Methane seepage leads to a specific microplastic aging process in the simulated cold seep environment.

Marine microplastics pose a significant threat to ecosystems, and deep-sea regions serve as critical sinks for these pollutants. Among these regions, cold seeps harbor relatively high concentrations of microplastics. However, research on the aging of microplastics under low-temperature, dark, methane-abundant, and high-pressure conditions remains limited. Seawater and sediment were collected from various Haima cold seepage sites to simulate seepage environments in 200-mL high-pressure reactors. Four types of microplastics at high concentrations (approximately 10 %) were cultured and monitored over two months to explore how they aged. The key findings are as follows: (1) Compared to areas of weak seepage, methane seepage accelerated microplastic aging, as evidenced by increased surface roughness, enhanced C-O and (CO)-O bond formation, increased microbial colonization, and reduced contact angles. (2) Microplastic aging is more pronounced in sediments than in seawater, with biodegradable polylactic acid (PLA) exhibiting the most significant aging characteristics and carbon contribution. (3) Aged microplastics induce greater disturbances in inorganic nutrient levels than in organic matter, impacting nitrogen cycle processes involving nitrate, nitrite, and ammonium. This study results reveal the fundamental aging characteristics of microplastics in extremely deep seas and highlight their potential ecological effects.