Endophytic bacteria and fungi from atmospheric mercury bioindicator Tillandsia usneoides (Spanish moss) alleviate mercury stress and promote plant growth

Gaseous elemental mercury (Hg 0 ) poses significant risks to ecosystems due to its high volatility and bioavailability. However, it remains largely unknown how the endophytes of Tillandsia usneoides (Spanish moss), a biological indicator responds to Hg 0 . In this study, after 14 days of exposure to the vapor from 0.22 mL of liquid Hg 0 , T. usneoides accumulated a markedly elevated Hg content of 164, 900 ± 28, 900 μg kg −1 . High-throughput sequencing of 16S rRNA and ITS genes revealed that Hg 0 exposure significantly reduced the α-diversity of endophytic bacteria and altered the β-diversity of both bacterial and fungal communities. Under Hg 0 stress, taxonomic shifts included increased relative abundances of Pseudomonas , Enterobacter , and Acidiella . Functional predictions further indicated upregulated expression of key enzymes involved in Hg detoxification and antioxidant defense, such as mercuric reductase (MerA), glutathione S-transferase (GST), catalase (CAT), and superoxide dismutase (SOD). From Hg 0 -exposed T. usneoides , we isolated five endophytic bacteria ( Staphylococcus sp. L3, Pseudomonas sp. L9, Enterobacter L19, Enterobacter L6, Bacillus LE) and two fungi ( Aspergillus G1, G2), all of which demonstrated strong Hg 2+ transformation and tolerance. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) confirmed morphological adaptations and Hg enrichment on microbial surfaces. Foliar inoculation with strains L6, LE, and G2 significantly alleviated Hg 0 -induced growth inhibition in Arabidopsis thaliana , enhancing biomass, leaf/root length, and root surface area, while mitigating auxin suppression. Overall, this study clarifies how T. usneoides endophytes respond to Hg 0 and highlights their promising role in microbial-assisted phytoremediation of Hg 0 pollution. • Endophytic bacterial and fungal communities of Hg 0 bioindicator T. usneoide to Hg 0 is analyzed. • Hg 0 significantly reduces the diversity of endophytic bacterial communities. • Ascomycota, Pseudomonas , Enterobacter and Acidiella are significantly enriched under Hg 0 . • Endophytic bacteria isolated mitigates Hg toxicity by reducing Hg 2+ to Hg 0 . • Endophytic fungus G2 alleviates Hg stress by adsorbing Hg 2+ . • Foliar application of endophytes alleviates Hg 0 stress and improves plant growth.