Microbial integration of plastics in the circular economy

In this proposal we will develop the technology required to use two recalcitrant plastic polymers, polyethylene terephthalate (PET) and polyurethane (PU) as feedstock for microbial transformations required for a circular economy. In particular, we will focus on the transformation of PET and PU waste into the industrially relevant and more sustainable moieties for Bio-PU polymers. With this approach we will also enable the recycling of the Bio-PU product, completing this way a fully circular strategy for the sustainable production of an important material.
The success of PET as a packaging material is only comparable to its resilience to degradation in the environment. Likewise, production rates of PU are also increasing, with the caveat that the recycling procedures for PU are virtually non-existing. PET can be degraded to constituent monomers using physicochemical processes such as pyrolysis and, more recently, by enzymatic hydrolysis. PU, on the other hand, is not so well characterised in terms of enzymatic hydrolysis, but microbial activities against the polymer have been recently described.
The synthesis of the monomers required for Bio-PU has been achieved using standard carbon sources (e.g. glucose) in different organisms. In MIPLACE we will synthesise them in biotransformations using the products of hydrolysis of PET as the sole energy and carbon source. Preliminary results from members in the consortium show that this can be achieved although with low yields.
Despite their limited scope, these previous observations support that PET and PU are feasible substrates for bacterial growth. In this proposal we will expand on those works and establish PET and PU as general substrates for biotechnological applications including the development of bacterial communities that can directly feed on PET and PU as the sole carbon source without the need of a prior lytic step.
In MIPLACE we have devised a multidisciplinary strategy based on the use of microbial communities for the effective transformation of PET and PU into Bio-PU. Our workflow is based on the traditional design-build-test cycle of engineering disciplines. We will use a combination of environmental screening of microorganisms, rational design of strains and lab directed evolution to achieve this goal. We build on the complementary strengths of the partners involved, which include enzymology, synthetic biology, metabolic engineering, mathematical modelling, chemical engineering and biodegradation. At the same time, we will address the societal concerns associated to this research that are related to the public perception of the approach taken and the possibility of influencing changes in consumer behaviour.
Overall, MIPLACE will deliver the technological development required for incorporating plastic polymers in the circular economy, paving the way for new biotechnological approaches based on a material that would otherwise be condemned to end up as waste otherwise. By developing efficient microbial PET transformation processes, we aim to use these waste streams in a completely new approach as fossil-based carbon sources for the production of value-added materials, providing the economic incentive to increase end-of-life plastic collection in favour of a sustainable bio-based economy.

The main goal of MIPLACE is to develop an efficient bio-based process that uses plastic waste as a feedstock to produce molecules of industrial interest. A fully circular approach will be applied to turn two types of plastic polymers, polyethylene terephthalate (PET) and polyurethane (PU), into the more environmentally friendly Bio-PU used as a construction and insulation material. For this, we will harness the potential of microbial communities to achieve the main goals of the proposal, namely: i) develop strategies for the effective hydrolysis of PET and PU monomers by taking advantage of natural or modified bacterial species expressing polyester hydrolases; ii) use engineered microbial communities to transform the hydrolysis products into products of interest; iii) use monomers resulting from PET and PU hydrolysis potentially in addition to other building blocks obtained from renewable sources (such as glycerol and acetate) to synthesise Bio-PU. As a result of this proposal we will contribute to the incorporation in the circular economy of two materials with limited recycling rates that are eventually discarded as waste. We will thereby create a novel path for their use as carbon source for microbial transformations that will expand beyond the focused applications described in this proposal.
We chose PET and PU as substrates because they are the only two types of plastic polymers derived from oil that can be readily degraded enzymatically to some extent. Both are polyesters, and a number of polyester hydrolase enzymes have been isolated capable of using PET and PU as substrates. In fact, members of this consortium have achieved the complete enzymatic hydrolysis of urban PET waste materials in vitro and used the resulting lysates to feed strains of the soil bacterium Pseudomas putida for the production of hydroxyalkanoyloxy-alkanoic acids (HAAs) and bioplastic polyhydroxyalkanoate (PHA) demonstrating the feasibility of the approach.

Plastics are extremely successful materials with many properties that make them almost essential in our lives. They are versatile, lightweight, waterproof, easy and affordable to synthesise and difficult to degrade. As a consequence, it is estimated that to date 8.3 billion metric tons (MT) of virgin plastics have been produced directly from oil, out of which 6.3 billion MT ended up as waste. Out of all the plastic waste only 9% was recycled, whereas 12% was incinerated. The remaining 79% of plastic waste is stored in landfills or has been directly released into the environment. This has led to alarming levels of pollution due to environmental accumulation of plastics, especially in marine environments, with devastating effects on the fauna and flora and numerous toxicological effects derived from the intake of microplastics resulting from the degradation of larger polymers.
In MIPLACE we consider plastics, mainly of urban origin, as alternative microbial feedstocks. We propose the biological up-cycling of these recalcitrant polymers, for which we do not only require enzymatic hydrolysis but also the metabolisation of its products to produce the biomass required for the transformations. In addition, MIPLACE will create a similar stream for PU, for which there are currently no suitable collection or recycling technologies. As a result we will deliver a complete pipeline for the transformation of waste into Bio-PU.
As opposed to consolidated bioprocesses using only one strain that are commonplace in industry, in MIPLACE we will rely on microbial communities and their interactions to deliver our research agenda. This is motivated by the complexity of the plastic substrates used as a feedstock as well as by the relatively large numbers of target molecules that can be produced. Furthermore, a community approach will be advantageous for a number of reasons such as: i) the enzymes required for polymer hydrolysis are secreted and are accessible to the whole population, therefore enabling complex dynamics in which the desired outcome may not always be favoured and, for this reason we will devise ways to stabilise microbial cooperation in our setup; ii) the production of target molecules from mixed substrates is likely to benefit from the distribution of tasks within a community that can split the cost of production and boost the reactions involved through metabolic exchanges; iii) the division of tasks will also help to devise strategies for lab directed evolution of individual modules of the pipeline such as enzymes for improved hydrolysis without affecting other components of the pipeline.
We have identified a market pull (the development of sustainable PU-based materials) with a large potential for producing revenue as is evidenced from the PU market size (expected to be $26.24 billion by 2024 in Europe; projection by Grand View Research). Although it is unclear how much of that market would correspond to Bio-PU, its production is likely to increase in the coming years aligning with social awareness and implementation of policies to prevent plastic misuse. SOPREMA is one of the European companies leading this change and MIPLACE will contribute towards this goal by satisfying an increasing demand.
MIPLACE will also act as a technology push. Considering the problems associated to a saturated recycling industry, the enablemenet of alternative paths to standard recycling is of utmost importance for a number of reasons. They allow to recycle PET polymers that are normally discarded such as opaque PET. In addition, they could also contribute to increasing the recycling rates of PU specially of resilient crosslinked PU polymers. In MIPLACE we have opted for a process that is fully compatible with microbial transformations that can go beyond our main goal Bio-PU. We envisage MIPLACE yielding a 'jack of all trades' process for the production of other added-value molecules out plastic waste.