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Microbial degradation and the persistence paradox of conventional and biodegradable microplastics
Summary
"Biodegradable" plastics are often marketed as an eco-friendly alternative to regular plastic, but this review of existing research finds that in real-world conditions, many of them don't actually fully break down—they just crumble into smaller microplastic pieces, just like conventional plastic does. This matters because these lingering microplastic fragments (regardless of what type of plastic they came from) can end up in soil, water, food, and ultimately our bodies, meaning switching to "biodegradable" products may not reduce microplastic exposure as much as people assume.
Microplastics (MPs), defined as plastic particles smaller than 5 mm, arise both from intentionally manufactured small plastic particles and the fragmentation of larger plastic debris. While conventional microplastics (CMPs) from fossil-based polymers have been widely studied, the increasing use of biodegradable plastics introduces a new category of contaminants known as bio-microplastics (BMPs). Crucially, under ambient conditions, BMP degradation often stalls at the fragmentation stage, failing to reach complete mineralization and leading to environmental persistence. This critical narrative review synthesizes current knowledge on the environmental fate and transport of microplastics, their ecotoxicological interactions across trophic levels, and the enzymatic and microbial processes governing polymer depolymerization and mineralization of both CMPs and BMPs. Particular emphasis is placed on the ‘persistence paradox’, whereby biodegradable plastics fragment into micro-sized particles under natural conditions without achieving complete mineralization, allowing these particles to remain in the environment for extended periods. Microbial degradation pathways involving bacterial and fungal taxa, enzymatic mechanisms, and physicochemical constraints are critically evaluated alongside measurable degradation endpoints, including mass loss, molecular-weight reduction, and mineralization evidence. Current findings indicate that many reported biodegradation studies demonstrate partial fragmentation or surface erosion rather than complete mineralization under environmentally realistic conditions. Future progress will require standardized degradation assessment frameworks, environmentally relevant testing conditions, and regulatory criteria that distinguish true mineralization from fragmentation alone to ensure that biodegradable plastics provide measurable environmental benefits without contributing to persistent microplastic pollution.