Microplastics threaten seagrass carbon sinks through microbial changes

Seagrass beds are key blue carbon ecosystems but their capacity to sequester carbon is threatened by microplastic (MP) pollution in the marine environment. A 28-day mesocosm experiment examined the effect of microplastics and nutrient enrichment (NE) on eelgrass (Zostera marina). We tested concentrations of 320 mg MPs per 100 g DW sediment^-1 and 70 mg of total nitrogen per 100 g DW sediment^-1 to evaluate impacts on plant performance, microbiome composition and detritus decomposition (after 60-days assay). MPs reduced leaf growth rate (-39 %), net production (-57 %) and rhizome elongation (-35 %), while NE shifted microbial assemblages, increasing phyllosphere and rhizosphere by 1.6-fold. MPs favored the growth of microbial taxa related with the consumption of plant-derived polysaccharides and lignin compounds (1.7-fold higher), accelerating detritus decomposition by 1.5-fold. Critically, MPs produced a notable decrease of bacteria involved in sulfur-cycle (0.45-fold lower under combined MPs + NE vs. NE alone), disrupting REDOX processes essential for seagrass survival in reducing sediments. By reducing plant growth, altering microbial communities and accelerating organic matter turnover, MPs compromise the carbon sequestration capacity of eelgrass meadows. These findings evidence the need for policies addressing plastic pollution to safeguard seagrass ecosystem services while prompt further research on mechanisms of interaction between MPs and biota.