Microplastics reduce eelgrass tolerance to heat stress with implications for restoration and blue carbon

Seagrass beds are key blue-carbon ecosystems, yet their resilience is increasingly challenged by microplastic (MP) pollution and marine heatwaves (MHWs). We experimentally tested how these stressors, alone and combined, affect the seagrass Zostera marina (eelgrass) using a controlled mesocosm experiment grounded in multiple-stressor and trait-based ecological theory. Plants were grown for 43 days in sediments with or without polyethylene/polypropylene MPs and a simulated MHW, (+5°C for 15 days) was imposed in the final phase. MP exposure significantly reduced rhizome elongation (-35%), total root length (-65%), and below-ground biomass, and depleted non-structural carbohydrates (NSC) in leaves and rhizomes (-35% to -40%). Warming alone stimulated leaf growth but further reduced NSC, while the MP × MHW interaction produced the lowest below-ground growth and carbohydrate reserves, consistent with synergistic stress predicted by multiple-stressor theory. MP exposure also reshaped the microbiome enriching putative sulfur-cycling taxa in the rhizosphere and indicating more reducing sediment conditions. With a carbon-balance and holobiont framework, MPs appear to constrain resource supply (oxygen and nutrients) and increase maintenance costs, whereas warming amplifies metabolic demand. The resulting carbon deficit limits below-ground growth, traits that underpin restoration success and blue-carbon function. These findings show the importance of incorporating microplastic monitoring into seagrass management to anticipate cumulative stress under a warming ocean. • Zostera marina was exposed to microplastics (28 d) and a marine heatwave (15 d) • Microplastics reduced growth and carbon reserves, while warming enhanced leaf growth • Combined stress impaired thermal tolerance and amplified microplastic effects • Microbiome shifts indicated partial adaptation and redox-related remediation • Reduced below-ground traits suggest lower blue-carbon and restoration potential