Plastics undergo various aging processes in the environment, yet information on their behavior in real marine settings over time is limited. This study conducted a 12-month outdoor seawater exposure experiment in Wuyuan Bay, Xiamen, using polypropylene (PP) and polylactic acid (PLA) microplastics, along with their products (PP lunch boxes and PLA straws). Results indicated that prolonged seawater exposure significantly degrades polymers' surface and chemical structure, with degradation rates differing by time and polymer type. Plastic products were more susceptible to seawater erosion, fragmentation, and oxidation than microparticles. The first-order kinetic and logarithmic models effectively described the aging rates, with PP microparticles showing significantly higher rates than PLA microparticles due to PP's lower antioxidant performance. Distinct microbial community structures were found between PLA and PP plastics, with cyanobacteria exclusively colonizing PP and absent from PLA. A significant positive correlation (p < 0.001) was observed between the carbonyl index and the number of operational taxonomic units in plastic-associated biofilms, suggesting that aging may be influenced by natural sunlight and microbial colonization. This research improves our understanding of marine plastics' ageing behaviour and long-term environmental fate.
 

Plastics, Seawater Exposure, Degradation Rates, Biofilm Dynamics, Microbial Colonization