Engineered microbes to enable a circular economy for biodegradable plastics.

Plastic pollution resulting from the continued dominance of fossil-derived polymers is a major global environmental challenge. Although biodegradable plastics, such as polylactic acid (PLA), polyhydroxyalkanoates (PHAs), polybutylene succinate (PBS), polybutylene adipate terephthalate (PBAT), and related materials, are increasingly being deployed as alternatives, their environmental performance is frequently constrained by infrastructure gaps, uncontrolled carbon loss, and incomplete degradation under realistic conditions. Therefore, biodegradability alone does not guarantee circularity of the material. To address this, intentional rerouting of plastics and their monomers into upcycling streams offers a widely applicable solution. This review advances the circular bioeconomy framework built on engineered depolymerization and metabolic bio-funneling of biodegradable and selected synthetic plastics. We present recent progress in enzyme-mediated polyester breakdown, emphasizing hydrolases and oxidoreductases, the kinetic and structural determinants of activity, and protein engineering strategies that broaden substrate scope and enhance operational stability. We then organize bio-upcycling strategies according to key metabolic entry nodes: pyruvate, acetyl-CoA/β-oxidation, and aromatic/dicarboxylate pathways, to demonstrate how plastic-derived monomers can be systematically redirected toward platform chemicals, fuels, specialty monomers, and next-generation biopolymers through pathway rewiring, flux control, and redox balance. In addition to biological conversion, we evaluate chemo-biological hybrid systems and integrated techno-economic and life cycle considerations, including process efficiency, enzyme cost, toxicity mitigation, and infrastructure compatibility. We further highlight emerging tools, such as systems biology, adaptive laboratory evolution, synthetic consortia design, and machine-learning-guided protein optimization, which accelerate the design-build-test-learn cycle for scalable microbial platforms for plastic upcycling. Collectively, this study reframes biodegradable plastics not as materials designed merely to disappear but as programmable carbon reservoirs that can be captured, upgraded, and reintegrated into biomanufacturing value chains. Actively closing the loop through engineered bio-upcycling, rather than relying on passive environmental degradation, offers a practical pathway to align plastic utility with environmental sustainability and achieve a truly circular bioeconomy.