Microbial and enzymatic biodegradation of synthetic plastics: mechanisms, environmental constraints, and translational limits

Synthetic plastics are among the most persistent anthropogenic pollutants due to their high molecular weight, hydrophobicity, and structural recalcitrance, driving global interest in biologically mediated degradation strategies. This narrative review critically examines current knowledge on microbial and enzymatic degradation of synthetic plastics, integrating insights from polymer chemistry, microbial ecology, enzymology, and environmental constraints. We synthesize evidence demonstrating that most reported microbial interactions with plastics represent biodeterioration and surface conditioning, rather than true depolymerization, assimilation, or mineralization. While plastisphere-associated microbial communities and extracellular enzymes can initiate oxidative and hydrolytic surface modifications, intrinsic polymer properties, environmental heterogeneity, and methodological limitations severely restrict progression toward complete biodegradation under realistic conditions. Among enzymatic systems, polyesterases, particularly PETase–MHETase pathways, represent a rare and well-validated case of true depolymerization, whereas polyolefin degradation remains fundamentally constrained. We further highlight how reliance on indirect proxies, such as weight loss and surface erosion, has contributed to overestimation of biodegradation potential, underscoring the need for rigorous validation. To advance the field, we advocate respirometric CO 2 evolution assays and 13 C-labeled polymers as gold-standard approaches for demonstrating microbial mineralization, alongside greater alignment with existing international standards. Finally, we discuss translational barriers, including scalability, biosafety, and regulatory challenges, and position cell-free enzymatic systems as a promising pathway bridging laboratory advances with environmentally and regulatorily acceptable applications.