Over the past decades, industrial biotechnology has been shaped by the ever-increasing demands of either replacing or supplanting petroleum-based fuels and chemicals with lower carbon footprints and more sustainable bio-based products using renewable feedstocks. One of the pioneering biotechnological applications in the development of bio-based products is the production of biofuels by native and genetically-engineered microbes through fermentation of biomass-based feedstocks including first-generation biomass (Edible crops) and second-generation biomass (Lignocellulosic biomass) [1]. Importantly, metabolic engineering and synthetic biology strategies were implemented with the aim of improving the innate metabolic and bioprocess capacities as well as expanding the repertoire of microbial growth substrates [2], [3], [4]. Aided by states and industrial interests in the use of bio-ethanol as the designated bio-fuel and potential gasoline substitute, microbial strain developments were much focused on maximizing ethanol titer and improving the conversion of cellulosic biomass as part of non-food-based feedstock [5], [6]. To date, several bioprocessing strategies have been proposed and developed including biological and chemical pretreatments of lignocellulosic biomass to release fermentable sugars from the complex components of cellulose and hemicellulose [7], [8]. The key feature of strain development in industrial biotechnology is the establishment of biorefinery and consolidated bioprocessing through the conferment of enhanced and often new-found abilities to overproduce desired products and utilize low-cost non-native substrates [9]. These whole-cell biocatalysis strategies have been the cornerstone of many bioproduct developments, especially with regards to the conversion of industrial and agricultural wastes including glycerol and cooking oils as renewable carbon resources [10], [11]

Microbial engineering as advanced biotechnology platform for circularizing plastic economy

The emergence of synthetic biology technology with biofoundry-style alliance has brought about the convergence of advanced biotechnology platforms dedicated for providing bio-based solutions to the community and industry [12]. Guided with iterative Design, Build, Test and Learn cycle, huge strides have been made especially in the use of engineered microbial chassis for the production of industrially-important chemicals using model microbes specifically Escherichia coli and Saccharomyces cerevisiae [13], [14]. A host of metabolically-versatile microbes such as Corynebacterium glutamicum and Pseudomonas putida were increasingly employed where the focuses were on attaining a robust chassis with high bioproduction capacity as well innate tolerance to unwanted and toxic fermentation byproducts [15]. Genetic and pathway engineering tools are now becoming more expansive and readily available, thereby enabling researchers to create modular and orthogonal expression of genetic parts across different bacterial chassis [16], [17]. However, although advancements in synthetic biology, rapid prototyping and reverse engineering have been made, the titer, rate, and yield (TRY) of the biochemical reactions remain the bottlenecks in the bio-based industry [18], [19]. In this regard, stoichiometric modelling of metabolic networks and flux balance analysis (FBA) have enabled in silico-guided pathway engineering and redesigning aimed at obtaining stoichiometry-balanced metabolic conversions with high specificity and maximum product formation in bioengineered hosts [20], [21]. With the establishment of many microbial hosts and chassis for the bioconversion of cellulosic substrates, the attention now turns towards the production of commodity, platform and specialty chemicals that serve as the building blocks of various polyesters and synthetic plastics.

Increasing demands in the bioplastic markets and growing calls for decarbonization strategy and sustainability in the petrochemical industries have culminated in notable shifts of circularity among plastic manufacturers [22], [23], [24]. Among the plastic markets, polyethylene (PE including low-density LDPE and high-density HDPE) and poly(ethylene terephthalate) (PET) are considered as the two most lucrative plastic markets where these plastics are manufactured through steam cracking and catalytic refining processes [25]. PE plastic is derived from catalytic polymerization of gaseous ethylene monomer which also serves as building blocks for (mono)ethylene glycol (EG), one of the two monomers of PET plastic. For PET plastic manufacturing, EG is polymerized with terephthalic acid (TPA), another PET monomer obtained from naphtha-derived xylene. In 2021, the global market size was estimated to be worth 163 billion USD and 32.33 billion USD for PE and PET plastics, respectively [26], [27]. Driven by the environmental concerns of the steam cracking technology and growing global demands of circular bioeconomy and new plastics economy, the global production capacities of bioplastics including bio-PE and bio-PET was projected to reach 6.3 million tonnes in 2027 [28].

In the bio-based plastics market, PEF plastics emerge as a 100% bio-based polymer alternative to PET plastics. PEF is produced via the polymerization of EG and furan dicarboxylic acid (FDCA) monomers that can be completely produced from bioresources. Compared to PET, PEF plastic has superior thermal and barrier properties with significant life-cycle GHG emission reduction of 50-74% for PEF bottles when compared to an equivalent PET 250 mL bottle [29]. The bio-based PEF plastic was projected to enter the market through the commercialization activities of Avantium and partnering companies [30]. Despite the adverse impacts of the COVID-19 pandemic on global bioplastics production capacities, PEF plastic is anticipated to experience market growth. By 2032, it is projected to reach a market value of USD 76.7 Million, compared to USD 34.6 Million in 2022 [31]. The estimated production of PEF plastic in 2024 is 2.40 kilotons, with expectations of continual growth to 6.01 kilotons by 2029 [32]. Meanwhile bio-PET market size is projected to be reduced to 1.8% in 2027 from 4.2% in 2022 [28]. The declining interest in bio-PET production can be attributed to the lack of feasible technology to produce bio-based TPA whereby fossil-based TPA constitutes 70% of the PET plastic as compared to the 100% renewable bio-PEF plastic [33]. Although considered non-biodegradable plastics, commercial manufacturing of bio-PEF is expected to contribute in advancing the new plastics economy owed to the renewability, recyclability and decarbonization potentials of bioplastics [29], [34]. However, the existing process for FDCA production necessitates costly and environmentally hazardous catalysts, along with high temperature and pressure conditions, raising significant environmental concerns [35]. Therefore, adopting a more sustainable approach involving enzymes or genetically engineered cells can mitigate these challenges. Circularizing the bio-based plastics economy requires the improvement of existing post-consumer waste management for the overall manufacturing processes of plastic products as well as the feedstock used in the production of the plastic monomers. Considering that bio-feedstocks such as molasses (1 G PEF) and sugarcane bagasse (2 G PEF) contribute at least 20% to overall PEF production [34], similar hurdles need to be addressed in order to realize a viable and circular PEF plastic economy.

Biorefinery and biotechnological approach in the reuse of plastic waste has gained considerable interest whereby plastic wastes are recycled and given added value to produce new products. This will ultimately reduce the environmental impacts of the current linear plastic economy and conserve biological resources [36], [37]. In 2015, a combined total of 149 million tonnes of PE and PET plastics were manufactured, representing 39% of the overall plastic production [25]. The persistence of plastics discarded in the environment has been attributed to a number of detrimental consequences including the accumulation of hormone-disrupting microplastics within the food chain [25], [38]. Upcycling and converting these plastic wastes into value-added products via biorefinery-based technology would therefore provide a green biotechnological solution. This is achieved by using plastic-derived substrates as new reusable resources, in addition to the utilization of lignocellulosic biomass as renewable feedstocks [39], [40]. Principally, monomers obtained from enzymatically oxidized or hydrolyzed PE and PET plastics can be used as substrates for microbial growth, biomass formation and bioproduction of desired compounds. Importantly, the breakdown of the plastic polymer into its constituent monomers must be thorough, as incomplete degradation could yield compounds potentially toxic to other microorganisms [41]. Using microbial enzymes, PEF monomers specifically EG and FDCA can both be produced via fermentation through the conversion of biomass-derived substrates such as xylose and 5-hydroxymethylfurfural (HMF) [42].

Aided by computational and high throughput analytical tools, current progress in the fields of systems and synthetic biology have been instrumental in the efforts to accelerate bioprocess development for bio-chemicals manufacturing. Towards providing an alternative feedstock for bio-PEF polymer production, this review highlights recent advancements in microbial systems biology and synthetic biology approaches for industrial biotechnology applications. An emphasis is placed on the employment of omics technologies in elucidating the conversion and assimilation of substrates from cellulosic biomass and synthetic plastics, particularly PE and PET plastics. The importance of transporter proteins in the uptake and assimilation of substrates from treated biomass and plastic will also be discussed. Importantly, this work proposed the development of metabolic engineering-driven artificial microbial consortia for bio-PEF monomers from biomass and plastics as biorefinery feedstocks.