Optimization of a Laccase Biosynthesis and Mediator System: A Potential Headway for Sustainable Microplastic Degradation

The global accumulation of microplastics has become a critical environmental challenge due to their persistence, widespread distribution, and adverse effects on ecosystems and human health. Although enzymatic degradation has emerged as a sustainable alternative to conventional physicochemical remediation methods, the practical application of laccase (Lac) remains constrained by low enzyme productivity, limited stability, high production costs, and insufficient activity towards recalcitrant non-phenolic polymers such as polyethylene and polystyrene. Furthermore, current studies are often fragmented, focusing on either enzyme production or degradation performance, while lacking an integrated assessment of catalytic optimization, mediator systems, scalability, and environmental sustainability. This review provides a critical and comprehensive evaluation of recent advances in optimizing Lac biosynthesis and laccase-mediator systems (LMS) for microplastic degradation. Unlike previous reviews, it integrates developments in strain engineering, heterologous expression, protein engineering, omics-guided process optimization, enzyme immobilization, mediator selection, and techno-economic considerations within a single framework. Particular attention is given to how LMS expands the oxidative capacity of Lac towards resistant plastic polymers and to the evidence required to distinguish surface modification from true polymer depolymerization. Current evidence suggests that combining optimized Lac production, suitable mediators, and advanced immobilization strategies can substantially enhance catalytic efficiency, operational stability, and process feasibility. However, challenges remain regarding degradation-product toxicity, mediator recovery, standardization of degradation assessment, and large-scale implementation. Future research should focus on pilot-scale validation, comprehensive ecotoxicological assessment, standardized life-cycle assessment, and integrated techno-economic analysis to identify environmental trade-offs associated with enzyme production, mediator utilization, and degradation-product management. Collectively, this review provides a forward-looking framework for advancing laccase-based technologies toward sustainable and scalable microplastic remediation.