Critical effect of biodegradation on long-term microplastic weathering in sediment environments: A systematic review

Microplastic (MP) pollution in global sediment has been intensely studied and recognized as the ultimate sink for residual MPs in terrestrial and aquatic ecosystems. During MP long-term retention in sediments, plastic-degrading bacteria (i.e., Flavobacteriaceae, Bacillus, Rhodobacteraceae, and Desulfobacteraceae) can utilize those MPs as their carbon and energy sources through enzyme (hydrolase and oxidoreductase) reactions, which further alter or transform high molecular weight MP polymers into lower molecular weight biodegradation byproducts (i.e., monomers and oligomers) and release toxic additives. In other words, MPs can act as durable substrates for plastic-degrading bacteria in sediments. However, to date, the biodegradation rates of MPs in sediment environments are still poorly understood due to their limited degradation efficiency. Herein, we review the enzyme-induced biodegradation processes of MPs in sediment environments, which is important for accessing the alteration of MP properties and their potential ecological risks after undergoing long-term weathering processes. In addition, the factors associated with the MP properties (polymer type, molecular weight, crystallinity, and hydrophobicity) and sediment conditions (sediment type, temperature, pH, salinity, and oxygen content) that influence plastic degradation processes are also reviewed. The mechanisms may relate to the MP properties and sediment conditions that can influence microbial abundance, enzyme concentrations, and enzyme activities, thus altering MP biodegradation ratios. We anticipate that the observations reviewed in this study will pose a new issue to better understand the formation process, fate, and potential ecological risks associated with aged MPs in sediment environments.