Microbial degradation of plastics in the environment: Mechanisms, enzymatic pathways, and constraints from laboratory studies to environmental reality

Microplastic (MP) pollution has become a persistent environmental challenge, raising increasing concern due to its global distribution and potential risks to human health. Biological degradation, including microbial and insect mediated processes, represents a promising and environmentally sustainable strategy for mitigating plastic and MP pollution; however, its effectiveness remains highly variable and strongly context dependent. This review systematically summarizes recent advances in microbial degradation of plastic, as well as emerging research on insect mediated plastic biodegradation, focusing on degrading microorganisms, key enzymes, metagenomic discovery, and enzyme engineering strategies. A wide range of bacterial and fungal taxa capable of degrading major plastic polymers, including polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), polyvinyl chloride (PVC), polyurethane (PU), and polylactic acid (PLA), has been reported. In addition, several insect species capable of ingesting and transforming plastics have been identified as model systems for studying plastic degradation, where mechanical fragmentation, host digestive processes, and gut microbial metabolism jointly contribute to polymer transformation. Among these, PET degrading enzymes, particularly PETase, are the most extensively characterized, benefiting from detailed structural insights and intensive protein engineering efforts that have markedly enhanced catalytic efficiency and thermostability. In contrast, enzymatic mechanisms involved in the biodegradation of polyolefins such as PE and PP remain poorly understood, representing a major knowledge gap. Recent metagenomic approaches have substantially expanded the repertoire of candidate plastic degrading enzymes by accessing uncultured MP associated microbial communities and insect gut microbiomes. Nevertheless, functional validation and evaluation under environmentally relevant conditions remain critical bottlenecks. Moreover, most reported degradation efficiencies are derived from optimized laboratory settings and may substantially overestimate microbial performance under natural environmental constraints, including low temperature, high salinity, nutrient limitation, and mixed polymer substrates. Overall, this review highlights the gap between laboratory based biodegradation studies and real world applications and emphasizes the need for integrated strategies to advance scalable plastic and MP remediation solutions.