Successive stressors alter microbiome composition and reduce resilience in the eelgrass Zostera marina.

Seagrass meadows are among the most threatened ecosystems worldwide, facing multiple anthropogenic stressors that often occur in succession. While plant-physiological responses to multiple stressors are well documented, the role of microbial symbionts in mediating consecutive stressors events remains poorly understood. Using a mesocosm experiment, Zostera marina (eelgrass) was exposed to sequential stressors: nutrient enriched sediments (NE; 70 mg of total N per 100 gDW sediment-1 for 28 days), followed by a simulated marine heatwave (MHW, 23.3 °C for 15 days) and subsequent storm event (25 cm/s flow, 12 days). Nutrient enrichment resulted in a microbiome shift, specifically a 49.2-fold enrichment of sulfur-oxidizing Arcobacteraceae and a 4.7-fold increase in Sulfurimonadaceae, suggesting possible microbiome-mediated responses mitigating sulfide toxicity. In contrast, warming responses were primarily physiological: aboveground biomass increased by 41.5 % and net production increased by 37.1 % (mg FW shoot-1 day-1), with synergistic effects under combined enrichment and heat stress (up to 175 % higher production), indicating that temperature outweighs nutrient stress. Storm exposure triggered a 114 % increase in belowground biomass via root elongation, which increases the resilience of these plants to higher flow velocities, but this acclimation was diminished by 51 % in plants previously exposed to the MHW, indicating environmental legacy effects. Our results demonstrate that eelgrass resilience depends critically on stressors sequence, where legacy effects alter both plant-microbe interactions and physiological responses. These findings emphasize the need to incorporate both a consecutive-stressor approach and microbiome dynamics into seagrass research and conservation strategies under climate change.