Distribution characteristics and potential microbial degradation mechanisms of microplastics in oyster aquaculture areas of southern China

Microplastic (MP) pollution in coastal aquaculture is a growing environmental and public health concern. Despite increasing reports, the cross-regional and cross-media pollution patterns, ecological risks, and microbial degradation potentials in aquaculture ecosystems remain poorly understood. We investigated oyster farming systems in South China: Zhanjiang Bay (ZJB, semi-enclosed) and Xuwen (XW, open coast). MP abundances ranged from 20 to 54 items/L in seawater and 950-6483 items/kg in sediment, with particles < 50 μm and granular shapes dominant in both media, as determined by Laser Direct Infrared Imaging. MP spatial patterns differed markedly between regions; XW exhibited higher seawater MP levels attributed to larger farming scales, whereas ZJB showed greater sediment MP accumulation owing to weaker water exchange and a longer farming history. Source apportionment identified aquaculture facilities as the primary source (44.86%). Notably, while the overall pollution load was relatively low, the potential ecological risk index reached 866.51 (classified as "dangerous"), driven predominantly by highly toxic polymers such as polyurethane (PU) and polyvinyl chloride (PVC). The distribution of plastic-degrading genes (PDGs) and their host microbial communities was primarily determined by these aquaculture facilities and the environmental medium (sediment vs. seawater), rather than by localized water-quality conditions. Metagenomic analysis identified sediments as key metabolic hotspots, harboring diverse functional genes involved in polyethylene β-oxidation, polystyrene aromatic ring cleavage, and PU hydrolysis. These findings bridge the gap in understanding MP dynamics between diverse aquaculture habitats and highlight the potential of indigenous microbes in natural attenuation, providing critical insights for MP risk management.