and chemical plastic recycling show limits.