An Evidence-Level Framework for Evaluating Enzyme-Mediated Plastic and Microplastic Transformation

Microbial enzymes have attracted considerable attention as biocatalysts for plastic transformation, yet the experimental evidence supporting reported biodegradation varies substantially in quality and interpretation. Surface-sensitive techniques, molecular-weight analyses, identification of transformation products, microbial assimilation assays, and carbon-tracking approaches each validate different stages of polymer transformation, but they are often treated as equivalent evidence of biodegradation. This review critically examines the analytical basis of enzyme-mediated plastic and microplastic transformation and introduces the Evidence-Level Framework (ELF), which classifies studies according to the highest experimentally validated transformation endpoint, from microbial colonization (ELF 0) to polymer-derived carbon conversion (ELF 5). Systematic screening identified 102 eligible experimental studies from an initial dataset of 150 publications, all of which were classified using the ELF to provide a comprehensive assessment of the current evidence landscape. Most studies clustered within intermediate evidence levels (ELF 2-3), where analytical validation was limited to polymer chain scission or the detection of soluble transformation products. By contrast, only a small proportion demonstrated microbial assimilation or unequivocal polymer-derived carbon conversion. Hydrolysable polyesters and their associated hydrolytic enzymes consistently reached the highest ELF categories because their chemical structure, enzymatic accessibility, and analytical tractability facilitate validation of successive transformation stages. Conventional plastics, however, remain constrained by polymer recalcitrance, limited substrate accessibility, microbial metabolic capacity, and the scarcity of analytical approaches capable of tracking polymer-derived carbon through biological systems. By providing a common framework for interpreting transformation claims, the ELF establishes objective criteria for experimental design, analytical validation, and comparison across independent studies, offering a stronger foundation for more reproducible, mechanistically robust, and environmentally relevant research on microbial plastic transformation.