Assessing the effect of microplastics on the marine ecosysteḿs carbon sequestration potential in life cycle assessment

• Microplastics impair marine carbon sequestration by inhibiting microalgal growth. • Impacts on the carbon sequestration vary spatially. • Arid and tropical ecosystems are most affected by plastics on a per kilogram basis. • The global carbon sequestration loss in 2020 is 75 kt CO2, worth $ 5.5 million. Marine ecosystems provide vital services, including carbon sequestration, yet face growing threats from microplastics (<5 mm), which disrupt microalgae growth—the foundation of oceanic carbon sinks. While life cycle assessment (LCA) is widely used to evaluate environmental impacts, specific methods for assessing impacts on ecosystem services, particularly within the marine ecosystems, are lacking. Our research aims to start bridging this gap by developing effect factors (EFs) for the impacts of microplastics on marine carbon sequestration within LCA. By integrating algae occurrence, climate zones, and laboratory results on microalgal growth inhibition by microplastics, we constructed five regionalized and one global species sensitivity distribution (SSD). These SSDs are then coupled with regional carbon sequestration, resulting in five spatially-differentiated and one global EF. Arid and tropical marine ecosystems have both the highest carbon sequestration and the greatest vulnerability to microplastic impacts. The potentially not sequestered carbon for 2020 in these zones is estimated at 25 kt of CO 2 (equivalent to 1.82 million dollars) and 48 kt of CO 2 (equivalent to 3.55 million dollars), respectively. Applying our EFs on a global scale reveals potentially not sequestered carbon of 75 kt CO2, translating to a financial loss of approximately 5.5 million U.S. dollars. This study offers the first global and spatially-distinct EFs for microplastic impacts on marine carbon sequestration in LCA. Our work enhances the understanding of microplastic impacts and underscores the importance of the marine carbon sequestration, representing a significant step towards comprehensive characterization factors.