Alternation magnitudes of organic matter composition determines priming effect of biodegradable microplastics on lake carbon emission

The ubiquitous existence of biodegradable microplastics (BMPs) in aquatic ecosystems necessitates a thorough understanding of their impact on aquatic carbon cycling. Although BMPs are known to release dissolved organic matter (DOM) into aquatic environment, their priming effect (PE) on native organic carbon remains largely unexplored. Here, we employed C-stable isotope labeling to quantify PEs induced by three common BMPs-polyhydroxyalkanoates (PHA), polylactic acid (PLA), and polybutylene succinate (PBS) in artificial lake water. The addition of all three BMPs to lake DOM enhanced CO emission but their PE differed: PLA and PHA induced positive PE, whereas PBS induced negative PE. Moreover, pre-solar irradiation of BMPs further enhanced CO emission and strengthened PEs. DOM leached from BMPs significantly altered both the quantity and quality of the lake DOM pool. Microbial respiration was primarily driven by the quantity of chromophoric DOM (CDOM), whereas the PE was governed by the quality of DOM, as reflected by the response ratios of freshness index (BIX) and the relative abundance of protein-like component following BMP addition. Collectively, our findings revealed that BMPs' impacts on freshwater carbon cycling are mediated by the compostion of DOM and further modulated by solar irradiation, which severs as an environmental magnifier. This underscores the urgent need to re-evaluate the environmental risks of BMPs, explicitly considering their effects on carbon cycling and ecosystem functioning, rather than assessing their sustainability solely based on degradability.