Metabolic Modulation in Phyllosphere Exiguobacterium aurantiacum-Seagrass Symbiosis Enhances Thermotolerance during Marine Heatwaves

Marine heatwaves driven by climatic changes are accelerating seagrass bed degradation. Harnessing microbiome technology to enhance seagrass stress resilience has emerged as a promising strategy for ecological conservation. Our investigation of microbial communities in Swan Lake (Weihai, China) revealed convergent rhizosphere profiles between Zostera marina and Z. japonica , both enriched with Propionigenium . Notably, the intertidal Z. japonica exhibited phyllosphere enrichment of Exiguobacterium aurantiacum and Bacillus aryabhattai compared to subtidal Z. marina . Functional characterization of isolated E. aurantiacum demonstrated thermotolerance, halotolerance, and indole-3-acetic acid (IAA) production. Microcosm experiments confirmed that E. aurantiacum inoculation elevated leaf elongation rates and chlorophyll retention under heat stress. KEGG analysis identified 19 significantly upregulated metabolic pathways—including tryptophan metabolism, flavonoid/isoflavonoid biosynthesis, and alkaloid synthesis—that collectively enhance thermal damage resistance. Genomic analysis further revealed multifunctional traits including antioxidation capacity, siderophore biosynthesis, facilitated nutrient acquisition, and osmoregulatory mechanisms. These findings deliver crucial theoretical and practical advances for understanding microbial-mediated host adaptation and developing microbiome-based conservation strategies for temperate seagrasses.